Appendix A: Model Definition Reference

Chapter 4 explains the basics of defining models, and we use them throughout the rest of the book. There is, however, a *huge* range of model options available not covered elsewhere. This appendix explains each possible model definition option. Note that although these APIs are considered stable, the Django developers consistently add new shortcuts and conveniences to the model definition. It’s a good idea to always check the latest documentation online at [http://docs.djangoproject.com/](http://docs.djangoproject.com/). [TOC=3] ## Fields The most important part of a model – and the only required part of a model – is the list of database fields it defines. Field Name Restrictions Django places only two restrictions on model field names: 1. A field name cannot be a Python reserved word, because that would result in a Python syntax error. For example: ~~~ class Example(models.Model): pass = models.IntegerField() # 'pass' is a reserved word! ~~~ 2. A field name cannot contain more than one underscore in a row, due to the way Django’s query lookup syntax works. For example: ~~~ class Example(models.Model): foo__bar = models.IntegerField() # 'foo__bar' has two underscores! ~~~ Each field in your model should be an instance of the appropriate `Field` class. Django uses the field class types to determine a few things: * The database column type (e.g., `INTEGER`, `VARCHAR`). * The widget to use in Django’s forms and admin site, if you care to use it (e.g., `<input type="text">`,`<select>`). * The minimal validation requirements, which are used in Django’s admin interface and by forms. A complete list of field classes follows, sorted alphabetically. Note that relationship fields (`ForeignKey`, etc.) are handled in the next section. Note Technically, these models are defined in `django.db.models.fields`, but for convenience they’re imported into`django.db.models`; the standard convention is to use `from django.db import models` and refer to fields as`models.<Foo>Field`. ### `AutoField` *class *`AutoField`(***options*) An `IntegerField` that automatically increments according to available IDs. You usually won’t need to use this directly; a primary key field will automatically be added to your model if you don’t specify otherwise. ### `BigIntegerField` *class *`BigIntegerField`([***options*]) A 64 bit integer, much like an `IntegerField` except that it is guaranteed to fit numbers from`-9223372036854775808` to `9223372036854775807`. The default form widget for this field is a `TextInput`. ### `BinaryField` *class *`BinaryField`([***options*]) A field to store raw binary data. It only supports `bytes` assignment. Be aware that this field has limited functionality. For example, it is not possible to filter a queryset on a `BinaryField` value. Abusing `BinaryField` Although you might think about storing files in the database, consider that it is bad design in 99% of the cases. This field is *not* a replacement for proper static files handling. ### `BooleanField` *class *`BooleanField`(***options*) A true/false field. The default form widget for this field is a `CheckboxInput`. If you need to accept `null` values then use `NullBooleanField` instead. The default value of `BooleanField` is `None` when `Field.default` isn’t defined. ### `CharField` *class *`CharField`(*max_length=None*[, ***options*]) A string field, for small- to large-sized strings. For large amounts of text, use [`TextField`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.TextField "django.db.models.TextField"). The default form widget for this field is a `TextInput`. [`CharField`](http://masteringdjango.com/django-model-definition-reference/#CharField "CharField") has one extra required argument: `CharField.``max_length` The maximum length (in characters) of the field. The max_length is enforced at the database level and in Django’s validation. Note If you are writing an application that must be portable to multiple database backends, you should be aware that there are restrictions on `max_length` for some backends. MySQL users If you are using this field with MySQLdb 1.2.2 and the `utf8_bin` collation (which is *not* the default), there are some issues to be aware of. Refer to Chapter 23 for details. ### `CommaSeparatedIntegerField` *class *`CommaSeparatedIntegerField`(*max_length=None*[, ***options*]) A field of integers separated by commas. As in [`CharField`](http://masteringdjango.com/django-model-definition-reference/#CharField "CharField"), the [`max_length`](http://masteringdjango.com/django-model-definition-reference/#CharField.max_length "CharField.max_length") argument is required and the note about database portability mentioned there should be heeded. ### `DateField` *class *`DateField`([*auto_now=False*, *auto_now_add=False*, ***options*]) A date, represented in Python by a `datetime.date` instance. Has a few extra, optional arguments: `DateField.``auto_now` Automatically set the field to now every time the object is saved. Useful for “last-modified” timestamps. Note that the current date is *always* used; it’s not just a default value that you can override. `DateField.``auto_now_add` Automatically set the field to now when the object is first created. Useful for creation of timestamps. Note that the current date is *always* used; it’s not just a default value that you can override. The default form widget for this field is a `TextInput`. The admin adds a JavaScript calendar, and a shortcut for “Today”. Includes an additional `invalid_date` error message key. The options `auto_now_add`, `auto_now`, and `default` are mutually exclusive. Any combination of these options will result in an error. Note As currently implemented, setting `auto_now` or `auto_now_add` to `True` will cause the field to have`editable=False` and `blank=True` set. ### `DateTimeField` *class *`DateTimeField`([*auto_now=False*, *auto_now_add=False*, ***options*]) A date and time, represented in Python by a `datetime.datetime` instance. Takes the same extra arguments as [`DateField`](http://masteringdjango.com/django-model-definition-reference/#DateField "DateField"). The default form widget for this field is a single `TextInput`. The admin uses two separate `TextInput` widgets with JavaScript shortcuts. ### `DecimalField` *class *`DecimalField`(*max_digits=None*, *decimal_places=None*[, ***options*]) A fixed-precision decimal number, represented in Python by a `Decimal` instance. Has two requiredarguments: `DecimalField.``max_digits` The maximum number of digits allowed in the number. Note that this number must be greater than or equal to `decimal_places`. `DecimalField.``decimal_places` The number of decimal places to store with the number. For example, to store numbers up to `999` with a resolution of 2 decimal places, you’d use: ~~~ models.DecimalField(..., max_digits=5, decimal_places=2) ~~~ And to store numbers up to approximately one billion with a resolution of 10 decimal places: ~~~ models.DecimalField(..., max_digits=19, decimal_places=10) ~~~ The default form widget for this field is a `TextInput`. Note For more information about the differences between the `FloatField` and [`DecimalField`](http://masteringdjango.com/django-model-definition-reference/#DecimalField "DecimalField") classes, please see FloatField vs. DecimalField. ### `DurationField` *class *`DurationField`([***options*]) A field for storing periods of time – modeled in Python by `timedelta`. When used on PostgreSQL, the data type used is an `interval` and on Oracle the data type is `INTERVAL DAY(9) TO SECOND(6)`. Otherwise a `bigint` of microseconds is used. Note Arithmetic with `DurationField` works in most cases. However on all databases other than PostgreSQL, comparing the value of a `DurationField` to arithmetic on `DateTimeField` instances will not work as expected. ### `EmailField` *class *`EmailField`([*max_length=254*, ***options*]) A [`CharField`](http://masteringdjango.com/django-model-definition-reference/#CharField "CharField") that checks that the value is a valid email address. It uses `EmailValidator` to validate the input. ### `FileField` *class *`FileField`([*upload_to=None*, *max_length=100*, ***options*]) A file-upload field. Note The `primary_key` and `unique` arguments are not supported, and will raise a `TypeError` if used. Has two optional arguments: `FileField.``upload_to` A local filesystem path that will be appended to your `MEDIA_ROOT` setting to determine the value of the [`url`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.fields.files.FieldFile.url "django.db.models.fields.files.FieldFile.url")attribute. This path may contain `strftime()` formatting, which will be replaced by the date/time of the file upload (so that uploaded files don’t fill up the given directory). This may also be a callable, such as a function, which will be called to obtain the upload path, including the filename. This callable must be able to accept two arguments, and return a Unix-style path (with forward slashes) to be passed along to the storage system. The two arguments that will be passed are: | Argument | Description | | --- | --- | | `instance` | An instance of the model where the `FileField` is defined. More specifically, this is the particular instance where the current file is being attached. In most cases, this object will not have been saved to the database yet, so if it uses the default `AutoField`, *it might not yet have a value for its primary key field*. | | `filename` | The filename that was originally given to the file. This may or may not be taken into account when determining the final destination path. | `FileField.``storage` A storage object, which handles the storage and retrieval of your files. The default form widget for this field is a `ClearableFileInput`. Using a [`FileField`](http://masteringdjango.com/django-model-definition-reference/#FileField "FileField") or an `ImageField` (see below) in a model takes a few steps: 1. In your settings file, you’ll need to define `MEDIA_ROOT` as the full path to a directory where you’d like Django to store uploaded files. (For performance, these files are not stored in the database.) Define `MEDIA_URL` as the base public URL of that directory. Make sure that this directory is writable by the Web server’s user account. 2. Add the [`FileField`](http://masteringdjango.com/django-model-definition-reference/#FileField "FileField") or `ImageField` to your model, defining the [`upload_to`](http://masteringdjango.com/django-model-definition-reference/#FileField.upload_to "FileField.upload_to") option to specify a subdirectory of`MEDIA_ROOT` to use for uploaded files. 3. All that will be stored in your database is a path to the file (relative to `MEDIA_ROOT`). You’ll most likely want to use the convenience [`url`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.fields.files.FieldFile.url "django.db.models.fields.files.FieldFile.url") attribute provided by Django. For example, if your `ImageField` is called `mug_shot`, you can get the absolute path to your image in a template with `{{ object.mug_shot.url }}`. For example, say your `MEDIA_ROOT` is set to `'/home/media'`, and [`upload_to`](http://masteringdjango.com/django-model-definition-reference/#FileField.upload_to "FileField.upload_to") is set to `'photos/%Y/%m/%d'`. The`'%Y/%m/%d'` part of [`upload_to`](http://masteringdjango.com/django-model-definition-reference/#FileField.upload_to "FileField.upload_to") is `strftime()` formatting; `'%Y'` is the four-digit year, `'%m'` is the two-digit month and `'%d'` is the two-digit day. If you upload a file on Jan. 15, 2007, it will be saved in the directory`/home/media/photos/2007/01/15`. If you wanted to retrieve the uploaded file’s on-disk filename, or the file’s size, you could use the `name` and`size` attributes respectively; for more information on the available attributes and methods, see the `File`class reference and the Files topic guide. Note The file is saved as part of saving the model in the database, so the actual file name used on disk cannot be relied on until after the model has been saved. The uploaded file’s relative URL can be obtained using the [`url`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.fields.files.FieldFile.url "django.db.models.fields.files.FieldFile.url") attribute. Internally, this calls the `url()`method of the underlying `Storage` class. Note that whenever you deal with uploaded files, you should pay close attention to where you’re uploading them and what type of files they are, to avoid security holes. *Validate all uploaded files* so that you’re sure the files are what you think they are. For example, if you blindly let somebody upload files, without validation, to a directory that’s within your Web server’s document root, then somebody could upload a CGI or PHP script and execute that script by visiting its URL on your site. Don’t allow that. Also note that even an uploaded HTML file, since it can be executed by the browser (though not by the server), can pose security threats that are equivalent to XSS or CSRF attacks. [`FileField`](http://masteringdjango.com/django-model-definition-reference/#FileField "FileField") instances are created in your database as `varchar` columns with a default max length of 100 characters. As with other fields, you can change the maximum length using the [`max_length`](http://masteringdjango.com/django-model-definition-reference/#CharField.max_length "CharField.max_length") argument. #### FILEFIELD AND FIELDFILE *class *`django.db.models.fields.files.``FieldFile` When you access a `FileField` on a model, you are given an instance of [`FieldFile`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.fields.files.FieldFile "django.db.models.fields.files.FieldFile") as a proxy for accessing the underlying file. In addition to the functionality inherited from `django.core.files.File`, this class has several attributes and methods that can be used to interact with file data: `FieldFile.``url` A read-only property to access the file’s relative URL by calling the `url()` method of the underlying `Storage`class. `FieldFile.``open`(*mode=’rb’*) Behaves like the standard Python `open()` method and opens the file associated with this instance in the mode specified by `mode`. `FieldFile.``close`() Behaves like the standard Python `file.close()` method and closes the file associated with this instance. `FieldFile.``save`(*name*, *content*, *save=True*) This method takes a filename and file contents and passes them to the storage class for the field, then associates the stored file with the model field. If you want to manually associate file data with `FileField`instances on your model, the `save()` method is used to persist that file data. Takes two required arguments: `name` which is the name of the file, and `content` which is an object containing the file’s contents. The optional `save` argument controls whether or not the model instance is saved after the file associated with this field has been altered. Defaults to `True`. Note that the `content` argument should be an instance of `django.core.files.File`, not Python’s built-in file object. You can construct a `File` from an existing Python file object like this: ~~~ from django.core.files import File # Open an existing file using Python's built-in open() f = open('/tmp/hello.world') myfile = File(f) ~~~ Or you can construct one from a Python string like this: ~~~ from django.core.files.base import ContentFile myfile = ContentFile("hello world") ~~~ `FieldFile.``delete`(*save=True*) Deletes the file associated with this instance and clears all attributes on the field. Note: This method will close the file if it happens to be open when `delete()` is called. The optional `save` argument controls whether or not the model instance is saved after the file associated with this field has been deleted. Defaults to `True`. Note that when a model is deleted, related files are not deleted. If you need to cleanup orphaned files, you’ll need to handle it yourself (for instance, with a custom management command that can be run manually or scheduled to run periodically via e.g. cron). ### `FilePathField` *class *`django.db.models.``FilePathField`(*path=None*[, *match=None*, *recursive=False*, *max_length=100*,***options*]) A [`CharField`](http://masteringdjango.com/django-model-definition-reference/#CharField "CharField") whose choices are limited to the filenames in a certain directory on the filesystem. Has three special arguments, of which the first is required: `FilePathField.``path` Required. The absolute filesystem path to a directory from which this [`FilePathField`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.FilePathField "django.db.models.FilePathField") should get its choices. Example: `"/home/images"`. `FilePathField.``match` Optional. A regular expression, as a string, that [`FilePathField`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.FilePathField "django.db.models.FilePathField") will use to filter filenames. Note that the regex will be applied to the base filename, not the full path. Example: `"foo.*\.txt$"`, which will match a file called `foo23.txt` but not `bar.txt` or `foo23.png`. `FilePathField.``recursive` Optional. Either `True` or `False`. Default is `False`. Specifies whether all subdirectories of [`path`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.FilePathField.path "django.db.models.FilePathField.path") should be included `FilePathField.``allow_files` Optional. Either `True` or `False`. Default is `True`. Specifies whether files in the specified location should be included. Either this or [`allow_folders`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.FilePathField.allow_folders "django.db.models.FilePathField.allow_folders") must be `True`. `FilePathField.``allow_folders` Optional. Either `True` or `False`. Default is `False`. Specifies whether folders in the specified location should be included. Either this or [`allow_files`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.FilePathField.allow_files "django.db.models.FilePathField.allow_files") must be `True`. Of course, these arguments can be used together. The one potential gotcha is that [`match`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.FilePathField.match "django.db.models.FilePathField.match") applies to the base filename, not the full path. So, this example: ~~~ FilePathField(path="/home/images", match="foo.*", recursive=True) ~~~ …will match `/home/images/foo.png` but not `/home/images/foo/bar.png` because the [`match`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.FilePathField.match "django.db.models.FilePathField.match") applies to the base filename (`foo.png` and `bar.png`). [`FilePathField`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.FilePathField "django.db.models.FilePathField") instances are created in your database as `varchar` columns with a default max length of 100 characters. As with other fields, you can change the maximum length using the [`max_length`](http://masteringdjango.com/django-model-definition-reference/#CharField.max_length "CharField.max_length") argument. ### `FloatField` *class *`django.db.models.``FloatField`([***options*]) A floating-point number represented in Python by a `float` instance. The default form widget for this field is a `TextInput`. `FloatField` vs. `DecimalField` The [`FloatField`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.FloatField "django.db.models.FloatField") class is sometimes mixed up with the [`DecimalField`](http://masteringdjango.com/django-model-definition-reference/#DecimalField "DecimalField") class. Although they both represent real numbers, they represent those numbers differently. `FloatField` uses Python’s `float` type internally, while`DecimalField` uses Python’s `Decimal` type. For information on the difference between the two, see Python’s documentation for the `decimal` module. ### `ImageField` *class *`django.db.models.``ImageField`([*upload_to=None*, *height_field=None*, *width_field=None*, *max_length=100*,***options*]) Inherits all attributes and methods from [`FileField`](http://masteringdjango.com/django-model-definition-reference/#FileField "FileField"), but also validates that the uploaded object is a valid image. In addition to the special attributes that are available for [`FileField`](http://masteringdjango.com/django-model-definition-reference/#FileField "FileField"), an [`ImageField`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.ImageField "django.db.models.ImageField") also has `height` and `width`attributes. To facilitate querying on those attributes, [`ImageField`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.ImageField "django.db.models.ImageField") has two extra optional arguments: `ImageField.``height_field` Name of a model field which will be auto-populated with the height of the image each time the model instance is saved. `ImageField.``width_field` Name of a model field which will be auto-populated with the width of the image each time the model instance is saved. Requires the [Pillow](http://pillow.readthedocs.org/en/latest/) library. [`ImageField`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.ImageField "django.db.models.ImageField") instances are created in your database as `varchar` columns with a default max length of 100 characters. As with other fields, you can change the maximum length using the [`max_length`](http://masteringdjango.com/django-model-definition-reference/#CharField.max_length "CharField.max_length") argument. The default form widget for this field is a `ClearableFileInput`. ### `IntegerField` *class *`django.db.models.``IntegerField`([***options*]) An integer. Values from `-2147483648` to `2147483647` are safe in all databases supported by Django. The default form widget for this field is a `TextInput`. ### `GenericIPAddressField` *class *`django.db.models.``GenericIPAddressField`([*protocol=both*, *unpack_ipv4=False*, ***options*]) An IPv4 or IPv6 address, in string format (e.g. `192.0.2.30` or `2a02:42fe::4`). The default form widget for this field is a `TextInput`. The IPv6 address normalization follows [RFC 4291#section-2.2](https://tools.ietf.org/html/rfc4291.html#section-2.2) section 2.2, including using the IPv4 format suggested in paragraph 3 of that section, like `::ffff:192.0.2.0`. For example, `2001:0::0:01` would be normalized to `2001::1`, and `::ffff:0a0a:0a0a` to `::ffff:10.10.10.10`. All characters are converted to lowercase. `GenericIPAddressField.``protocol` Limits valid inputs to the specified protocol. Accepted values are `'both'` (default), `'IPv4'` or `'IPv6'`. Matching is case insensitive. `GenericIPAddressField.``unpack_ipv4` Unpacks IPv4 mapped addresses like `::ffff:192.0.2.1`. If this option is enabled that address would be unpacked to `192.0.2.1`. Default is disabled. Can only be used when `protocol` is set to `'both'`. If you allow for blank values, you have to allow for null values since blank values are stored as null. ### `NullBooleanField` *class *`django.db.models.``NullBooleanField`([***options*]) Like a [`BooleanField`](http://masteringdjango.com/django-model-definition-reference/#BooleanField "BooleanField"), but allows `NULL` as one of the options. Use this instead of a [`BooleanField`](http://masteringdjango.com/django-model-definition-reference/#BooleanField "BooleanField") with `null=True`. The default form widget for this field is a `NullBooleanSelect`. ### `PositiveIntegerField` *class *`django.db.models.``PositiveIntegerField`([***options*]) Like an [`IntegerField`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.IntegerField "django.db.models.IntegerField"), but must be either positive or zero (`0`). Values from `0` to `2147483647` are safe in all databases supported by Django. The value `0` is accepted for backward compatibility reasons. ### `PositiveSmallIntegerField` *class *`django.db.models.``PositiveSmallIntegerField`([***options*]) Like a [`PositiveIntegerField`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.PositiveIntegerField "django.db.models.PositiveIntegerField"), but only allows values under a certain (database-dependent) point. Values from `0` to `32767` are safe in all databases supported by Django. ### `SlugField` *class *`django.db.models.``SlugField`([*max_length=50*, ***options*]) Slug is a newspaper term. A slug is a short label for something, containing only letters, numbers, underscores or hyphens. They’re generally used in URLs. Like a CharField, you can specify [`max_length`](http://masteringdjango.com/django-model-definition-reference/#CharField.max_length "CharField.max_length") (read the note about database portability and [`max_length`](http://masteringdjango.com/django-model-definition-reference/#CharField.max_length "CharField.max_length") in that section, too). If [`max_length`](http://masteringdjango.com/django-model-definition-reference/#CharField.max_length "CharField.max_length") is not specified, Django will use a default length of 50. Implies setting [`Field.db_index`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.Field.db_index "django.db.models.Field.db_index") to `True`. It is often useful to automatically prepopulate a SlugField based on the value of some other value. You can do this automatically in the admin using `prepopulated_fields`. ### `SmallIntegerField` *class *`django.db.models.``SmallIntegerField`([***options*]) Like an [`IntegerField`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.IntegerField "django.db.models.IntegerField"), but only allows values under a certain (database-dependent) point. Values from`-32768` to `32767` are safe in all databases supported by Django. ### `TextField` *class *`django.db.models.``TextField`([***options*]) A large text field. The default form widget for this field is a `Textarea`. If you specify a `max_length` attribute, it will be reflected in the `Textarea` widget of the auto-generated form field. However it is not enforced at the model or database level. Use a [`CharField`](http://masteringdjango.com/django-model-definition-reference/#CharField "CharField") for that. MySQL users If you are using this field with MySQLdb 1.2.1p2 and the `utf8_bin` collation (which is *not* the default), there are some issues to be aware of. Refer to Chapter 23 for details. ### `TimeField` *class *`django.db.models.``TimeField`([*auto_now=False*, *auto_now_add=False*, ***options*]) A time, represented in Python by a `datetime.time` instance. Accepts the same auto-population options as[`DateField`](http://masteringdjango.com/django-model-definition-reference/#DateField "DateField"). The default form widget for this field is a `TextInput`. The admin adds some JavaScript shortcuts. ### `URLField` *class *`django.db.models.``URLField`([*max_length=200*, ***options*]) A [`CharField`](http://masteringdjango.com/django-model-definition-reference/#CharField "CharField") for a URL. The default form widget for this field is a `TextInput`. Like all [`CharField`](http://masteringdjango.com/django-model-definition-reference/#CharField "CharField") subclasses, [`URLField`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.URLField "django.db.models.URLField") takes the optional [`max_length`](http://masteringdjango.com/django-model-definition-reference/#CharField.max_length "CharField.max_length") argument. If you don’t specify[`max_length`](http://masteringdjango.com/django-model-definition-reference/#CharField.max_length "CharField.max_length"), a default of 200 is used. ### `UUIDField` *class *`django.db.models.``UUIDField`([***options*]) A field for storing universally unique identifiers. Uses Python’s `UUID` class. When used on PostgreSQL, this stores in a `uuid` datatype, otherwise in a `char(32)`. Universally unique identifiers are a good alternative to [`AutoField`](http://masteringdjango.com/django-model-definition-reference/#AutoField "AutoField") for [`primary_key`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.Field.primary_key "django.db.models.Field.primary_key"). The database will not generate the UUID for you, so it is recommended to use [`default`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.Field.default "django.db.models.Field.default"): ~~~ import uuid from django.db import models class MyUUIDModel(models.Model): id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False) # other fields ~~~ Note that a callable (with the parentheses omitted) is passed to `default`, not an instance of `UUID`. ## Universal Field Options The following arguments are available to all field types. All are optional. ### `null` `Field.``null` If `True`, Django will store empty values as `NULL` in the database. Default is `False`. Avoid using [`null`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.Field.null "django.db.models.Field.null") on string-based fields such as [`CharField`](http://masteringdjango.com/django-model-definition-reference/#CharField "CharField") and [`TextField`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.TextField "django.db.models.TextField") because empty string values will always be stored as empty strings, not as `NULL`. If a string-based field has `null=True`, that means it has two possible values for “no data”: `NULL`, and the empty string. In most cases, it’s redundant to have two possible values for “no data;” the Django convention is to use the empty string, not `NULL`. For both string-based and non-string-based fields, you will also need to set `blank=True` if you wish to permit empty values in forms, as the [`null`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.Field.null "django.db.models.Field.null") parameter only affects database storage (see [`blank`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.Field.blank "django.db.models.Field.blank")). Note When using the Oracle database backend, the value `NULL` will be stored to denote the empty string regardless of this attribute. If you want to accept [`null`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.Field.null "django.db.models.Field.null") values with [`BooleanField`](http://masteringdjango.com/django-model-definition-reference/#BooleanField "BooleanField"), use [`NullBooleanField`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.NullBooleanField "django.db.models.NullBooleanField") instead. ### `blank` `Field.``blank` If `True`, the field is allowed to be blank. Default is `False`. Note that this is different than [`null`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.Field.null "django.db.models.Field.null"). [`null`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.Field.null "django.db.models.Field.null") is purely database-related, whereas [`blank`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.Field.blank "django.db.models.Field.blank") is validation-related. If a field has `blank=True`, form validation will allow entry of an empty value. If a field has `blank=False`, the field will be required. ### `choices` `Field.``choices` An iterable (e.g., a list or tuple) consisting itself of iterables of exactly two items (e.g. `[(A, B), (A, B)...]`) to use as choices for this field. If this is given, the default form widget will be a select box with these choices instead of the standard text field. The first element in each tuple is the actual value to be set on the model, and the second element is the human-readable name. For example: ~~~ YEAR_IN_SCHOOL_CHOICES = ( ('FR', 'Freshman'), ('SO', 'Sophomore'), ('JR', 'Junior'), ('SR', 'Senior'), ) ~~~ Generally, it’s best to define choices inside a model class, and to define a suitably-named constant for each value: ~~~ from django.db import models class Student(models.Model): FRESHMAN = 'FR' SOPHOMORE = 'SO' JUNIOR = 'JR' SENIOR = 'SR' YEAR_IN_SCHOOL_CHOICES = ( (FRESHMAN, 'Freshman'), (SOPHOMORE, 'Sophomore'), (JUNIOR, 'Junior'), (SENIOR, 'Senior'), ) year_in_school = models.CharField(max_length=2, choices=YEAR_IN_SCHOOL_CHOICES, default=FRESHMAN) def is_upperclass(self): return self.year_in_school in (self.JUNIOR, self.SENIOR) ~~~ Though you can define a choices list outside of a model class and then refer to it, defining the choices and names for each choice inside the model class keeps all of that information with the class that uses it, and makes the choices easy to reference (e.g, `Student.SOPHOMORE` will work anywhere that the `Student` model has been imported). You can also collect your available choices into named groups that can be used for organizational purposes: ~~~ MEDIA_CHOICES = ( ('Audio', ( ('vinyl', 'Vinyl'), ('cd', 'CD'), ) ), ('Video', ( ('vhs', 'VHS Tape'), ('dvd', 'DVD'), ) ), ('unknown', 'Unknown'), ) ~~~ The first element in each tuple is the name to apply to the group. The second element is an iterable of 2-tuples, with each 2-tuple containing a value and a human-readable name for an option. Grouped options may be combined with ungrouped options within a single list (such as the unknown option in this example). For each model field that has [`choices`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.Field.choices "django.db.models.Field.choices") set, Django will add a method to retrieve the human-readable name for the field’s current value. See `get_FOO_display()` in the database API documentation. Note that choices can be any iterable object – not necessarily a list or tuple. This lets you construct choices dynamically. But if you find yourself hacking [`choices`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.Field.choices "django.db.models.Field.choices") to be dynamic, you’re probably better off using a proper database table with a [`ForeignKey`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.ForeignKey "django.db.models.ForeignKey"). [`choices`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.Field.choices "django.db.models.Field.choices") is meant for static data that doesn’t change much, if ever. Unless [`blank=False`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.Field.blank "django.db.models.Field.blank") is set on the field along with a [`default`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.Field.default "django.db.models.Field.default") then a label containing `"---------"` will be rendered with the select box. To override this behavior, add a tuple to `choices` containing `None`; e.g. `(None,'Your String For Display')`. Alternatively, you can use an empty string instead of `None` where this makes sense – such as on a `CharField`. ### `db_column` `Field.``db_column` The name of the database column to use for this field. If this isn’t given, Django will use the field’s name. If your database column name is an SQL reserved word, or contains characters that aren’t allowed in Python variable names – notably, the hyphen – that’s OK. Django quotes column and table names behind the scenes. ### `db_index` `Field.``db_index` If `True`, a database index will be created for this field. ### `db_tablespace` `Field.``db_tablespace` The name of the database tablespace to use for this field’s index, if this field is indexed. The default is the project’s `DEFAULT_INDEX_TABLESPACE` setting, if set, or the [`db_tablespace`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.Options.db_tablespace "django.db.models.Options.db_tablespace") of the model, if any. If the backend doesn’t support tablespaces for indexes, this option is ignored. ### `default` `Field.``default` The default value for the field. This can be a value or a callable object. If callable it will be called every time a new object is created. The default cannot be a mutable object (model instance, list, set, etc.), as a reference to the same instance of that object would be used as the default value in all new model instances. Instead, wrap the desired default in a callable. For example, if you had a custom `JSONField` and wanted to specify a dictionary as the default, use a function as follows: ~~~ def contact_default(): return {"email": "to1@example.com"} contact_info = JSONField("ContactInfo", default=contact_default) ~~~ Note that `lambda`s cannot be used for field options like `default` because they cannot be serialized by migrations. See the Django documentation for other caveats. The default value is used when new model instances are created and a value isn’t provided for the field. When the field is a primary key, the default is also used when the field is set to `None`. ### `editable` `Field.``editable` If `False`, the field will not be displayed in the admin or any other `ModelForm`. They are also skipped during model validation. Default is `True`. ### `error_messages` `Field.``error_messages` The `error_messages` argument lets you override the default messages that the field will raise. Pass in a dictionary with keys matching the error messages you want to override. Error message keys include `null`, `blank`, `invalid`, `invalid_choice`, `unique`, and `unique_for_date`. Additional error message keys are specified for each field in the Field types section below. ### `help_text` `Field.``help_text` Extra “help” text to be displayed with the form widget. It’s useful for documentation even if your field isn’t used on a form. Note that this value is *not* HTML-escaped in automatically-generated forms. This lets you include HTML in [`help_text`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.Field.help_text "django.db.models.Field.help_text") if you so desire. For example: ~~~ help_text="Please use the following format: <em>YYYY-MM-DD</em>." ~~~ Alternatively you can use plain text and `django.utils.html.escape()` to escape any HTML special characters. Ensure that you escape any help text that may come from untrusted users to avoid a cross-site scripting attack. ### `primary_key` `Field.``primary_key` If `True`, this field is the primary key for the model. If you don’t specify `primary_key=True` for any field in your model, Django will automatically add an [`AutoField`](http://masteringdjango.com/django-model-definition-reference/#AutoField "AutoField")to hold the primary key, so you don’t need to set `primary_key=True` on any of your fields unless you want to override the default primary-key behavior. `primary_key=True` implies [`null=False`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.Field.null "django.db.models.Field.null") and [`unique=True`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.Field.unique "django.db.models.Field.unique"). Only one primary key is allowed on an object. The primary key field is read-only. If you change the value of the primary key on an existing object and then save it, a new object will be created alongside the old one. ### `unique` `Field.``unique` If `True`, this field must be unique throughout the table. This is enforced at the database level and by model validation. If you try to save a model with a duplicate value in a [`unique`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.Field.unique "django.db.models.Field.unique") field, a `django.db.IntegrityError` will be raised by the model’s `save()` method. This option is valid on all field types except [`ManyToManyField`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.ManyToManyField "django.db.models.ManyToManyField"), [`OneToOneField`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.OneToOneField "django.db.models.OneToOneField"), and [`FileField`](http://masteringdjango.com/django-model-definition-reference/#FileField "FileField"). Note that when `unique` is `True`, you don’t need to specify [`db_index`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.Field.db_index "django.db.models.Field.db_index"), because `unique` implies the creation of an index. ### `unique_for_date` `Field.``unique_for_date` Set this to the name of a [`DateField`](http://masteringdjango.com/django-model-definition-reference/#DateField "DateField") or [`DateTimeField`](http://masteringdjango.com/django-model-definition-reference/#DateTimeField "DateTimeField") to require that this field be unique for the value of the date field. For example, if you have a field `title` that has `unique_for_date="pub_date"`, then Django wouldn’t allow the entry of two records with the same `title` and `pub_date`. Note that if you set this to point to a [`DateTimeField`](http://masteringdjango.com/django-model-definition-reference/#DateTimeField "DateTimeField"), only the date portion of the field will be considered. Besides, when `USE_TZ` is `True`, the check will be performed in the current time zone at the time the object gets saved. This is enforced by `Model.validate_unique()` during model validation but not at the database level. If any[`unique_for_date`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.Field.unique_for_date "django.db.models.Field.unique_for_date") constraint involves fields that are not part of a `ModelForm` (for example, if one of the fields is listed in `exclude` or has [`editable=False`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.Field.editable "django.db.models.Field.editable")), `Model.validate_unique()` will skip validation for that particular constraint. ### `unique_for_month` `Field.``unique_for_month` Like [`unique_for_date`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.Field.unique_for_date "django.db.models.Field.unique_for_date"), but requires the field to be unique with respect to the month. ### `unique_for_year` `Field.``unique_for_year` Like [`unique_for_date`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.Field.unique_for_date "django.db.models.Field.unique_for_date") and [`unique_for_month`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.Field.unique_for_month "django.db.models.Field.unique_for_month"). ### `verbose_name` `Field.``verbose_name` A human-readable name for the field. If the verbose name isn’t given, Django will automatically create it using the field’s attribute name, converting underscores to spaces. See Verbose field names. ### `validators` `Field.``validators` A list of validators to run for this field. ## Field attribute reference Every `Field` instance contains several attributes that allow introspecting its behavior. Use these attributes instead of `isinstance` checks when you need to write code that depends on a field’s functionality. These attributes can be used together with the Model._meta API to narrow down a search for specific field types. Custom model fields should implement these flags. ## Attributes for fields `Field.``auto_created` Boolean flag that indicates if the field was automatically created, such as the `OneToOneField` used by model inheritance. `Field.``concrete` Boolean flag that indicates if the field has a database column associated with it. `Field.``hidden` Boolean flag that indicates if a field is used to back another non-hidden field’s functionality (e.g. the`content_type` and `object_id` fields that make up a `GenericForeignKey`). The `hidden` flag is used to distinguish what constitutes the public subset of fields on the model from all the fields on the model. Note `Options.get_fields()` excludes hidden fields by default. Pass in `include_hidden=True` to return hidden fields in the results. `Field.``is_relation` Boolean flag that indicates if a field contains references to one or more other models for its functionality (e.g. `ForeignKey`, `ManyToManyField`, `OneToOneField`, etc.). `Field.``model` Returns the model on which the field is defined. If a field is defined on a superclass of a model, `model` will refer to the superclass, not the class of the instance. ## Attributes for fields with relations These attributes are used to query for the cardinality and other details of a relation. These attribute are present on all fields; however, they will only have meaningful values if the field is a relation type ([`Field.is_relation=True`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.Field.is_relation "django.db.models.Field.is_relation")). `Field.``many_to_many` Boolean flag that is `True` if the field has a many-to-many relation; `False` otherwise. The only field included with Django where this is `True` is `ManyToManyField`. `Field.``many_to_one` Boolean flag that is `True` if the field has a many-to-one relation, such as a `ForeignKey`; `False` otherwise. `Field.``one_to_many` Boolean flag that is `True` if the field has a one-to-many relation, such as a `GenericRelation` or the reverse of a `ForeignKey`; `False` otherwise. `Field.``one_to_one` Boolean flag that is `True` if the field has a one-to-one relation, such as a `OneToOneField`; `False` otherwise. `Field.``related_model` Points to the model the field relates to. For example, `Author` in `ForeignKey(Author)`. If a field has a generic relation (such as a `GenericForeignKey` or a `GenericRelation`) then `related_model` will be `None`. ## Relationships ### `ForeignKey` *class *`django.db.models.``ForeignKey`(*othermodel*[, ***options*]) A many-to-one relationship. Requires a positional argument: the class to which the model is related. To create a recursive relationship – an object that has a many-to-one relationship with itself – use`models.ForeignKey('self')`. If you need to create a relationship on a model that has not yet been defined, you can use the name of the model, rather than the model object itself: ~~~ from django.db import models class Car(models.Model): manufacturer = models.ForeignKey('Manufacturer') # ... class Manufacturer(models.Model): # ... pass ~~~ To refer to models defined in another application, you can explicitly specify a model with the full application label. For example, if the `Manufacturer` model above is defined in another application called`production`, you’d need to use: ~~~ class Car(models.Model): manufacturer = models.ForeignKey('production.Manufacturer') ~~~ This sort of reference can be useful when resolving circular import dependencies between two applications. A database index is automatically created on the `ForeignKey`. You can disable this by setting [`db_index`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.Field.db_index "django.db.models.Field.db_index") to`False`. You may want to avoid the overhead of an index if you are creating a foreign key for consistency rather than joins, or if you will be creating an alternative index like a partial or multiple column index. #### DATABASE REPRESENTATION Behind the scenes, Django appends `"_id"` to the field name to create its database column name. In the above example, the database table for the `Car` model will have a `manufacturer_id` column. (You can change this explicitly by specifying [`db_column`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.Field.db_column "django.db.models.Field.db_column")) However, your code should never have to deal with the database column name, unless you write custom SQL. You’ll always deal with the field names of your model object. #### ARGUMENTS [`ForeignKey`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.ForeignKey "django.db.models.ForeignKey") accepts an extra set of arguments – all optional – that define the details of how the relation works. `ForeignKey.``limit_choices_to` Sets a limit to the available choices for this field when this field is rendered using a `ModelForm` or the admin (by default, all objects in the queryset are available to choose). Either a dictionary, a `Q` object, or a callable returning a dictionary or `Q` object can be used. For example: ~~~ staff_member = models.ForeignKey(User, limit_choices_to={'is_staff': True}) ~~~ causes the corresponding field on the `ModelForm` to list only `Users` that have `is_staff=True`. This may be helpful in the Django admin. The callable form can be helpful, for instance, when used in conjunction with the Python `datetime` module to limit selections by date range. For example: ~~~ def limit_pub_date_choices(): return {'pub_date__lte': datetime.date.utcnow()} limit_choices_to = limit_pub_date_choices ~~~ If `limit_choices_to` is or returns a `Q object`, which is useful for complex queries , then it will only have an effect on the choices available in the admin when the field is not listed in `raw_id_fields` in the `ModelAdmin`for the model. Note If a callable is used for `limit_choices_to`, it will be invoked every time a new form is instantiated. It may also be invoked when a model is validated, for example by management commands or the admin. The admin constructs querysets to validate its form inputs in various edge cases multiple times, so there is a possibility your callable may be invoked several times. `ForeignKey.``related_name` The name to use for the relation from the related object back to this one. It’s also the default value for[`related_query_name`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.ForeignKey.related_query_name "django.db.models.ForeignKey.related_query_name") (the name to use for the reverse filter name from the target model). See the related objects documentation for a full explanation and example. Note that you must set this value when defining relations on abstract models; and when you do so some special syntax is available. If you’d prefer Django not to create a backwards relation, set `related_name` to `'+'` or end it with `'+'`. For example, this will ensure that the `User` model won’t have a backwards relation to this model: ~~~ user = models.ForeignKey(User, related_name='+') ~~~ `ForeignKey.``related_query_name` The name to use for the reverse filter name from the target model. Defaults to the value of [`related_name`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.ForeignKey.related_name "django.db.models.ForeignKey.related_name") if it is set, otherwise it defaults to the name of the model: ~~~ # Declare the ForeignKey with related_query_name class Tag(models.Model): article = models.ForeignKey(Article, related_name="tags", related_query_name="tag") name = models.CharField(max_length=255) # That's now the name of the reverse filter Article.objects.filter(tag__name="important") ~~~ `ForeignKey.``to_field` The field on the related object that the relation is to. By default, Django uses the primary key of the related object. `ForeignKey.``db_constraint` Controls whether or not a constraint should be created in the database for this foreign key. The default is`True`, and that’s almost certainly what you want; setting this to `False` can be very bad for data integrity. That said, here are some scenarios where you might want to do this: * You have legacy data that is not valid. * You’re sharding your database. If this is set to `False`, accessing a related object that doesn’t exist will raise its `DoesNotExist` exception. `ForeignKey.``on_delete` When an object referenced by a [`ForeignKey`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.ForeignKey "django.db.models.ForeignKey") is deleted, Django by default emulates the behavior of the SQL constraint `ON DELETE CASCADE` and also deletes the object containing the `ForeignKey`. This behavior can be overridden by specifying the [`on_delete`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.ForeignKey.on_delete "django.db.models.ForeignKey.on_delete") argument. For example, if you have a nullable [`ForeignKey`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.ForeignKey "django.db.models.ForeignKey") and you want it to be set null when the referenced object is deleted: ~~~ user = models.ForeignKey(User, blank=True, null=True, on_delete=models.SET_NULL) ~~~ The possible values for [`on_delete`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.ForeignKey.on_delete "django.db.models.ForeignKey.on_delete") are found in `django.db.models`: * `django.db.models.``CASCADE` Cascade deletes; the default. * `django.db.models.``PROTECT` Prevent deletion of the referenced object by raising `ProtectedError`, a subclass of `django.db.IntegrityError`. * `django.db.models.``SET_NULL` Set the [`ForeignKey`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.ForeignKey "django.db.models.ForeignKey") null; this is only possible if [`null`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.Field.null "django.db.models.Field.null") is `True`. * `django.db.models.``SET_DEFAULT` Set the [`ForeignKey`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.ForeignKey "django.db.models.ForeignKey") to its default value; a default for the [`ForeignKey`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.ForeignKey "django.db.models.ForeignKey") must be set. * `django.db.models.``SET`() Set the [`ForeignKey`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.ForeignKey "django.db.models.ForeignKey") to the value passed to [`SET()`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.SET "django.db.models.SET"), or if a callable is passed in, the result of calling it. In most cases, passing a callable will be necessary to avoid executing queries at the time your models.py is imported: ~~~ from django.conf import settings from django.contrib.auth import get_user_model from django.db import models def get_sentinel_user(): return get_user_model().objects.get_or_create(username='deleted')[0] class MyModel(models.Model): user = models.ForeignKey(settings.AUTH_USER_MODEL, on_delete=models.SET(get_sentinel_user)) ~~~ * `django.db.models.``DO_NOTHING` Take no action. If your database backend enforces referential integrity, this will cause an `IntegrityError`unless you manually add an SQL `ON DELETE` constraint to the database field. `ForeignKey.``swappable` Controls the migration framework’s reaction if this [`ForeignKey`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.ForeignKey "django.db.models.ForeignKey") is pointing at a swappable model. If it is `True`– the default – then if the [`ForeignKey`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.ForeignKey "django.db.models.ForeignKey") is pointing at a model which matches the current value of`settings.AUTH_USER_MODEL` (or another swappable model setting) the relationship will be stored in the migration using a reference to the setting, not to the model directly. You only want to override this to be `False` if you are sure your model should always point towards the swapped-in model – for example, if it is a profile model designed specifically for your custom user model. Setting it to `False` does not mean you can reference a swappable model even if it is swapped out – `False`just means that the migrations made with this ForeignKey will always reference the exact model you specify (so it will fail hard if the user tries to run with a User model you don’t support, for example). If in doubt, leave it to its default of `True`. ### `ManyToManyField` *class *`django.db.models.``ManyToManyField`(*othermodel*[, ***options*]) A many-to-many relationship. Requires a positional argument: the class to which the model is related, which works exactly the same as it does for [`ForeignKey`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.ForeignKey "django.db.models.ForeignKey"), including recursive and lazy relationships. Related objects can be added, removed, or created with the field’s `RelatedManager`. #### DATABASE REPRESENTATION Behind the scenes, Django creates an intermediary join table to represent the many-to-many relationship. By default, this table name is generated using the name of the many-to-many field and the name of the table for the model that contains it. Since some databases don’t support table names above a certain length, these table names will be automatically truncated to 64 characters and a uniqueness hash will be used. This means you might see table names like `author_books_9cdf4`; this is perfectly normal. You can manually provide the name of the join table using the [`db_table`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.ManyToManyField.db_table "django.db.models.ManyToManyField.db_table") option. #### ARGUMENTS [`ManyToManyField`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.ManyToManyField "django.db.models.ManyToManyField") accepts an extra set of arguments – all optional – that control how the relationship functions. `ManyToManyField.``related_name` Same as [`ForeignKey.related_name`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.ForeignKey.related_name "django.db.models.ForeignKey.related_name"). `ManyToManyField.``related_query_name` Same as [`ForeignKey.related_query_name`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.ForeignKey.related_query_name "django.db.models.ForeignKey.related_query_name"). `ManyToManyField.``limit_choices_to` Same as [`ForeignKey.limit_choices_to`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.ForeignKey.limit_choices_to "django.db.models.ForeignKey.limit_choices_to"). `limit_choices_to` has no effect when used on a `ManyToManyField` with a custom intermediate table specified using the [`through`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.ManyToManyField.through "django.db.models.ManyToManyField.through") parameter. `ManyToManyField.``symmetrical` Only used in the definition of ManyToManyFields on self. Consider the following model: ~~~ from django.db import models class Person(models.Model): friends = models.ManyToManyField("self") ~~~ When Django processes this model, it identifies that it has a [`ManyToManyField`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.ManyToManyField "django.db.models.ManyToManyField") on itself, and as a result, it doesn’t add a `person_set` attribute to the `Person` class. Instead, the [`ManyToManyField`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.ManyToManyField "django.db.models.ManyToManyField") is assumed to be symmetrical – that is, if I am your friend, then you are my friend. If you do not want symmetry in many-to-many relationships with `self`, set [`symmetrical`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.ManyToManyField.symmetrical "django.db.models.ManyToManyField.symmetrical") to `False`. This will force Django to add the descriptor for the reverse relationship, allowing [`ManyToManyField`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.ManyToManyField "django.db.models.ManyToManyField") relationships to be non-symmetrical. `ManyToManyField.``through` Django will automatically generate a table to manage many-to-many relationships. However, if you want to manually specify the intermediary table, you can use the [`through`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.ManyToManyField.through "django.db.models.ManyToManyField.through") option to specify the Django model that represents the intermediate table that you want to use. The most common use for this option is when you want to associate extra data with a many-to-many relationship. If you don’t specify an explicit `through` model, there is still an implicit `through` model class you can use to directly access the table created to hold the association. It has three fields: * `id`: the primary key of the relation. * `<containing_model>_id`: the `id` of the model that declares the `ManyToManyField`. * `<other_model>_id`: the `id` of the model that the `ManyToManyField` points to. This class can be used to query associated records for a given model instance like a normal model. `ManyToManyField.``through_fields` Only used when a custom intermediary model is specified. Django will normally determine which fields of the intermediary model to use in order to establish a many-to-many relationship automatically. However, consider the following models: ~~~ from django.db import models class Person(models.Model): name = models.CharField(max_length=50) class Group(models.Model): name = models.CharField(max_length=128) members = models.ManyToManyField(Person, through='Membership', through_fields=('group', 'person')) class Membership(models.Model): group = models.ForeignKey(Group) person = models.ForeignKey(Person) inviter = models.ForeignKey(Person, related_name="membership_invites") invite_reason = models.CharField(max_length=64) ~~~ `Membership` has *two* foreign keys to `Person` (`person` and `inviter`), which makes the relationship ambiguous and Django can’t know which one to use. In this case, you must explicitly specify which foreign keys Django should use using `through_fields`, as in the example above. `through_fields` accepts a 2-tuple `('field1', 'field2')`, where `field1` is the name of the foreign key to the model the [`ManyToManyField`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.ManyToManyField "django.db.models.ManyToManyField") is defined on (`group` in this case), and `field2` the name of the foreign key to the target model (`person` in this case). When you have more than one foreign key on an intermediary model to any (or even both) of the models participating in a many-to-many relationship, you *must* specify `through_fields`. This also applies to recursive relationships when an intermediary model is used and there are more than two foreign keys to the model, or you want to explicitly specify which two Django should use. Recursive relationships using an intermediary model are always defined as non-symmetrical – that is, with[`symmetrical=False`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.ManyToManyField.symmetrical "django.db.models.ManyToManyField.symmetrical") – therefore, there is the concept of a “source” and a “target”. In that case `'field1'` will be treated as the “source” of the relationship and `'field2'` as the “target”. `ManyToManyField.``db_table` The name of the table to create for storing the many-to-many data. If this is not provided, Django will assume a default name based upon the names of: the table for the model defining the relationship and the name of the field itself. `ManyToManyField.``db_constraint` Controls whether or not constraints should be created in the database for the foreign keys in the intermediary table. The default is `True`, and that’s almost certainly what you want; setting this to `False` can be very bad for data integrity. That said, here are some scenarios where you might want to do this: * You have legacy data that is not valid. * You’re sharding your database. It is an error to pass both `db_constraint` and `through`. `ManyToManyField.``swappable` Controls the migration framework’s reaction if this [`ManyToManyField`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.ManyToManyField "django.db.models.ManyToManyField") is pointing at a swappable model. If it is `True` – the default – then if the [`ManyToManyField`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.ManyToManyField "django.db.models.ManyToManyField") is pointing at a model which matches the current value of `settings.AUTH_USER_MODEL` (or another swappable model setting) the relationship will be stored in the migration using a reference to the setting, not to the model directly. You only want to override this to be `False` if you are sure your model should always point towards the swapped-in model – for example, if it is a profile model designed specifically for your custom user model. If in doubt, leave it to its default of `True`. [`ManyToManyField`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.ManyToManyField "django.db.models.ManyToManyField") does not support [`validators`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.Field.validators "django.db.models.Field.validators"). [`null`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.Field.null "django.db.models.Field.null") has no effect since there is no way to require a relationship at the database level. ### `OneToOneField` *class *`django.db.models.``OneToOneField`(*othermodel*[, *parent_link=False*, ***options*]) A one-to-one relationship. Conceptually, this is similar to a [`ForeignKey`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.ForeignKey "django.db.models.ForeignKey") with [`unique=True`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.Field.unique "django.db.models.Field.unique"), but the “reverse” side of the relation will directly return a single object. This is most useful as the primary key of a model which “extends” another model in some way; multi table inheritance is implemented by adding an implicit one-to-one relation from the child model to the parent model, for example. One positional argument is required: the class to which the model will be related. This works exactly the same as it does for [`ForeignKey`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.ForeignKey "django.db.models.ForeignKey"), including all the options regarding recursive and lazy relationships. If you do not specify the [`related_name`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.ForeignKey.related_name "django.db.models.ForeignKey.related_name") argument for the `OneToOneField`, Django will use the lower-case name of the current model as default value. With the following example: ~~~ from django.conf import settings from django.db import models class MySpecialUser(models.Model): user = models.OneToOneField(settings.AUTH_USER_MODEL) supervisor = models.OneToOneField(settings.AUTH_USER_MODEL, related_name='supervisor_of') ~~~ your resulting `User` model will have the following attributes: ~~~ >>> user = User.objects.get(pk=1) >>> hasattr(user, 'myspecialuser') True >>> hasattr(user, 'supervisor_of') True ~~~ A `DoesNotExist` exception is raised when accessing the reverse relationship if an entry in the related table doesn’t exist. For example, if a user doesn’t have a supervisor designated by `MySpecialUser`: ~~~ >>> user.supervisor_of Traceback (most recent call last): ... DoesNotExist: User matching query does not exist. ~~~ Additionally, `OneToOneField` accepts all of the extra arguments accepted by [`ForeignKey`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.ForeignKey "django.db.models.ForeignKey"), plus one extra argument: `OneToOneField.``parent_link` When `True` and used in a model which inherits from another concrete model, indicates that this field should be used as the link back to the parent class, rather than the extra `OneToOneField` which would normally be implicitly created by subclassing. See One-to-one relationships for usage examples of `OneToOneField`. ## Model Metadata Options ### `abstract` `Options.``abstract` If `abstract = True`, this model will be an abstract base class. ### `app_label` `Options.``app_label` If a model is defined outside of an application in `INSTALLED_APPS`, it must declare which app it belongs to: ~~~ app_label = 'myapp' ~~~ ### `db_table` `Options.``db_table` The name of the database table to use for the model: ~~~ db_table = 'music_album' ~~~ #### TABLE NAMES To save you time, Django automatically derives the name of the database table from the name of your model class and the app that contains it. A model’s database table name is constructed by joining the model’s “app label” – the name you used in `manage.py startapp` – to the model’s class name, with an underscore between them. For example, if you have an app `bookstore` (as created by `manage.py startapp bookstore`), a model defined as`class Book` will have a database table named `bookstore_book`. To override the database table name, use the `db_table` parameter in `class Meta`. If your database table name is an SQL reserved word, or contains characters that aren’t allowed in Python variable names – notably, the hyphen – that’s OK. Django quotes column and table names behind the scenes. Use lowercase table names for MySQL It is strongly advised that you use lowercase table names when you override the table name via `db_table`, particularly if you are using the MySQL backend. See Chapter 23 for more details. Table name quoting for Oracle In order to meet the 30-char limitation Oracle has on table names, and match the usual conventions for Oracle databases, Django may shorten table names and turn them all-uppercase. To prevent such transformations, use a quoted name as the value for `db_table`: ~~~ db_table = '"name_left_in_lowercase"' ~~~ Such quoted names can also be used with Django’s other supported database backends; except for Oracle, however, the quotes have no effect. See Chapter 23 for more details. ### `db_tablespace` `Options.``db_tablespace` The name of the database tablespace to use for this model. The default is the project’s `DEFAULT_TABLESPACE`setting, if set. If the backend doesn’t support tablespaces, this option is ignored. ### `default_related_name` `Options.``default_related_name` The name that will be used by default for the relation from a related object back to this one. The default is`<model_name>_set`. As the reverse name for a field should be unique, be careful if you intend to subclass your model. To work around name collisions, part of the name should contain `'%(app_label)s'` and `'%(model_name)s'`, which are replaced respectively by the name of the application the model is in, and the name of the model, both lowercased. See the paragraph on related names for abstract models. ### `get_latest_by` `Options.``get_latest_by` The name of an orderable field in the model, typically a [`DateField`](http://masteringdjango.com/django-model-definition-reference/#DateField "DateField"), [`DateTimeField`](http://masteringdjango.com/django-model-definition-reference/#DateTimeField "DateTimeField"), or [`IntegerField`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.IntegerField "django.db.models.IntegerField"). This specifies the default field to use in your model `Manager`’s `latest()` and `earliest()` methods. Example: ~~~ get_latest_by = "order_date" ~~~ See the `latest()` docs for more. ### `managed` `Options.``managed` Defaults to `True`, meaning Django will create the appropriate database tables in `migrate` or as part of migrations and remove them as part of a `flush` management command. That is, Django *manages* the database tables’ lifecycles. If `False`, no database table creation or deletion operations will be performed for this model. This is useful if the model represents an existing table or a database view that has been created by some other means. This is the *only* difference when `managed=False`. All other aspects of model handling are exactly the same as normal. This includes 1. Adding an automatic primary key field to the model if you don’t declare it. To avoid confusion for later code readers, it’s recommended to specify all the columns from the database table you are modeling when using unmanaged models. 2. If a model with `managed=False` contains a [`ManyToManyField`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.ManyToManyField "django.db.models.ManyToManyField") that points to another unmanaged model, then the intermediate table for the many-to-many join will also not be created. However, the intermediary table between one managed and one unmanaged model *will* be created. If you need to change this default behavior, create the intermediary table as an explicit model (with`managed` set as needed) and use the [`ManyToManyField.through`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.ManyToManyField.through "django.db.models.ManyToManyField.through") attribute to make the relation use your custom model. For tests involving models with `managed=False`, it’s up to you to ensure the correct tables are created as part of the test setup. If you’re interested in changing the Python-level behavior of a model class, you *could* use `managed=False` and create a copy of an existing model. However, there’s a better approach for that situation: proxy models. ### `order_with_respect_to` `Options.``order_with_respect_to` Marks this object as “orderable” with respect to the given field. This is almost always used with related objects to allow them to be ordered with respect to a parent object. For example, if an `Answer` relates to a`Question` object, and a question has more than one answer, and the order of answers matters, you’d do this: ~~~ from django.db import models class Question(models.Model): text = models.TextField() # ... class Answer(models.Model): question = models.ForeignKey(Question) # ... class Meta: order_with_respect_to = 'question' ~~~ When `order_with_respect_to` is set, two additional methods are provided to retrieve and to set the order of the related objects: `get_RELATED_order()` and `set_RELATED_order()`, where `RELATED` is the lowercased model name. For example, assuming that a `Question` object has multiple related `Answer` objects, the list returned contains the primary keys of the related `Answer` objects: ~~~ >>> question = Question.objects.get(id=1) >>> question.get_answer_order() [1, 2, 3] ~~~ The order of a `Question` object’s related `Answer` objects can be set by passing in a list of `Answer` primary keys: ~~~ >>> question.set_answer_order([3, 1, 2]) ~~~ The related objects also get two methods, `get_next_in_order()` and `get_previous_in_order()`, which can be used to access those objects in their proper order. Assuming the `Answer` objects are ordered by `id`: ~~~ >>> answer = Answer.objects.get(id=2) >>> answer.get_next_in_order() <Answer: 3> >>> answer.get_previous_in_order() <Answer: 1> ~~~ Changing order_with_respect_to `order_with_respect_to` adds an additional field/database column named `_order`, so be sure to make and apply the appropriate migrations if you add or change `order_with_respect_to` after your initial `migrate`. ### `ordering` `Options.``ordering` The default ordering for the object, for use when obtaining lists of objects: ~~~ ordering = ['-order_date'] ~~~ This is a tuple or list of strings. Each string is a field name with an optional “-” prefix, which indicates descending order. Fields without a leading “-” will be ordered ascending. Use the string ”?” to order randomly. For example, to order by a `pub_date` field ascending, use this: ~~~ ordering = ['pub_date'] ~~~ To order by `pub_date` descending, use this: ~~~ ordering = ['-pub_date'] ~~~ To order by `pub_date` descending, then by `author` ascending, use this: ~~~ ordering = ['-pub_date', 'author'] ~~~ Warning Ordering is not a free operation. Each field you add to the ordering incurs a cost to your database. Each foreign key you add will implicitly include all of its default orderings as well. ### `permissions` `Options.``permissions` Extra permissions to enter into the permissions table when creating this object. Add, delete and change permissions are automatically created for each model. This example specifies an extra permission,`can_deliver_pizzas`: ~~~ permissions = (("can_deliver_pizzas", "Can deliver pizzas"),) ~~~ This is a list or tuple of 2-tuples in the format `(permission_code, human_readable_permission_name)`. ### `default_permissions` `Options.``default_permissions` Defaults to `('add', 'change', 'delete')`. You may customize this list, for example, by setting this to an empty list if your app doesn’t require any of the default permissions. It must be specified on the model before the model is created by `migrate` in order to prevent any omitted permissions from being created. ### `proxy` `Options.``proxy` If `proxy = True`, a model which subclasses another model will be treated as a proxy model. ### `select_on_save` `Options.``select_on_save` Determines if Django will use the pre-1.6 `django.db.models.Model.save()` algorithm. The old algorithm uses`SELECT` to determine if there is an existing row to be updated. The new algorithm tries an `UPDATE` directly. In some rare cases the `UPDATE` of an existing row isn’t visible to Django. An example is the PostgreSQL `ONUPDATE` trigger which returns `NULL`. In such cases the new algorithm will end up doing an `INSERT` even when a row exists in the database. Usually there is no need to set this attribute. The default is `False`. See `django.db.models.Model.save()` for more about the old and new saving algorithm. ### `unique_together` `Options.``unique_together` Sets of field names that, taken together, must be unique: ~~~ unique_together = (("driver", "restaurant"),) ~~~ This is a tuple of tuples that must be unique when considered together. It’s used in the Django admin and is enforced at the database level (i.e., the appropriate `UNIQUE` statements are included in the `CREATE TABLE`statement). For convenience, unique_together can be a single tuple when dealing with a single set of fields: ~~~ unique_together = ("driver", "restaurant") ~~~ A [`ManyToManyField`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.ManyToManyField "django.db.models.ManyToManyField") cannot be included in unique_together. (It’s not clear what that would even mean!) If you need to validate uniqueness related to a [`ManyToManyField`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.ManyToManyField "django.db.models.ManyToManyField"), try using a signal or an explicit [`through`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.ManyToManyField.through "django.db.models.ManyToManyField.through") model. The `ValidationError` raised during model validation when the constraint is violated has the `unique_together`error code. ### `index_together` `Options.``index_together` Sets of field names that, taken together, are indexed: ~~~ index_together = [ ["pub_date", "deadline"], ] ~~~ This list of fields will be indexed together (i.e. the appropriate `CREATE INDEX` statement will be issued.) For convenience, `index_together` can be a single list when dealing with a single set of fields: ~~~ index_together = ["pub_date", "deadline"] ~~~ ### `verbose_name` `Options.``verbose_name` A human-readable name for the object, singular: ~~~ verbose_name = "pizza" ~~~ If this isn’t given, Django will use a munged version of the class name: `CamelCase` becomes `camel case`. ### `verbose_name_plural` `Options.``verbose_name_plural` The plural name for the object: ~~~ verbose_name_plural = "stories" ~~~ If this isn’t given, Django will use [`verbose_name`](http://masteringdjango.com/django-model-definition-reference/#django.db.models.Options.verbose_name "django.db.models.Options.verbose_name") + `"s"`.