Appendix B: Database API Reference

Django’’s database API is the other half of the model API discussed in Appendix A. Once you’ve defined a model, you’ll use this API any time you need to access the database. You’ve seen examples of this API in use throughout the book; this appendix explains all the various options in detail. Like the model APIs discussed in Appendix A, though these APIs are considered very stable, the Django developers consistently add new shortcuts and conveniences. It’s a good idea to always check the latest documentation online, available at [http://docs.djangoproject.com/](http://docs.djangoproject.com/). Throughout this guide (and in the reference), we’ll refer to the following models, which comprise a Weblog application: ~~~ from django.db import models class Blog(models.Model): name = models.CharField(max_length=100) tagline = models.TextField() def __str__(self): return self.name class Author(models.Model): name = models.CharField(max_length=50) email = models.EmailField() def __str__(self): return self.name class Entry(models.Model): blog = models.ForeignKey(Blog) headline = models.CharField(max_length=255) body_text = models.TextField() pub_date = models.DateField() mod_date = models.DateField() authors = models.ManyToManyField(Author) n_comments = models.IntegerField() n_pingbacks = models.IntegerField() rating = models.IntegerField() def __str__(self): # __unicode__ on Python 2 return self.headline ~~~ [TOC=3] ## Creating objects To represent database-table data in Python objects, Django uses an intuitive system: A model class represents a database table, and an instance of that class represents a particular record in the database table. To create an object, instantiate it using keyword arguments to the model class, then call `save()` to save it to the database. Assuming models live in a file `mysite/blog/models.py`, here’s an example: ~~~ >>> from blog.models import Blog >>> b = Blog(name='Beatles Blog', tagline='All the latest Beatles news.') >>> b.save() ~~~ This performs an `INSERT` SQL statement behind the scenes. Django doesn’t hit the database until you explicitly call `save()`. The `save()` method has no return value. See also `save()` takes a number of advanced options not described here. See the documentation for `save()` for complete details. To create and save an object in a single step, use the `create()` method. ## Saving changes to objects To save changes to an object that’s already in the database, use `save()`. Given a `Blog` instance `b5` that has already been saved to the database, this example changes its name and updates its record in the database: ~~~ >>> b5.name = 'New name' >>> b5.save() ~~~ This performs an `UPDATE` SQL statement behind the scenes. Django doesn’t hit the database until you explicitly call `save()`. ### Saving `ForeignKey` and `ManyToManyField` fields Updating a [`ForeignKey`](http://masteringdjango.com/django-database-api-reference/appendix_A.html#django.db.models.ForeignKey "django.db.models.ForeignKey") field works exactly the same way as saving a normal field – simply assign an object of the right type to the field in question. This example updates the `blog` attribute of an `Entry` instance `entry`, assuming appropriate instances of `Entry` and `Blog` are already saved to the database (so we can retrieve them below): ~~~ >>> from blog.models import Entry >>> entry = Entry.objects.get(pk=1) >>> cheese_blog = Blog.objects.get(name="Cheddar Talk") >>> entry.blog = cheese_blog >>> entry.save() ~~~ Updating a [`ManyToManyField`](http://masteringdjango.com/django-database-api-reference/appendix_A.html#django.db.models.ManyToManyField "django.db.models.ManyToManyField") works a little differently – use the `add()` method on the field to add a record to the relation. This example adds the `Author` instance `joe` to the `entry` object: ~~~ >>> from blog.models import Author >>> joe = Author.objects.create(name="Joe") >>> entry.authors.add(joe) ~~~ To add multiple records to a [`ManyToManyField`](http://masteringdjango.com/django-database-api-reference/appendix_A.html#django.db.models.ManyToManyField "django.db.models.ManyToManyField") in one go, include multiple arguments in the call to `add()`, like this: ~~~ >>> john = Author.objects.create(name="John") >>> paul = Author.objects.create(name="Paul") >>> george = Author.objects.create(name="George") >>> ringo = Author.objects.create(name="Ringo") >>> entry.authors.add(john, paul, george, ringo) ~~~ Django will complain if you try to assign or add an object of the wrong type. ## Retrieving objects To retrieve objects from your database, construct a `QuerySet` via a `Manager` on your model class. A `QuerySet` represents a collection of objects from your database. It can have zero, one or many *filters*. Filters narrow down the query results based on the given parameters. In SQL terms, a `QuerySet` equates to a`SELECT` statement, and a filter is a limiting clause such as `WHERE` or `LIMIT`. You get a `QuerySet` by using your model’s `Manager`. Each model has at least one `Manager`, and it’s called `objects`by default. Access it directly via the model class, like so: ~~~ >>> Blog.objects <django.db.models.manager.Manager object at ...> >>> b = Blog(name='Foo', tagline='Bar') >>> b.objects Traceback: ... AttributeError: "Manager isn't accessible via Blog instances." ~~~ Note `Managers` are accessible only via model classes, rather than from model instances, to enforce a separation between “table-level” operations and “record-level” operations. The `Manager` is the main source of `QuerySets` for a model. For example, `Blog.objects.all()` returns a `QuerySet`that contains all `Blog` objects in the database. ### Retrieving all objects The simplest way to retrieve objects from a table is to get all of them. To do this, use the `all()` method on a `Manager`: ~~~ >>> all_entries = Entry.objects.all() ~~~ The `all()` method returns a `QuerySet` of all the objects in the database. ### Retrieving specific objects with filters The `QuerySet` returned by `all()` describes all objects in the database table. Usually, though, you’ll need to select only a subset of the complete set of objects. To create such a subset, you refine the initial `QuerySet`, adding filter conditions. The two most common ways to refine a `QuerySet` are: `filter(**kwargs)` Returns a new `QuerySet` containing objects that match the given lookup parameters. `exclude(**kwargs)` Returns a new `QuerySet` containing objects that do *not* match the given lookup parameters. The lookup parameters (`**kwargs` in the above function definitions) should be in the format described in[Field lookups](http://masteringdjango.com/django-database-api-reference/#field-lookups) below. For example, to get a `QuerySet` of blog entries from the year 2006, use `filter()` like so: ~~~ Entry.objects.filter(pub_date__year=2006) ~~~ With the default manager class, it is the same as: ~~~ Entry.objects.all().filter(pub_date__year=2006) ~~~ #### CHAINING FILTERS The result of refining a `QuerySet` is itself a `QuerySet`, so it’s possible to chain refinements together. For example: ~~~ >>> Entry.objects.filter( ... headline__startswith='What' ... ).exclude( ... pub_date__gte=datetime.date.today() ... ).filter( ... pub_date__gte=datetime(2005, 1, 30) ... ) ~~~ This takes the initial `QuerySet` of all entries in the database, adds a filter, then an exclusion, then another filter. The final result is a `QuerySet` containing all entries with a headline that starts with “What”, that were published between January 30, 2005, and the current day. #### FILTERED QUERYSETS ARE UNIQUE Each time you refine a `QuerySet`, you get a brand-new `QuerySet` that is in no way bound to the previous`QuerySet`. Each refinement creates a separate and distinct `QuerySet` that can be stored, used and reused. Example: ~~~ >>> q1 = Entry.objects.filter(headline__startswith="What") >>> q2 = q1.exclude(pub_date__gte=datetime.date.today()) >>> q3 = q1.filter(pub_date__gte=datetime.date.today()) ~~~ These three `QuerySets` are separate. The first is a base `QuerySet` containing all entries that contain a headline starting with “What”. The second is a subset of the first, with an additional criteria that excludes records whose `pub_date` is today or in the future. The third is a subset of the first, with an additional criteria that selects only the records whose `pub_date` is today or in the future. The initial `QuerySet` (`q1`) is unaffected by the refinement process. #### QUERYSETS ARE LAZY `QuerySets` are lazy – the act of creating a `QuerySet` doesn’t involve any database activity. You can stack filters together all day long, and Django won’t actually run the query until the `QuerySet` is *evaluated*. Take a look at this example: ~~~ >>> q = Entry.objects.filter(headline__startswith="What") >>> q = q.filter(pub_date__lte=datetime.date.today()) >>> q = q.exclude(body_text__icontains="food") >>> print(q) ~~~ Though this looks like three database hits, in fact it hits the database only once, at the last line (`print(q)`). In general, the results of a `QuerySet` aren’t fetched from the database until you “ask” for them. When you do, the `QuerySet` is *evaluated* by accessing the database. For more details on exactly when evaluation takes place, see ‘when querysets are evaluated’. ### Retrieving a single object with get `filter()` will always give you a `QuerySet`, even if only a single object matches the query – in this case, it will be a `QuerySet` containing a single element. If you know there is only one object that matches your query, you can use the `get()` method on a `Manager`which returns the object directly: ~~~ >>> one_entry = Entry.objects.get(pk=1) ~~~ You can use any query expression with `get()`, just like with `filter()` – again, see [Field lookups](http://masteringdjango.com/django-database-api-reference/#field-lookups) below. Note that there is a difference between using `get()`, and using `filter()` with a slice of `[0]`. If there are no results that match the query, `get()` will raise a `DoesNotExist` exception. This exception is an attribute of the model class that the query is being performed on – so in the code above, if there is no `Entry` object with a primary key of 1, Django will raise `Entry.DoesNotExist`. Similarly, Django will complain if more than one item matches the `get()` query. In this case, it will raise`MultipleObjectsReturned`, which again is an attribute of the model class itself. ### Other QuerySet methods Most of the time you’ll use `all()`, `get()`, `filter()` and `exclude()` when you need to look up objects from the database. However, that’s far from all there is; see the QuerySet API Reference  for a complete list of all the various `QuerySet` methods. ### Limiting QuerySets Use a subset of Python’s array-slicing syntax to limit your `QuerySet` to a certain number of results. This is the equivalent of SQL’s `LIMIT` and `OFFSET` clauses. For example, this returns the first 5 objects (`LIMIT 5`): ~~~ >>> Entry.objects.all()[:5] ~~~ This returns the sixth through tenth objects (`OFFSET 5 LIMIT 5`): ~~~ >>> Entry.objects.all()[5:10] ~~~ Negative indexing (i.e. `Entry.objects.all()[-1]`) is not supported. Generally, slicing a `QuerySet` returns a new `QuerySet` – it doesn’t evaluate the query. An exception is if you use the “step” parameter of Python slice syntax. For example, this would actually execute the query in order to return a list of every *second* object of the first 10: ~~~ >>> Entry.objects.all()[:10:2] ~~~ To retrieve a *single* object rather than a list (e.g. `SELECT foo FROM bar LIMIT 1`), use a simple index instead of a slice. For example, this returns the first `Entry` in the database, after ordering entries alphabetically by headline: ~~~ >>> Entry.objects.order_by('headline')[0] ~~~ This is roughly equivalent to: ~~~ >>> Entry.objects.order_by('headline')[0:1].get() ~~~ Note, however, that the first of these will raise `IndexError` while the second will raise `DoesNotExist` if no objects match the given criteria. See `get()` for more details. ### Field lookups Field lookups are how you specify the meat of an SQL `WHERE` clause. They’re specified as keyword arguments to the `QuerySet` methods `filter()`, `exclude()` and `get()`. Basic lookups keyword arguments take the form `field__lookuptype=value`. (That’s a double-underscore). For example: ~~~ >>> Entry.objects.filter(pub_date__lte='2006-01-01') ~~~ translates (roughly) into the following SQL: ~~~ SELECT * FROM blog_entry WHERE pub_date <= '2006-01-01'; ~~~ How this is possible Python has the ability to define functions that accept arbitrary name-value arguments whose names and values are evaluated at runtime. For more information, see [Keyword Arguments](https://docs.python.org/tutorial/controlflow.html#keyword-arguments) in the official Python tutorial. The field specified in a lookup has to be the name of a model field. There’s one exception though, in case of a [`ForeignKey`](http://masteringdjango.com/django-database-api-reference/appendix_A.html#django.db.models.ForeignKey "django.db.models.ForeignKey") you can specify the field name suffixed with `_id`. In this case, the value parameter is expected to contain the raw value of the foreign model’s primary key. For example: ~~~ >>> Entry.objects.filter(blog_id=4) ~~~ If you pass an invalid keyword argument, a lookup function will raise `TypeError`. The database API supports about two dozen lookup types; a complete reference can be found in the field lookup reference . To give you a taste of what’s available, here’s some of the more common lookups you’ll probably use: `exact` An “exact” match. For example: ~~~ >>> Entry.objects.get(headline__exact="Man bites dog") ~~~ Would generate SQL along these lines: ~~~ SELECT ... WHERE headline = 'Man bites dog'; ~~~ If you don’t provide a lookup type – that is, if your keyword argument doesn’t contain a double underscore – the lookup type is assumed to be `exact`. For example, the following two statements are equivalent: ~~~ >>> Blog.objects.get(id__exact=14) # Explicit form >>> Blog.objects.get(id=14) # __exact is implied ~~~ This is for convenience, because `exact` lookups are the common case. `iexact` A case-insensitive match. So, the query: ~~~ >>> Blog.objects.get(name__iexact="beatles blog") ~~~ Would match a `Blog` titled `"Beatles Blog"`, `"beatles blog"`, or even `"BeAtlES blOG"`. `contains` Case-sensitive containment test. For example: ~~~ Entry.objects.get(headline__contains='Lennon') ~~~ Roughly translates to this SQL: ~~~ SELECT ... WHERE headline LIKE '%Lennon%'; ~~~ Note this will match the headline `'Today Lennon honored'` but not `'today lennon honored'`. There’s also a case-insensitive version, `icontains`. `startswith`, `endswith` Starts-with and ends-with search, respectively. There are also case-insensitive versions called `istartswith`and `iendswith`. Again, this only scratches the surface. A complete reference can be found in the field lookup reference . ### Lookups that span relationships Django offers a powerful and intuitive way to “follow” relationships in lookups, taking care of the SQL `JOIN`s for you automatically, behind the scenes. To span a relationship, just use the field name of related fields across models, separated by double underscores, until you get to the field you want. This example retrieves all `Entry` objects with a `Blog` whose `name` is `'Beatles Blog'`: ~~~ >>> Entry.objects.filter(blog__name='Beatles Blog') ~~~ This spanning can be as deep as you’d like. It works backwards, too. To refer to a “reverse” relationship, just use the lowercase name of the model. This example retrieves all `Blog` objects which have at least one `Entry` whose `headline` contains `'Lennon'`: ~~~ >>> Blog.objects.filter(entry__headline__contains='Lennon') ~~~ If you are filtering across multiple relationships and one of the intermediate models doesn’t have a value that meets the filter condition, Django will treat it as if there is an empty (all values are `NULL`), but valid, object there. All this means is that no error will be raised. For example, in this filter: ~~~ Blog.objects.filter(entry__authors__name='Lennon') ~~~ (if there was a related `Author` model), if there was no `author` associated with an entry, it would be treated as if there was also no `name` attached, rather than raising an error because of the missing `author`. Usually this is exactly what you want to have happen. The only case where it might be confusing is if you are using `isnull`. Thus: ~~~ Blog.objects.filter(entry__authors__name__isnull=True) ~~~ will return `Blog` objects that have an empty `name` on the `author` and also those which have an empty `author`on the `entry`. If you don’t want those latter objects, you could write: ~~~ Blog.objects.filter(entry__authors__isnull=False, entry__authors__name__isnull=True) ~~~ #### SPANNING MULTI-VALUED RELATIONSHIPS When you are filtering an object based on a [`ManyToManyField`](http://masteringdjango.com/django-database-api-reference/appendix_A.html#django.db.models.ManyToManyField "django.db.models.ManyToManyField") or a reverse [`ForeignKey`](http://masteringdjango.com/django-database-api-reference/appendix_A.html#django.db.models.ForeignKey "django.db.models.ForeignKey"), there are two different sorts of filter you may be interested in. Consider the `Blog`/`Entry` relationship (`Blog` to `Entry` is a one-to-many relation). We might be interested in finding blogs that have an entry which has both *“Lennon”* in the headline and was published in 2008\. Or we might want to find blogs that have an entry with *“Lennon”* in the headline as well as an entry that was published in 2008\. Since there are multiple entries associated with a single `Blog`, both of these queries are possible and make sense in some situations. The same type of situation arises with a [`ManyToManyField`](http://masteringdjango.com/django-database-api-reference/appendix_A.html#django.db.models.ManyToManyField "django.db.models.ManyToManyField"). For example, if an `Entry` has a [`ManyToManyField`](http://masteringdjango.com/django-database-api-reference/appendix_A.html#django.db.models.ManyToManyField "django.db.models.ManyToManyField")called `tags`, we might want to find entries linked to tags called *“music”* and *“bands”* or we might want an entry that contains a tag with a name of *“music”* and a status of *“public”*. To handle both of these situations, Django has a consistent way of processing `filter()` and `exclude()` calls. Everything inside a single `filter()` call is applied simultaneously to filter out items matching all those requirements. Successive `filter()` calls further restrict the set of objects, but for multi-valued relations, they apply to any object linked to the primary model, not necessarily those objects that were selected by an earlier `filter()` call. That may sound a bit confusing, so hopefully an example will clarify. To select all blogs that contain entries with both *“Lennon”* in the headline and that were published in 2008 (the same entry satisfying both conditions), we would write: ~~~ Blog.objects.filter(entry__headline__contains='Lennon', entry__pub_date__year=2008) ~~~ To select all blogs that contain an entry with *“Lennon”* in the headline as well as an entry that was published in 2008, we would write: ~~~ Blog.objects.filter(entry__headline__contains='Lennon').filter( entry__pub_date__year=2008) ~~~ Suppose there is only one blog that had both entries containing *“Lennon”* and entries from 2008, but that none of the entries from 2008 contained *“Lennon”*. The first query would not return any blogs, but the second query would return that one blog. In the second example, the first filter restricts the queryset to all those blogs linked to entries with*“Lennon”* in the headline. The second filter restricts the set of blogs *further* to those that are also linked to entries that were published in 2008\. The entries selected by the second filter may or may not be the same as the entries in the first filter. We are filtering the `Blog` items with each filter statement, not the `Entry`items. All of this behavior also applies to `exclude()`: all the conditions in a single `exclude()` statement apply to a single instance (if those conditions are talking about the same multi-valued relation). Conditions in subsequent `filter()` or `exclude()` calls that refer to the same relation may end up filtering on different linked objects. ### Filters can reference fields on the model In the examples given so far, we have constructed filters that compare the value of a model field with a constant. But what if you want to compare the value of a model field with another field on the same model? Django provides `F expressions` to allow such comparisons. Instances of `F()` act as a reference to a model field within a query. These references can then be used in query filters to compare the values of two different fields on the same model instance. For example, to find a list of all blog entries that have had more comments than pingbacks, we construct an `F()` object to reference the pingback count, and use that `F()` object in the query: ~~~ >>> from django.db.models import F >>> Entry.objects.filter(n_comments__gt=F('n_pingbacks')) ~~~ Django supports the use of addition, subtraction, multiplication, division, modulo, and power arithmetic with `F()` objects, both with constants and with other `F()` objects. To find all the blog entries with more than *twice* as many comments as pingbacks, we modify the query: ~~~ >>> Entry.objects.filter(n_comments__gt=F('n_pingbacks') * 2) ~~~ To find all the entries where the rating of the entry is less than the sum of the pingback count and comment count, we would issue the query: ~~~ >>> Entry.objects.filter(rating__lt=F('n_comments') + F('n_pingbacks')) ~~~ You can also use the double underscore notation to span relationships in an `F()` object. An `F()` object with a double underscore will introduce any joins needed to access the related object. For example, to retrieve all the entries where the author’s name is the same as the blog name, we could issue the query: ~~~ >>> Entry.objects.filter(authors__name=F('blog__name')) ~~~ For date and date/time fields, you can add or subtract a `timedelta` object. The following would return all entries that were modified more than 3 days after they were published: ~~~ >>> from datetime import timedelta >>> Entry.objects.filter(mod_date__gt=F('pub_date') + timedelta(days=3)) ~~~ The `F()` objects support bitwise operations by `.bitand()` and `.bitor()`, for example: ~~~ >>> F('somefield').bitand(16) ~~~ ### The pk lookup shortcut For convenience, Django provides a `pk` lookup shortcut, which stands for “primary key”. In the example `Blog` model, the primary key is the `id` field, so these three statements are equivalent: ~~~ >>> Blog.objects.get(id__exact=14) # Explicit form >>> Blog.objects.get(id=14) # __exact is implied >>> Blog.objects.get(pk=14) # pk implies id__exact ~~~ The use of `pk` isn’t limited to `__exact` queries – any query term can be combined with `pk` to perform a query on the primary key of a model: ~~~ # Get blogs entries with id 1, 4 and 7 >>> Blog.objects.filter(pk__in=[1,4,7]) # Get all blog entries with id > 14 >>> Blog.objects.filter(pk__gt=14) ~~~ `pk` lookups also work across joins. For example, these three statements are equivalent: ~~~ >>> Entry.objects.filter(blog__id__exact=3) # Explicit form >>> Entry.objects.filter(blog__id=3) # __exact is implied >>> Entry.objects.filter(blog__pk=3) # __pk implies __id__exact ~~~ ### Escaping percent signs and underscores in LIKE statements The field lookups that equate to `LIKE` SQL statements (`iexact`, `contains`, `icontains`, `startswith`, `istartswith`,`endswith` and `iendswith`) will automatically escape the two special characters used in `LIKE` statements – the percent sign and the underscore. (In a `LIKE` statement, the percent sign signifies a multiple-character wildcard and the underscore signifies a single-character wildcard.) This means things should work intuitively, so the abstraction doesn’t leak. For example, to retrieve all the entries that contain a percent sign, just use the percent sign as any other character: ~~~ >>> Entry.objects.filter(headline__contains='%') ~~~ Django takes care of the quoting for you; the resulting SQL will look something like this: ~~~ SELECT ... WHERE headline LIKE '%\%%'; ~~~ Same goes for underscores. Both percentage signs and underscores are handled for you transparently. ### Caching and QuerySets Each `QuerySet` contains a cache to minimize database access. Understanding how it works will allow you to write the most efficient code. In a newly created `QuerySet`, the cache is empty. The first time a `QuerySet` is evaluated – and, hence, a database query happens – Django saves the query results in the `QuerySet`’s cache and returns the results that have been explicitly requested (e.g., the next element, if the `QuerySet` is being iterated over). Subsequent evaluations of the `QuerySet` reuse the cached results. Keep this caching behavior in mind, because it may bite you if you don’t use your `QuerySet`s correctly. For example, the following will create two `QuerySet`s, evaluate them, and throw them away: ~~~ >>> print([e.headline for e in Entry.objects.all()]) >>> print([e.pub_date for e in Entry.objects.all()]) ~~~ That means the same database query will be executed twice, effectively doubling your database load. Also, there’s a possibility the two lists may not include the same database records, because an `Entry` may have been added or deleted in the split second between the two requests. To avoid this problem, simply save the `QuerySet` and reuse it: ~~~ >>> queryset = Entry.objects.all() >>> print([p.headline for p in queryset]) # Evaluate the query set. >>> print([p.pub_date for p in queryset]) # Re-use the cache from the evaluation. ~~~ #### WHEN QUERYSETS ARE NOT CACHED Querysets do not always cache their results. When evaluating only *part* of the queryset, the cache is checked, but if it is not populated then the items returned by the subsequent query are not cached. Specifically, this means that limiting the queryset using an array slice or an index will not populate the cache. For example, repeatedly getting a certain index in a queryset object will query the database each time: ~~~ >>> queryset = Entry.objects.all() >>> print queryset[5] # Queries the database >>> print queryset[5] # Queries the database again ~~~ However, if the entire queryset has already been evaluated, the cache will be checked instead: ~~~ >>> queryset = Entry.objects.all() >>> [entry for entry in queryset] # Queries the database >>> print queryset[5] # Uses cache >>> print queryset[5] # Uses cache ~~~ Here are some examples of other actions that will result in the entire queryset being evaluated and therefore populate the cache: ~~~ >>> [entry for entry in queryset] >>> bool(queryset) >>> entry in queryset >>> list(queryset) ~~~ Note Simply printing the queryset will not populate the cache. This is because the call to `__repr__()` only returns a slice of the entire queryset. ## Complex lookups with Q objects Keyword argument queries – in `filter()`, etc. – are “AND”ed together. If you need to execute more complex queries (for example, queries with `OR` statements), you can use `Q objects`. A `Q object` (`django.db.models.Q`) is an object used to encapsulate a collection of keyword arguments. These keyword arguments are specified as in “Field lookups” above. For example, this `Q` object encapsulates a single `LIKE` query: ~~~ from django.db.models import Q Q(question__startswith='What') ~~~ `Q` objects can be combined using the `&` and `|` operators. When an operator is used on two `Q` objects, it yields a new `Q` object. For example, this statement yields a single `Q` object that represents the “OR” of two `"question__startswith"`queries: ~~~ Q(question__startswith='Who') | Q(question__startswith='What') ~~~ This is equivalent to the following SQL `WHERE` clause: ~~~ WHERE question LIKE 'Who%' OR question LIKE 'What%' ~~~ You can compose statements of arbitrary complexity by combining `Q` objects with the `&` and `|` operators and use parenthetical grouping. Also, `Q` objects can be negated using the `~` operator, allowing for combined lookups that combine both a normal query and a negated (`NOT`) query: ~~~ Q(question__startswith='Who') | ~Q(pub_date__year=2005) ~~~ Each lookup function that takes keyword-arguments (e.g. `filter()`, `exclude()`, `get()`) can also be passed one or more `Q` objects as positional (not-named) arguments. If you provide multiple `Q` object arguments to a lookup function, the arguments will be “AND”ed together. For example: ~~~ Poll.objects.get( Q(question__startswith='Who'), Q(pub_date=date(2005, 5, 2)) | Q(pub_date=date(2005, 5, 6)) ) ~~~ … roughly translates into the SQL: ~~~ SELECT * from polls WHERE question LIKE 'Who%' AND (pub_date = '2005-05-02' OR pub_date = '2005-05-06') ~~~ Lookup functions can mix the use of `Q` objects and keyword arguments. All arguments provided to a lookup function (be they keyword arguments or `Q` objects) are “AND”ed together. However, if a `Q` object is provided, it must precede the definition of any keyword arguments. For example: ~~~ Poll.objects.get( Q(pub_date=date(2005, 5, 2)) | Q(pub_date=date(2005, 5, 6)), question__startswith='Who') ~~~ … would be a valid query, equivalent to the previous example; but: ~~~ # INVALID QUERY Poll.objects.get( question__startswith='Who', Q(pub_date=date(2005, 5, 2)) | Q(pub_date=date(2005, 5, 6))) ~~~ … would not be valid. See also The [OR lookups examples](https://github.com/django/django/blob/master/tests/or_lookups/tests.py) in the Django unit tests show some possible uses of `Q`. ## Comparing objects To compare two model instances, just use the standard Python comparison operator, the double equals sign: `==`. Behind the scenes, that compares the primary key values of two models. Using the `Entry` example above, the following two statements are equivalent: ~~~ >>> some_entry == other_entry >>> some_entry.id == other_entry.id ~~~ If a model’s primary key isn’t called `id`, no problem. Comparisons will always use the primary key, whatever it’s called. For example, if a model’s primary key field is called `name`, these two statements are equivalent: ~~~ >>> some_obj == other_obj >>> some_obj.name == other_obj.name ~~~ ## Deleting objects The delete method, conveniently, is named `delete()`. This method immediately deletes the object and has no return value. Example: ~~~ e.delete() ~~~ You can also delete objects in bulk. Every `QuerySet` has a `delete()` method, which deletes all members of that `QuerySet`. For example, this deletes all `Entry` objects with a `pub_date` year of 2005: ~~~ Entry.objects.filter(pub_date__year=2005).delete() ~~~ Keep in mind that this will, whenever possible, be executed purely in SQL, and so the `delete()` methods of individual object instances will not necessarily be called during the process. If you’ve provided a custom`delete()` method on a model class and want to ensure that it is called, you will need to “manually” delete instances of that model (e.g., by iterating over a `QuerySet` and calling `delete()` on each object individually) rather than using the bulk `delete()` method of a `QuerySet`. When Django deletes an object, by default it emulates the behavior of the SQL constraint `ON DELETE CASCADE`– in other words, any objects which had foreign keys pointing at the object to be deleted will be deleted along with it. For example: ~~~ b = Blog.objects.get(pk=1) # This will delete the Blog and all of its Entry objects. b.delete() ~~~ This cascade behavior is customizable via the [`on_delete`](http://masteringdjango.com/django-database-api-reference/appendix_A.html#django.db.models.ForeignKey.on_delete "django.db.models.ForeignKey.on_delete") argument to the [`ForeignKey`](http://masteringdjango.com/django-database-api-reference/appendix_A.html#django.db.models.ForeignKey "django.db.models.ForeignKey"). Note that `delete()` is the only `QuerySet` method that is not exposed on a `Manager` itself. This is a safety mechanism to prevent you from accidentally requesting `Entry.objects.delete()`, and deleting *all* the entries. If you *do* want to delete all the objects, then you have to explicitly request a complete query set: ~~~ Entry.objects.all().delete() ~~~ ## Copying model instances Although there is no built-in method for copying model instances, it is possible to easily create new instance with all fields’ values copied. In the simplest case, you can just set `pk` to `None`. Using our blog example: ~~~ blog = Blog(name='My blog', tagline='Blogging is easy') blog.save() # blog.pk == 1 blog.pk = None blog.save() # blog.pk == 2 ~~~ Things get more complicated if you use inheritance. Consider a subclass of `Blog`: ~~~ class ThemeBlog(Blog): theme = models.CharField(max_length=200) django_blog = ThemeBlog(name='Django', tagline='Django is easy', theme='python') django_blog.save() # django_blog.pk == 3 ~~~ Due to how inheritance works, you have to set both `pk` and `id` to None: ~~~ django_blog.pk = None django_blog.id = None django_blog.save() # django_blog.pk == 4 ~~~ This process does not copy related objects. If you want to copy relations, you have to write a little bit more code. In our example, `Entry` has a many to many field to `Author`: ~~~ entry = Entry.objects.all()[0] # some previous entry old_authors = entry.authors.all() entry.pk = None entry.save() entry.authors = old_authors # saves new many2many relations ~~~ ## Updating multiple objects at once Sometimes you want to set a field to a particular value for all the objects in a `QuerySet`. You can do this with the `update()` method. For example: ~~~ # Update all the headlines with pub_date in 2007. Entry.objects.filter(pub_date__year=2007).update(headline='Everything is the same') ~~~ You can only set non-relation fields and [`ForeignKey`](http://masteringdjango.com/django-database-api-reference/appendix_A.html#django.db.models.ForeignKey "django.db.models.ForeignKey") fields using this method. To update a non-relation field, provide the new value as a constant. To update [`ForeignKey`](http://masteringdjango.com/django-database-api-reference/appendix_A.html#django.db.models.ForeignKey "django.db.models.ForeignKey") fields, set the new value to be the new model instance you want to point to. For example: ~~~ >>> b = Blog.objects.get(pk=1) # Change every Entry so that it belongs to this Blog. >>> Entry.objects.all().update(blog=b) ~~~ The `update()` method is applied instantly and returns the number of rows matched by the query (which may not be equal to the number of rows updated if some rows already have the new value). The only restriction on the `QuerySet` that is updated is that it can only access one database table, the model’s main table. You can filter based on related fields, but you can only update columns in the model’s main table. Example: ~~~ >>> b = Blog.objects.get(pk=1) # Update all the headlines belonging to this Blog. >>> Entry.objects.select_related().filter(blog=b).update(headline='Everything is the same') ~~~ Be aware that the `update()` method is converted directly to an SQL statement. It is a bulk operation for direct updates. It doesn’t run any `save()` methods on your models, or emit the `pre_save` or `post_save` signals (which are a consequence of calling `save()`), or honor the `auto_now` field option. If you want to save every item in a `QuerySet` and make sure that the `save()` method is called on each instance, you don’t need any special function to handle that. Just loop over them and call `save()`: ~~~ for item in my_queryset: item.save() ~~~ Calls to update can also use `F expressions` to update one field based on the value of another field in the model. This is especially useful for incrementing counters based upon their current value. For example, to increment the pingback count for every entry in the blog: ~~~ >>> Entry.objects.all().update(n_pingbacks=F('n_pingbacks') + 1) ~~~ However, unlike `F()` objects in filter and exclude clauses, you can’t introduce joins when you use `F()` objects in an update – you can only reference fields local to the model being updated. If you attempt to introduce a join with an `F()` object, a `FieldError` will be raised: ~~~ # THIS WILL RAISE A FieldError >>> Entry.objects.update(headline=F('blog__name')) ~~~ ## Related objects When you define a relationship in a model (i.e., a [`ForeignKey`](http://masteringdjango.com/django-database-api-reference/appendix_A.html#django.db.models.ForeignKey "django.db.models.ForeignKey"), [`OneToOneField`](http://masteringdjango.com/django-database-api-reference/appendix_A.html#django.db.models.OneToOneField "django.db.models.OneToOneField"), or [`ManyToManyField`](http://masteringdjango.com/django-database-api-reference/appendix_A.html#django.db.models.ManyToManyField "django.db.models.ManyToManyField")), instances of that model will have a convenient API to access the related object(s). Using the models at the top of this page, for example, an `Entry` object `e` can get its associated `Blog` object by accessing the `blog` attribute: `e.blog`. (Behind the scenes, this functionality is implemented by Python [descriptors](http://users.rcn.com/python/download/Descriptor.htm). This shouldn’t really matter to you, but we point it out here for the curious.) Django also creates API accessors for the “other” side of the relationship – the link from the related model to the model that defines the relationship. For example, a `Blog` object `b` has access to a list of all related `Entry`objects via the `entry_set` attribute: `b.entry_set.all()`. All examples in this section use the sample `Blog`, `Author` and `Entry` models defined at the top of this page. ### One-to-many relationships #### FORWARD If a model has a [`ForeignKey`](http://masteringdjango.com/django-database-api-reference/appendix_A.html#django.db.models.ForeignKey "django.db.models.ForeignKey"), instances of that model will have access to the related (foreign) object via a simple attribute of the model. Example: ~~~ >>> e = Entry.objects.get(id=2) >>> e.blog # Returns the related Blog object. ~~~ You can get and set via a foreign-key attribute. As you may expect, changes to the foreign key aren’t saved to the database until you call `save()`. Example: ~~~ >>> e = Entry.objects.get(id=2) >>> e.blog = some_blog >>> e.save() ~~~ If a [`ForeignKey`](http://masteringdjango.com/django-database-api-reference/appendix_A.html#django.db.models.ForeignKey "django.db.models.ForeignKey") field has `null=True` set (i.e., it allows `NULL` values), you can assign `None` to remove the relation. Example: ~~~ >>> e = Entry.objects.get(id=2) >>> e.blog = None >>> e.save() # "UPDATE blog_entry SET blog_id = NULL ...;" ~~~ Forward access to one-to-many relationships is cached the first time the related object is accessed. Subsequent accesses to the foreign key on the same object instance are cached. Example: ~~~ >>> e = Entry.objects.get(id=2) >>> print(e.blog) # Hits the database to retrieve the associated Blog. >>> print(e.blog) # Doesn't hit the database; uses cached version. ~~~ Note that the `select_related()` `QuerySet` method recursively prepopulates the cache of all one-to-many relationships ahead of time. Example: ~~~ >>> e = Entry.objects.select_related().get(id=2) >>> print(e.blog) # Doesn't hit the database; uses cached version. >>> print(e.blog) # Doesn't hit the database; uses cached version. ~~~ #### FOLLOWING RELATIONSHIPS “BACKWARD” If a model has a [`ForeignKey`](http://masteringdjango.com/django-database-api-reference/appendix_A.html#django.db.models.ForeignKey "django.db.models.ForeignKey"), instances of the foreign-key model will have access to a `Manager` that returns all instances of the first model. By default, this `Manager` is named `FOO_set`, where `FOO` is the source model name, lowercased. This `Manager` returns `QuerySets`, which can be filtered and manipulated as described in the “Retrieving objects” section above. Example: ~~~ >>> b = Blog.objects.get(id=1) >>> b.entry_set.all() # Returns all Entry objects related to Blog. # b.entry_set is a Manager that returns QuerySets. >>> b.entry_set.filter(headline__contains='Lennon') >>> b.entry_set.count() ~~~ You can override the `FOO_set` name by setting the [`related_name`](http://masteringdjango.com/django-database-api-reference/appendix_A.html#django.db.models.ForeignKey.related_name "django.db.models.ForeignKey.related_name") parameter in the [`ForeignKey`](http://masteringdjango.com/django-database-api-reference/appendix_A.html#django.db.models.ForeignKey "django.db.models.ForeignKey") definition. For example, if the `Entry` model was altered to `blog = ForeignKey(Blog, related_name='entries')`, the above example code would look like this: ~~~ >>> b = Blog.objects.get(id=1) >>> b.entries.all() # Returns all Entry objects related to Blog. # b.entries is a Manager that returns QuerySets. >>> b.entries.filter(headline__contains='Lennon') >>> b.entries.count() ~~~ #### USING A CUSTOM REVERSE MANAGER By default the `RelatedManager` used for reverse relations is a subclass of the default manager for that model. If you would like to specify a different manager for a given query you can use the following syntax: ~~~ from django.db import models class Entry(models.Model): #... objects = models.Manager() # Default Manager entries = EntryManager() # Custom Manager b = Blog.objects.get(id=1) b.entry_set(manager='entries').all() ~~~ If `EntryManager` performed default filtering in its `get_queryset()` method, that filtering would apply to the`all()` call. Of course, specifying a custom reverse manager also enables you to call its custom methods: ~~~ b.entry_set(manager='entries').is_published() ~~~ #### ADDITIONAL METHODS TO HANDLE RELATED OBJECTS In addition to the `QuerySet` methods defined in “Retrieving objects” above, the [`ForeignKey`](http://masteringdjango.com/django-database-api-reference/appendix_A.html#django.db.models.ForeignKey "django.db.models.ForeignKey") `Manager` has additional methods used to handle the set of related objects. A synopsis of each is below, and complete details can be found in the related objects reference . `add(obj1, obj2, ...)` Adds the specified model objects to the related object set. `create(**kwargs)` Creates a new object, saves it and puts it in the related object set. Returns the newly created object. `remove(obj1, obj2, ...)` Removes the specified model objects from the related object set. `clear()` Removes all objects from the related object set. `set(objs)` Replace the set of related objects. To assign the members of a related set in one fell swoop, just assign to it from any iterable object. The iterable can contain object instances, or just a list of primary key values. For example: ~~~ b = Blog.objects.get(id=1) b.entry_set = [e1, e2] ~~~ In this example, `e1` and `e2` can be full Entry instances, or integer primary key values. If the `clear()` method is available, any pre-existing objects will be removed from the `entry_set` before all objects in the iterable (in this case, a list) are added to the set. If the `clear()` method is *not* available, all objects in the iterable will be added without removing any existing elements. Each “reverse” operation described in this section has an immediate effect on the database. Every addition, creation and deletion is immediately and automatically saved to the database. ### Many-to-many relationships Both ends of a many-to-many relationship get automatic API access to the other end. The API works just as a “backward” one-to-many relationship, above. The only difference is in the attribute naming: The model that defines the [`ManyToManyField`](http://masteringdjango.com/django-database-api-reference/appendix_A.html#django.db.models.ManyToManyField "django.db.models.ManyToManyField") uses the attribute name of that field itself, whereas the “reverse” model uses the lowercased model name of the original model, plus `'_set'` (just like reverse one-to-many relationships). An example makes this easier to understand: ~~~ e = Entry.objects.get(id=3) e.authors.all() # Returns all Author objects for this Entry. e.authors.count() e.authors.filter(name__contains='John') a = Author.objects.get(id=5) a.entry_set.all() # Returns all Entry objects for this Author. ~~~ Like [`ForeignKey`](http://masteringdjango.com/django-database-api-reference/appendix_A.html#django.db.models.ForeignKey "django.db.models.ForeignKey"), [`ManyToManyField`](http://masteringdjango.com/django-database-api-reference/appendix_A.html#django.db.models.ManyToManyField "django.db.models.ManyToManyField") can specify [`related_name`](http://masteringdjango.com/django-database-api-reference/appendix_A.html#django.db.models.ManyToManyField.related_name "django.db.models.ManyToManyField.related_name"). In the above example, if the [`ManyToManyField`](http://masteringdjango.com/django-database-api-reference/appendix_A.html#django.db.models.ManyToManyField "django.db.models.ManyToManyField") in`Entry` had specified `related_name='entries'`, then each `Author` instance would have an `entries` attribute instead of `entry_set`. ### One-to-one relationships One-to-one relationships are very similar to many-to-one relationships. If you define a [`OneToOneField`](http://masteringdjango.com/django-database-api-reference/appendix_A.html#django.db.models.OneToOneField "django.db.models.OneToOneField") on your model, instances of that model will have access to the related object via a simple attribute of the model. For example: ~~~ class EntryDetail(models.Model): entry = models.OneToOneField(Entry) details = models.TextField() ed = EntryDetail.objects.get(id=2) ed.entry # Returns the related Entry object. ~~~ The difference comes in “reverse” queries. The related model in a one-to-one relationship also has access to a `Manager` object, but that `Manager` represents a single object, rather than a collection of objects: ~~~ e = Entry.objects.get(id=2) e.entrydetail # returns the related EntryDetail object ~~~ If no object has been assigned to this relationship, Django will raise a `DoesNotExist` exception. Instances can be assigned to the reverse relationship in the same way as you would assign the forward relationship: ~~~ e.entrydetail = ed ~~~ ### How are the backward relationships possible? Other object-relational mappers require you to define relationships on both sides. The Django developers believe this is a violation of the DRY (Don’t Repeat Yourself) principle, so Django only requires you to define the relationship on one end. But how is this possible, given that a model class doesn’t know which other model classes are related to it until those other model classes are loaded? The answer lies in the `app registry`. When Django starts, it imports each application listed in`INSTALLED_APPS`, and then the `models` module inside each application. Whenever a new model class is created, Django adds backward-relationships to any related models. If the related models haven’t been imported yet, Django keeps tracks of the relationships and adds them when the related models eventually are imported. For this reason, it’s particularly important that all the models you’re using be defined in applications listed in `INSTALLED_APPS`. Otherwise, backwards relations may not work properly. ### Queries over related objects Queries involving related objects follow the same rules as queries involving normal value fields. When specifying the value for a query to match, you may use either an object instance itself, or the primary key value for the object. For example, if you have a Blog object `b` with `id=5`, the following three queries would be identical: ~~~ Entry.objects.filter(blog=b) # Query using object instance Entry.objects.filter(blog=b.id) # Query using id from instance Entry.objects.filter(blog=5) # Query using id directly ~~~ ## Falling back to raw SQL If you find yourself needing to write an SQL query that is too complex for Django’s database-mapper to handle, you can fall back on writing SQL by hand. Finally, it’s important to note that the Django database layer is merely an interface to your database. You can access your database via other tools, programming languages or database frameworks; there’s nothing Django-specific about your database.