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+======================
+Database API reference
+======================
+Once you've created your `data models`_, Django automatically gives you a
+database-abstraction API that lets you create, retrieve, update and delete
+objects. This document explains that API.
+.. _`data models`: ../model_api/
+Throughout this reference, we'll refer to the following models, which comprise
+a weblog application::
+class Blog(models.Model):
+name = models.CharField(maxlength=100)
+tagline = models.TextField()
+def __str__(self):
+return self.name
+class Author(models.Model):
+name = models.CharField(maxlength=50)
+email = models.URLField()
+def __str__(self):
+return self.name
+class Entry(models.Model):
+blog = models.ForeignKey(Blog)
+headline = models.CharField(maxlength=255)
+body_text = models.TextField()
+pub_date = models.DateTimeField()
+authors = models.ManyToManyField(Author)
+def __str__(self):
+return self.headline
+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.
+You import the model class from wherever it lives on the Python path, as you
+may expect. (We point this out here because previous Django versions required
+funky model importing.)
+Assuming models live in a file ``mysite/blog/models.py``, here's an example::
+from mysite.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.
+To create an object and save it all in one step see the `create`__ method.
+__ `create(**kwargs)`_
+Auto-incrementing primary keys
+------------------------------
+If a model has an ``AutoField`` -- an auto-incrementing primary key -- then
+that auto-incremented value will be calculated and saved as an attribute on
+your object the first time you call ``save()``.
+Example::
+b2 = Blog(name='Cheddar Talk', tagline='Thoughts on cheese.')
+b2.id     # Returns None, because b doesn't have an ID yet.
+b2.save()
+b2.id     # Returns the ID of your new object.
+There's no way to tell what the value of an ID will be before you call
+``save()``, because that value is calculated by your database, not by Django.
+(For convenience, each model has an ``AutoField`` named ``id`` by default
+unless you explicitly specify ``primary_key=True`` on a field. See the
+`AutoField documentation`_.)
+.. _AutoField documentation: ../model_api/#autofield
+Explicitly specifying auto-primary-key values
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+If a model has an ``AutoField`` but you want to define a new object's ID
+explicitly when saving, just define it explicitly before saving, rather than
+relying on the auto-assignment of the ID.
+Example::
+b3 = Blog(id=3, name='Cheddar Talk', tagline='Thoughts on cheese.')
+b3.id     # Returns 3.
+b3.save()
+b3.id     # Returns 3.
+If you assign auto-primary-key values manually, make sure not to use an
+already-existing primary-key value! If you create a new object with an explicit
+primary-key value that already exists in the database, Django will assume
+you're changing the existing record rather than creating a new one.
+Given the above ``'Cheddar Talk'`` blog example, this example would override
+the previous record in the database::
+b4 = Blog(id=3, name='Not Cheddar', tagline='Anything but cheese.')
+b4.save()  # Overrides the previous blog with ID=3!
+See _`How Django knows to UPDATE vs. INSERT`, below, for the reason this
+happens.
+Explicitly specifying auto-primary-key values is mostly useful for bulk-saving
+objects, when you're confident you won't have primary-key collision.
+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()``.
+The ``save()`` method has no return value.
+How Django knows to UPDATE vs. INSERT
+-------------------------------------
+You may have noticed Django database objects use the same ``save()`` method
+for creating and changing objects. Django abstracts the need to use ``INSERT``
+or ``UPDATE`` SQL statements. Specifically, when you call ``save()``, Django
+follows this algorithm:
+* If the object's primary key attribute is set to a value that evaluates to
+``True`` (i.e., a value other than ``None`` or the empty string), Django
+executes a ``SELECT`` query to determine whether a record with the given
+primary key already exists.
+* If the record with the given primary key does already exist, Django
+executes an ``UPDATE`` query.
+* If the object's primary key attribute is *not* set, or if it's set but a
+record doesn't exist, Django executes an ``INSERT``.
+The one gotcha here is that you should be careful not to specify a primary-key
+value explicitly when saving new objects, if you cannot guarantee the
+primary-key value is unused. For more on this nuance, see
+"Explicitly specifying auto-primary-key values" above.
+Retrieving objects
+==================
+To retrieve objects from your database, you 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* -- criteria that narrow down the collection
+based on 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     # AttributeError: "Manager isn't accessible via Blog instances."
+(``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. It acts as a
+"root" ``QuerySet`` that describes all objects in the model's database table.
+For example, ``Blog.objects`` is the initial ``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``.
+Example::
+all_entries = Entry.objects.all()
+The ``all()`` method returns a ``QuerySet`` of all the objects in the database.
+(If ``Entry.objects`` is a ``QuerySet``, why can't we just do ``Entry.objects``?
+That's because ``Entry.objects``, the root ``QuerySet``, is a special case
+that cannot be evaluated. The ``all()`` method returns a ``QuerySet`` that
+*can* be evaluated.)
+Filtering objects
+-----------------
+The root ``QuerySet`` provided by the ``Manager`` 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`_ 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)
+(Note we don't have to add an ``all()`` -- ``Entry.objects.all().filter(...)``.
+That would still work, but you only need ``all()`` when you want all objects
+from the root ``QuerySet``.)
+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.now()).filter(
+pub_date__gte=datetime(2005, 1, 1))
+...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 1, 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.now())
+q3 = q1.filter(pub_date__gte=datetime.now())
+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 greater than now. The third is a subset of the first,
+with an additional criteria that selects only the records whose ``pub_date`` is
+greater than now. 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*.
+When QuerySets are evaluated
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+You can evaluate a ``QuerySet`` in the following ways:
+* **Iteration.** A ``QuerySet`` is iterable, and it executes its database
+query the first time you iterate over it. For example, this will print
+the headline of all entries in the database::
+for e in Entry.objects.all():
+print e.headline
+* **Slicing.** As explained in `Limiting QuerySets`_ below, a ``QuerySet``
+can be sliced, using Python's array-slicing syntax. Usually slicing a
+``QuerySet`` returns another (unevaluated )``QuerySet``, but Django will
+execute the database query if you use the "step" parameter of slice
+syntax.
+* **repr().** A ``QuerySet`` is evaluated when you call ``repr()`` on it.
+This is for convenience in the Python interactive interpreter, so you can
+immediately see your results when using the API interactively.
+* **len().** A ``QuerySet`` is evaluated when you call ``len()`` on it.
+This, as you might expect, returns the length of the result list.
+Note: *Don't* use ``len()`` on ``QuerySet``\s if all you want to do is
+determine the number of records in the set. It's much more efficient to
+handle a count at the database level, using SQL's ``SELECT COUNT(*)``,
+and Django provides a ``count()`` method for precisely this reason. See
+``count()`` below.
+* **list().** Force evaluation of a ``QuerySet`` by calling ``list()`` on
+it. For example::
+entry_list = list(Entry.objects.all())
+Be warned, though, that this could have a large memory overhead, because
+Django will load each element of the list into memory. In contrast,
+iterating over a ``QuerySet`` will take advantage of your database to
+load data and instantiate objects only as you need them.
+Limiting QuerySets
+------------------
+Use 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 fifth through tenth objects (``OFFSET 5 LIMIT 5``)::
+Entry.objects.all()[5:10]
+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.
+QuerySet methods that return new QuerySets
+------------------------------------------
+Django provides a range of ``QuerySet`` refinement methods that modify either
+the types of results returned by the ``QuerySet`` or the way its SQL query is
+executed.
+``filter(**kwargs)``
+~~~~~~~~~~~~~~~~~~~~
+Returns a new ``QuerySet`` containing objects that match the given lookup
+parameters.
+The lookup parameters (``**kwargs``) should be in the format described in
+`Field lookups`_ below. Multiple parameters are joined via ``AND`` in the
+underlying SQL statement.
+``exclude(**kwargs)``
+~~~~~~~~~~~~~~~~~~~~~
+Returns a new ``QuerySet`` containing objects that do *not* match the given
+lookup parameters.
+The lookup parameters (``**kwargs``) should be in the format described in
+`Field lookups`_ below. Multiple parameters are joined via ``AND`` in the
+underlying SQL statement, and the whole thing is enclosed in a ``NOT()``.
+This example excludes all entries whose ``pub_date`` is the current date/time
+AND whose ``headline`` is "Hello"::
+Entry.objects.exclude(pub_date__gt=datetime.date(2005, 1, 3), headline='Hello')
+In SQL terms, that evaluates to::
+SELECT ...
+WHERE NOT (pub_date > '2005-1-3' AND headline = 'Hello')
+This example excludes all entries whose ``pub_date`` is the current date/time
+OR whose ``headline`` is "Hello"::
+Entry.objects.exclude(pub_date__gt=datetime.date(2005, 1, 3)).exclude(headline='Hello')
+In SQL terms, that evaluates to::
+SELECT ...
+WHERE NOT pub_date > '2005-1-3'
+AND NOT headline = 'Hello'
+Note the second example is more restrictive.
+``order_by(*fields)``
+~~~~~~~~~~~~~~~~~~~~~
+By default, results returned by a ``QuerySet`` are ordered by the ordering
+tuple given by the ``ordering`` option in the model's ``Meta``. You can
+override this on a per-``QuerySet`` basis by using the ``order_by`` method.
+Example::
+Entry.objects.filter(pub_date__year=2005).order_by('-pub_date', 'headline')
+The result above will be ordered by ``pub_date`` descending, then by
+``headline`` ascending. The negative sign in front of ``"-pub_date"`` indicates
+*descending* order. Ascending order is implied. To order randomly, use ``"?"``,
+like so::
+Entry.objects.order_by('?')
+To order by a field in a different table, add the other table's name and a dot,
+like so::
+Entry.objects.order_by('blogs_blog.name', 'headline')
+There's no way to specify whether ordering should be case sensitive. With
+respect to case-sensitivity, Django will order results however your database
+backend normally orders them.
+``distinct()``
+~~~~~~~~~~~~~~
+Returns a new ``QuerySet`` that uses ``SELECT DISTINCT`` in its SQL query. This
+eliminates duplicate rows from the query results.
+By default, a ``QuerySet`` will not eliminate duplicate rows. In practice, this
+is rarely a problem, because simple queries such as ``Blog.objects.all()``
+don't introduce the possibility of duplicate result rows.
+However, if your query spans multiple tables, it's possible to get duplicate
+results when a ``QuerySet`` is evaluated. That's when you'd use ``distinct()``.
+``values(*fields)``
+~~~~~~~~~~~~~~~~~~~
+Returns a ``ValuesQuerySet`` -- a ``QuerySet`` that evaluates to a list of
+dictionaries instead of model-instance objects.
+Each of those dictionaries represents an object, with the keys corresponding to
+the attribute names of model objects.
+This example compares the dictionaries of ``values()`` with the normal model
+objects::
+# This list contains a Blog object.
+>>> Blog.objects.filter(name__startswith='Beatles')
+[Beatles Blog]
+# This list contains a dictionary.
+>>> Blog.objects.filter(name__startswith='Beatles').values()
+[{'id': 1, 'name': 'Beatles Blog', 'tagline': 'All the latest Beatles news.'}]
+``values()`` takes optional positional arguments, ``*fields``, which specify
+field names to which the ``SELECT`` should be limited. If you specify the
+fields, each dictionary will contain only the field keys/values for the fields
+you specify. If you don't specify the fields, each dictionary will contain a
+key and value for every field in the database table.
+Example::
+>>> Blog.objects.values()
+[{'id': 1, 'name': 'Beatles Blog', 'tagline': 'All the latest Beatles news.'}],
+>>> Blog.objects.values('id', 'name')
+[{'id': 1, 'name': 'Beatles Blog'}]
+A ``ValuesQuerySet`` is useful when you know you're only going to need values
+from a small number of the available fields and you won't need the
+functionality of a model instance object. It's more efficient to select only
+the fields you need to use.
+Finally, note a ``ValuesQuerySet`` is a subclass of ``QuerySet``, so it has all
+methods of ``QuerySet``. You can call ``filter()`` on it, or ``order_by()``, or
+whatever. Yes, that means these two calls are identical::
+Blog.objects.values().order_by('id')
+Blog.objects.order_by('id').values()
+The people who made Django prefer to put all the SQL-affecting methods first,
+followed (optionally) by any output-affecting methods (such as ``values()``),
+but it doesn't really matter. This is your chance to really flaunt your
+individualism.
+``dates(field, kind, order='ASC')``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Returns a ``DateQuerySet`` -- a ``QuerySet`` that evaluates to a list of
+``datetime.datetime`` objects representing all available dates of a particular
+kind within the contents of the ``QuerySet``.
+``field`` should be the name of a ``DateField`` or ``DateTimeField`` of your
+model.
+``kind`` should be either ``"year"``, ``"month"`` or ``"day"``. Each
+``datetime.datetime`` object in the result list is "truncated" to the given
+``type``.
+* ``"year"`` returns a list of all distinct year values for the field.
+* ``"month"`` returns a list of all distinct year/month values for the field.
+* ``"day"`` returns a list of all distinct year/month/day values for the field.
+``order``, which defaults to ``'ASC'``, should be either ``'ASC'`` or
+``'DESC'``. This specifies how to order the results.
+Examples::
+>>> Entry.objects.dates('pub_date', 'year')
+[datetime.datetime(2005, 1, 1)]
+>>> Entry.objects.dates('pub_date', 'month')
+[datetime.datetime(2005, 2, 1), datetime.datetime(2005, 3, 1)]
+>>> Entry.objects.dates('pub_date', 'day')
+[datetime.datetime(2005, 2, 20), datetime.datetime(2005, 3, 20)]
+>>> Entry.objects.dates('pub_date', 'day', order='DESC')
+[datetime.datetime(2005, 3, 20), datetime.datetime(2005, 2, 20)]
+>>> Entry.objects.filter(headline__contains='Lennon').dates('pub_date', 'day')
+[datetime.datetime(2005, 3, 20)]
+``none()``
+~~~~~~~~~~
+**New in Django development version**
+Returns an ``EmptyQuerySet`` -- a ``QuerySet`` that always evaluates to
+an empty list. This can be used in cases where you know that you should
+return an empty result set and your caller is expecting a ``QuerySet``
+object (instead of returning an empty list, for example.)
+Examples::
+>>> Entry.objects.none()
+[]
+``select_related()``
+~~~~~~~~~~~~~~~~~~~~
+Returns a ``QuerySet`` that will automatically "follow" foreign-key
+relationships, selecting that additional related-object data when it executes
+its query. This is a performance booster which results in (sometimes much)
+larger queries but means later use of foreign-key relationships won't require
+database queries.
+The following examples illustrate the difference between plain lookups and
+``select_related()`` lookups. Here's standard lookup::
+# Hits the database.
+e = Entry.objects.get(id=5)
+# Hits the database again to get the related Blog object.
+b = e.blog
+And here's ``select_related`` lookup::
+# Hits the database.
+e = Entry.objects.select_related().get(id=5)
+# Doesn't hit the database, because e.blog has been prepopulated
+# in the previous query.
+b = e.blog
+``select_related()`` follows foreign keys as far as possible. If you have the
+following models::
+class City(models.Model):
+# ...
+class Person(models.Model):
+# ...
+hometown = models.ForeignKey(City)
+class Book(models.Model):
+# ...
+author = models.ForeignKey(Person)
+...then a call to ``Book.objects.select_related().get(id=4)`` will cache the
+related ``Person`` *and* the related ``City``::
+b = Book.objects.select_related().get(id=4)
+p = b.author         # Doesn't hit the database.
+c = p.hometown       # Doesn't hit the database.
+sv = Book.objects.get(id=4) # No select_related() in this example.
+p = b.author         # Hits the database.
+c = p.hometown       # Hits the database.
+Note that ``select_related()`` does not follow foreign keys that have
+``null=True``.
+Usually, using ``select_related()`` can vastly improve performance because your
+app can avoid many database calls. However, in situations with deeply nested
+sets of relationships ``select_related()`` can sometimes end up following "too
+many" relations, and can generate queries so large that they end up being slow.
+In these situations, you can use the ``depth`` argument to ``select_related()``
+to control how many "levels" of relations ``select_related()`` will actually
+follow::
+b = Book.objects.select_related(depth=1).get(id=4)
+p = b.author         # Doesn't hit the database.
+c = p.hometown       # Requires a database call.
+The ``depth`` argument is new in the Django development version.
+``extra(select=None, where=None, params=None, tables=None)``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Sometimes, the Django query syntax by itself can't easily express a complex
+``WHERE`` clause. For these edge cases, Django provides the ``extra()``
+``QuerySet`` modifier -- a hook for injecting specific clauses into the SQL
+generated by a ``QuerySet``.
+By definition, these extra lookups may not be portable to different database
+engines (because you're explicitly writing SQL code) and violate the DRY
+principle, so you should avoid them if possible.
+Specify one or more of ``params``, ``select``, ``where`` or ``tables``. None
+of the arguments is required, but you should use at least one of them.
+``select``
+The ``select`` argument lets you put extra fields in the ``SELECT`` clause.
+It should be a dictionary mapping attribute names to SQL clauses to use to
+calculate that attribute.
+Example::
+Entry.objects.extra(select={'is_recent': "pub_date > '2006-01-01'"})
+As a result, each ``Entry`` object will have an extra attribute,
+``is_recent``, a boolean representing whether the entry's ``pub_date`` is
+greater than Jan. 1, 2006.
+Django inserts the given SQL snippet directly into the ``SELECT``
+statement, so the resulting SQL of the above example would be::
+SELECT blog_entry.*, (pub_date > '2006-01-01')
+FROM blog_entry;
+The next example is more advanced; it does a subquery to give each
+resulting ``Blog`` object an ``entry_count`` attribute, an integer count
+of associated ``Entry`` objects::
+Blog.objects.extra(
+select={
+'entry_count': 'SELECT COUNT(*) FROM blog_entry WHERE blog_entry.blog_id = blog_blog.id'
+},
+)
+(In this particular case, we're exploiting the fact that the query will
+already contain the ``blog_blog`` table in its ``FROM`` clause.)
+The resulting SQL of the above example would be::
+SELECT blog_blog.*, (SELECT COUNT(*) FROM blog_entry WHERE blog_entry.blog_id = blog_blog.id)
+FROM blog_blog;
+Note that the parenthesis required by most database engines around
+subqueries are not required in Django's ``select`` clauses. Also note that
+some database backends, such as some MySQL versions, don't support
+subqueries.
+``where`` / ``tables``
+You can define explicit SQL ``WHERE`` clauses -- perhaps to perform
+non-explicit joins -- by using ``where``. You can manually add tables to
+the SQL ``FROM`` clause by using ``tables``.
+``where`` and ``tables`` both take a list of strings. All ``where``
+parameters are "AND"ed to any other search criteria.
+Example::
+Entry.objects.extra(where=['id IN (3, 4, 5, 20)'])
+...translates (roughly) into the following SQL::
+SELECT * FROM blog_entry WHERE id IN (3, 4, 5, 20);
+``params``
+The ``select`` and ``where`` parameters described above may use standard
+Python database string placeholders -- ``'%s'`` to indicate parameters the
+database engine should automatically quote. The ``params`` argument is a
+list of any extra parameters to be substituted.
+Example::
+Entry.objects.extra(where=['headline=%s'], params=['Lennon'])
+Always use ``params`` instead of embedding values directly into ``select``
+or ``where`` because ``params`` will ensure values are quoted correctly
+according to your particular backend. (For example, quotes will be escaped
+correctly.)
+Bad::
+Entry.objects.extra(where=["headline='Lennon'"])
+Good::
+Entry.objects.extra(where=['headline=%s'], params=['Lennon'])
+QuerySet methods that do not return QuerySets
+---------------------------------------------
+The following ``QuerySet`` methods evaluate the ``QuerySet`` and return
+something *other than* a ``QuerySet``.
+These methods do not use a cache (see _`Caching and QuerySets` below). Rather,
+they query the database each time they're called.
+``get(**kwargs)``
+~~~~~~~~~~~~~~~~~
+Returns the object matching the given lookup parameters, which should be in
+the format described in `Field lookups`_.
+``get()`` raises ``AssertionError`` if more than one object was found.
+``get()`` raises a ``DoesNotExist`` exception if an object wasn't found for the
+given parameters. The ``DoesNotExist`` exception is an attribute of the model
+class. Example::
+Entry.objects.get(id='foo') # raises Entry.DoesNotExist
+The ``DoesNotExist`` exception inherits from
+``django.core.exceptions.ObjectDoesNotExist``, so you can target multiple
+``DoesNotExist`` exceptions. Example::
+from django.core.exceptions import ObjectDoesNotExist
+try:
+e = Entry.objects.get(id=3)
+b = Blog.objects.get(id=1)
+except ObjectDoesNotExist:
+print "Either the entry or blog doesn't exist."
+``create(**kwargs)``
+~~~~~~~~~~~~~~~~~~~~
+A convenience method for creating an object and saving it all in one step.  Thus::
+p = Person.objects.create(first_name="Bruce", last_name="Springsteen")
+and::
+p = Person(first_name="Bruce", last_name="Springsteen")
+p.save()
+are equivalent.
+``get_or_create(**kwargs)``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+A convenience method for looking up an object with the given kwargs, creating
+one if necessary.
+Returns a tuple of ``(object, created)``, where ``object`` is the retrieved or
+created object and ``created`` is a boolean specifying whether a new object was
+created.
+This is meant as a shortcut to boilerplatish code and is mostly useful for
+data-import scripts. For example::
+try:
+obj = Person.objects.get(first_name='John', last_name='Lennon')
+except Person.DoesNotExist:
+obj = Person(first_name='John', last_name='Lennon', birthday=date(1940, 10, 9))
+obj.save()
+This pattern gets quite unwieldy as the number of fields in a model goes up.
+The above example can be rewritten using ``get_or_create()`` like so::
+obj, created = Person.objects.get_or_create(first_name='John', last_name='Lennon',
+defaults={'birthday': date(1940, 10, 9)})
+Any keyword arguments passed to ``get_or_create()`` -- *except* an optional one
+called ``defaults`` -- will be used in a ``get()`` call. If an object is found,
+``get_or_create()`` returns a tuple of that object and ``False``. If an object
+is *not* found, ``get_or_create()`` will instantiate and save a new object,
+returning a tuple of the new object and ``True``. The new object will be
+created according to this algorithm::
+defaults = kwargs.pop('defaults', {})
+params = dict([(k, v) for k, v in kwargs.items() if '__' not in k])
+params.update(defaults)
+obj = self.model(**params)
+obj.save()
+In English, that means start with any non-``'defaults'`` keyword argument that
+doesn't contain a double underscore (which would indicate a non-exact lookup).
+Then add the contents of ``defaults``, overriding any keys if necessary, and
+use the result as the keyword arguments to the model class.
+If you have a field named ``defaults`` and want to use it as an exact lookup in
+``get_or_create()``, just use ``'defaults__exact'``, like so::
+Foo.objects.get_or_create(defaults__exact='bar', defaults={'defaults': 'baz'})
+Finally, a word on using ``get_or_create()`` in Django views. As mentioned
+earlier, ``get_or_create()`` is mostly useful in scripts that need to parse
+data and create new records if existing ones aren't available. But if you need
+to use ``get_or_create()`` in a view, please make sure to use it only in
+``POST`` requests unless you have a good reason not to. ``GET`` requests
+shouldn't have any effect on data; use ``POST`` whenever a request to a page
+has a side effect on your data. For more, see `Safe methods`_ in the HTTP spec.
+.. _Safe methods: http://www.w3.org/Protocols/rfc2616/rfc2616-sec9.html#sec9.1.1
+``count()``
+~~~~~~~~~~~
+Returns an integer representing the number of objects in the database matching
+the ``QuerySet``. ``count()`` never raises exceptions.
+Example::
+# Returns the total number of entries in the database.
+Entry.objects.count()
+# Returns the number of entries whose headline contains 'Lennon'
+Entry.objects.filter(headline__contains='Lennon').count()
+``count()`` performs a ``SELECT COUNT(*)`` behind the scenes, so you should
+always use ``count()`` rather than loading all of the record into Python
+objects and calling ``len()`` on the result.
+Depending on which database you're using (e.g. PostgreSQL vs. MySQL),
+``count()`` may return a long integer instead of a normal Python integer. This
+is an underlying implementation quirk that shouldn't pose any real-world
+problems.
+``in_bulk(id_list)``
+~~~~~~~~~~~~~~~~~~~~
+Takes a list of primary-key values and returns a dictionary mapping each
+primary-key value to an instance of the object with the given ID.
+Example::
+>>> Blog.objects.in_bulk([1])
+{1: Beatles Blog}
+>>> Blog.objects.in_bulk([1, 2])
+{1: Beatles Blog, 2: Cheddar Talk}
+>>> Blog.objects.in_bulk([])
+{}
+If you pass ``in_bulk()`` an empty list, you'll get an empty dictionary.
+``latest(field_name=None)``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Returns the latest object in the table, by date, using the ``field_name``
+provided as the date field.
+This example returns the latest ``Entry`` in the table, according to the
+``pub_date`` field::
+Entry.objects.latest('pub_date')
+If your model's ``Meta`` specifies ``get_latest_by``, you can leave off the
+``field_name`` argument to ``latest()``. Django will use the field specified in
+``get_latest_by`` by default.
+Like ``get()``, ``latest()`` raises ``DoesNotExist`` if an object doesn't
+exist with the given parameters.
+Note ``latest()`` exists purely for convenience and readability.
+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';
+.. admonition:: 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`_ in the official Python tutorial.
+.. _`Keyword Arguments`: http://docs.python.org/tut/node6.html#SECTION006720000000000000000
+If you pass an invalid keyword argument, a lookup function will raise
+``TypeError``.
+The database API supports the following lookup types:
+exact
+~~~~~
+Exact match. If the value provided for comparison is ``None``, it will
+be interpreted as an SQL ``NULL`` (See isnull_ for more details).
+Examples::
+Entry.objects.get(id__exact=14)
+Entry.objects.get(id__exact=None)
+SQL equivalents::
+SELECT ... WHERE id = 14;
+SELECT ... WHERE id = NULL;
+iexact
+~~~~~~
+Case-insensitive exact match.
+Example::
+Blog.objects.get(name__iexact='beatles blog')
+SQL equivalent::
+SELECT ... WHERE name ILIKE 'beatles blog';
+Note this will match ``'Beatles Blog'``, ``'beatles blog'``,
+``'BeAtLes BLoG'``, etc.
+contains
+~~~~~~~~
+Case-sensitive containment test.
+Example::
+Entry.objects.get(headline__contains='Lennon')
+SQL equivalent::
+SELECT ... WHERE headline LIKE '%Lennon%';
+Note this will match the headline ``'Today Lennon honored'`` but not
+``'today lennon honored'``.
+SQLite doesn't support case-sensitive ``LIKE`` statements; ``contains`` acts
+like ``icontains`` for SQLite.
+icontains
+~~~~~~~~~
+Case-insensitive containment test.
+Example::
+Entry.objects.get(headline__icontains='Lennon')
+SQL equivalent::
+SELECT ... WHERE headline ILIKE '%Lennon%';
+gt
+~~
+Greater than.
+Example::
+Entry.objects.filter(id__gt=4)
+SQL equivalent::
+SELECT ... WHERE id > 4;
+gte
+~~~
+Greater than or equal to.
+lt
+~~
+Less than.
+lte
+~~~
+Less than or equal to.
+in
+~~
+In a given list.
+Example::
+Entry.objects.filter(id__in=[1, 3, 4])
+SQL equivalent::
+SELECT ... WHERE id IN (1, 3, 4);
+startswith
+~~~~~~~~~~
+Case-sensitive starts-with.
+Example::
+Entry.objects.filter(headline__startswith='Will')
+SQL equivalent::
+SELECT ... WHERE headline LIKE 'Will%';
+SQLite doesn't support case-sensitive ``LIKE`` statements; ``startswith`` acts
+like ``istartswith`` for SQLite.
+istartswith
+~~~~~~~~~~~
+Case-insensitive starts-with.
+Example::
+Entry.objects.filter(headline__istartswith='will')
+SQL equivalent::
+SELECT ... WHERE headline ILIKE 'Will%';
+endswith
+~~~~~~~~
+Case-sensitive ends-with.
+Example::
+Entry.objects.filter(headline__endswith='cats')
+SQL equivalent::
+SELECT ... WHERE headline LIKE '%cats';
+SQLite doesn't support case-sensitive ``LIKE`` statements; ``endswith`` acts
+like ``iendswith`` for SQLite.
+iendswith
+~~~~~~~~~
+Case-insensitive ends-with.
+Example::
+Entry.objects.filter(headline__iendswith='will')
+SQL equivalent::
+SELECT ... WHERE headline ILIKE '%will'
+range
+~~~~~
+Range test (inclusive).
+Example::
+start_date = datetime.date(2005, 1, 1)
+end_date = datetime.date(2005, 3, 31)
+Entry.objects.filter(pub_date__range=(start_date, end_date))
+SQL equivalent::
+SELECT ... WHERE pub_date BETWEEN '2005-01-01' and '2005-03-31';
+You can use ``range`` anywhere you can use ``BETWEEN`` in SQL -- for dates,
+numbers and even characters.
+year
+~~~~
+For date/datetime fields, exact year match. Takes a four-digit year.
+Example::
+Entry.objects.filter(pub_date__year=2005)
+SQL equivalent::
+SELECT ... WHERE EXTRACT('year' FROM pub_date) = '2005';
+(The exact SQL syntax varies for each database engine.)
+month
+~~~~~
+For date/datetime fields, exact month match. Takes an integer 1 (January)
+through 12 (December).
+Example::
+Entry.objects.filter(pub_date__month=12)
+SQL equivalent::
+SELECT ... WHERE EXTRACT('month' FROM pub_date) = '12';
+(The exact SQL syntax varies for each database engine.)
+day
+~~~
+For date/datetime fields, exact day match.
+Example::
+Entry.objects.filter(pub_date__day=3)
+SQL equivalent::
+SELECT ... WHERE EXTRACT('day' FROM pub_date) = '3';
+(The exact SQL syntax varies for each database engine.)
+Note this will match any record with a pub_date on the third day of the month,
+such as January 3, July 3, etc.
+isnull
+~~~~~~
+Takes either ``True`` or ``False``, which correspond to SQL queries of
+``IS NULL`` and ``IS NOT NULL``, respectively.
+Example::
+Entry.objects.filter(pub_date__isnull=True)
+SQL equivalent::
+SELECT ... WHERE pub_date IS NULL;
+.. admonition:: ``__isnull=True`` vs ``__exact=None``
+There is an important difference between ``__isnull=True`` and
+``__exact=None``. ``__exact=None`` will *always* return an empty result
+set, because SQL requires that no value is equal to ``NULL``.
+``__isnull`` determines if the field is currently holding the value
+of ``NULL`` without performing a comparison.
+search
+~~~~~~
+A boolean full-text search, taking advantage of full-text indexing. This is
+like ``contains`` but is significantly faster due to full-text indexing.
+Note this is only available in MySQL and requires direct manipulation of the
+database to add the full-text index.
+Default lookups are exact
+-------------------------
+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.
+The pk lookup shortcut
+----------------------
+For convenience, Django provides a ``pk`` lookup type, 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
+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__exact='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')
+Escaping parenthesis 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. It's important
+to understand how it works, in order 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 = Poll.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.
+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
+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::
+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. You can also use parenthetical grouping.
+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 the `OR lookups examples page`_ for more examples.
+.. _OR lookups examples page: http://www.djangoproject.com/documentation/models/or_lookups/
+Related objects
+===============
+When you define a relationship in a model (i.e., a ``ForeignKey``,
+``OneToOneField``, or ``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_.
+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.
+.. _descriptors: http://users.rcn.com/python/download/Descriptor.htm
+One-to-many relationships
+-------------------------
+Forward
+~~~~~~~
+If a model has a ``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`` field has ``null=True`` set (i.e., it allows ``NULL``
+values), you can assign ``None`` to it. 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.
+``select_related()`` is documented in the "QuerySet methods that return new
+QuerySets" section above.
+Backward
+~~~~~~~~
+If a model has a ``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``
+parameter in the ``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()
+You cannot access a reverse ``ForeignKey`` ``Manager`` from the class; it must
+be accessed from an instance. Example::
+Blog.entry_set # Raises AttributeError: "Manager must be accessed via instance".
+In addition to the ``QuerySet`` methods defined in "Retrieving objects" above,
+the ``ForeignKey`` ``Manager`` has these additional methods:
+* ``add(obj1, obj2, ...)``: Adds the specified model objects to the related
+object set.
+Example::
+b = Blog.objects.get(id=1)
+e = Entry.objects.get(id=234)
+b.entry_set.add(e) # Associates Entry e with Blog b.
+* ``create(**kwargs)``: Creates a new object, saves it and puts it in the
+related object set. Returns the newly created object.
+Example::
+b = Blog.objects.get(id=1)
+e = b.entry_set.create(headline='Hello', body_text='Hi', pub_date=datetime.date(2005, 1, 1))
+# No need to call e.save() at this point -- it's already been saved.
+This is equivalent to (but much simpler than)::
+b = Blog.objects.get(id=1)
+e = Entry(blog=b, headline='Hello', body_text='Hi', pub_date=datetime.date(2005, 1, 1))
+e.save()
+Note that there's no need to specify the keyword argument of the model
+that defines the relationship. In the above example, we don't pass the
+parameter ``blog`` to ``create()``. Django figures out that the new
+``Entry`` object's ``blog`` field should be set to ``b``.
+* ``remove(obj1, obj2, ...)``: Removes the specified model objects from the
+related object set.
+Example::
+b = Blog.objects.get(id=1)
+e = Entry.objects.get(id=234)
+b.entry_set.remove(e) # Disassociates Entry e from Blog b.
+In order to prevent database inconsistency, this method only exists on
+``ForeignKey`` objects where ``null=True``. If the related field can't be
+set to ``None`` (``NULL``), then an object can't be removed from a
+relation without being added to another. In the above example, removing
+``e`` from ``b.entry_set()`` is equivalent to doing ``e.blog = None``,
+and because the ``blog`` ``ForeignKey`` doesn't have ``null=True``, this
+is invalid.
+* ``clear()``: Removes all objects from the related object set.
+Example::
+b = Blog.objects.get(id=1)
+b.entry_set.clear()
+Note this doesn't delete the related objects -- it just disassociates
+them.
+Just like ``remove()``, ``clear()`` is only available on ``ForeignKey``s
+where ``null=True``.
+To assign the members of a related set in one fell swoop, just assign to it
+from any iterable object. Example::
+b = Blog.objects.get(id=1)
+b.entry_set = [e1, e2]
+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. See Backward_
+above.
+The only difference is in the attribute naming: The model that defines the
+``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``, ``ManyToManyField`` can specify ``related_name``. In the
+above example, if the ``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
+------------------------
+The semantics of one-to-one relationships will be changing soon, so we don't
+recommend you use them.
+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 ``INSTALLED_APPS`` setting. The first time any model is
+loaded, Django iterates over every model in ``INSTALLED_APPS`` and creates the
+backward relationships in memory as needed. Essentially, one of the functions
+of ``INSTALLED_APPS`` is to tell Django the entire model domain.
+Queries over related objects
+----------------------------
+Queries involving related objects follow the same rules as queries involving
+normal value fields. When specifying the 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
+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()
+When Django deletes an object, 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()
+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()
+Extra instance methods
+======================
+In addition to ``save()``, ``delete()``, a model object might get any or all
+of the following methods:
+get_FOO_display()
+-----------------
+For every field that has ``choices`` set, the object will have a
+``get_FOO_display()`` method, where ``FOO`` is the name of the field. This
+method returns the "human-readable" value of the field. For example, in the
+following model::
+GENDER_CHOICES = (
+('M', 'Male'),
+('F', 'Female'),
+)
+class Person(models.Model):
+name = models.CharField(maxlength=20)
+gender = models.CharField(maxlength=1, choices=GENDER_CHOICES)
+...each ``Person`` instance will have a ``get_gender_display()`` method. Example::
+>>> p = Person(name='John', gender='M')
+>>> p.save()
+>>> p.gender
+'M'
+>>> p.get_gender_display()
+'Male'
+get_next_by_FOO(\**kwargs) and get_previous_by_FOO(\**kwargs)
+-------------------------------------------------------------
+For every ``DateField`` and ``DateTimeField`` that does not have ``null=True``,
+the object will have ``get_next_by_FOO()`` and ``get_previous_by_FOO()``
+methods, where ``FOO`` is the name of the field. This returns the next and
+previous object with respect to the date field, raising the appropriate
+``DoesNotExist`` exception when appropriate.
+Both methods accept optional keyword arguments, which should be in the format
+described in `Field lookups`_ above.
+Note that in the case of identical date values, these methods will use the ID
+as a fallback check. This guarantees that no records are skipped or duplicated.
+For a full example, see the `lookup API sample model`_.
+.. _lookup API sample model: http://www.djangoproject.com/documentation/models/lookup/
+get_FOO_filename()
+------------------
+For every ``FileField``, the object will have a ``get_FOO_filename()`` method,
+where ``FOO`` is the name of the field. This returns the full filesystem path
+to the file, according to your ``MEDIA_ROOT`` setting.
+Note that ``ImageField`` is technically a subclass of ``FileField``, so every
+model with an ``ImageField`` will also get this method.
+get_FOO_url()
+-------------
+For every ``FileField``, the object will have a ``get_FOO_url()`` method,
+where ``FOO`` is the name of the field. This returns the full URL to the file,
+according to your ``MEDIA_URL`` setting. If the value is blank, this method
+returns an empty string.
+get_FOO_size()
+--------------
+For every ``FileField``, the object will have a ``get_FOO_size()`` method,
+where ``FOO`` is the name of the field. This returns the size of the file, in
+bytes. (Behind the scenes, it uses ``os.path.getsize``.)
+save_FOO_file(filename, raw_contents)
+-------------------------------------
+For every ``FileField``, the object will have a ``save_FOO_file()`` method,
+where ``FOO`` is the name of the field. This saves the given file to the
+filesystem, using the given filename. If a file with the given filename already
+exists, Django adds an underscore to the end of the filename (but before the
+extension) until the filename is available.
+get_FOO_height() and get_FOO_width()
+------------------------------------
+For every ``ImageField``, the object will have ``get_FOO_height()`` and
+``get_FOO_width()`` methods, where ``FOO`` is the name of the field. This
+returns the height (or width) of the image, as an integer, in pixels.
+Shortcuts
+=========
+As you develop views, you will discover a number of common idioms in the
+way you use the database API. Django encodes some of these idioms as
+shortcuts that can be used to simplify the process of writing views.
+get_object_or_404()
+-------------------
+One common idiom to use ``get()`` and raise ``Http404`` if the
+object doesn't exist. This idiom is captured by ``get_object_or_404()``.
+This function takes a Django model as its first argument and an
+arbitrary number of keyword arguments, which it passes to the manager's
+``get()`` function. It raises ``Http404`` if the object doesn't
+exist. For example::
+# Get the Entry with a primary key of 3
+e = get_object_or_404(Entry, pk=3)
+When you provide a model to this shortcut function, the default manager
+is used to execute the underlying ``get()`` query. If you don't want to
+use the default manager, or you want to search a list of related objects,
+you can provide ``get_object_or_404()`` with a manager object, instead.
+For example::
+# Get the author of blog instance `e` with a name of 'Fred'
+a = get_object_or_404(e.authors, name='Fred')
+# Use a custom manager 'recent_entries' in the search for an
+# entry with a primary key of 3
+e = get_object_or_404(Entry.recent_entries, pk=3)
+get_list_or_404()
+-----------------
+``get_list_or_404`` behaves the same was as ``get_object_or_404()``
+-- except the it uses using ``filter()`` instead of ``get()``. It raises
+``Http404`` if the list is empty.
+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 into raw-SQL statement
+mode.
+The preferred way to do this is by giving your model custom methods or custom
+manager methods that execute queries. Although there's nothing in Django that
+*requires* database queries to live in the model layer, this approach keeps all
+your data-access logic in one place, which is smart from an code-organization
+standpoint. For instructions, see `Executing custom SQL`_.
+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.
+.. _Executing custom SQL: ../model_api/#executing-custom-sql