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=========================
Related objects reference
=========================
.. currentmodule:: django.db.models.fields.related
.. class:: RelatedManager
A "related manager" is a manager used in a one-to-many or many-to-many
related context. This happens in two cases:
* The "other side" of a :class:`~django.db.models.ForeignKey` relation.
That is::
from django.db import models
class Reporter(models.Model):
# ...
pass
class Article(models.Model):
reporter = models.ForeignKey(Reporter)
In the above example, the methods below will be available on
the manager ``reporter.article_set``.
* Both sides of a :class:`~django.db.models.ManyToManyField` relation::
class Topping(models.Model):
# ...
pass
class Pizza(models.Model):
toppings = models.ManyToManyField(Topping)
In this example, the methods below will be available both on
``topping.pizza_set`` and on ``pizza.toppings``.
.. method:: 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.
In the example above, in the case of a
:class:`~django.db.models.ForeignKey` relationship,
``e.save()`` is called by the related manager to perform the update.
Using ``add()`` with a many-to-many relationship, however, will not
call any ``save()`` methods, but rather create the relationships
using :meth:`QuerySet.bulk_create()
<django.db.models.query.QuerySet.bulk_create>`. If you need to execute
some custom logic when a relationship is created, listen to the
:data:`~django.db.models.signals.m2m_changed` signal.
.. method:: create(**kwargs)
Creates a new object, saves it and puts it in the related object set.
Returns the newly created object::
>>> 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(force_insert=True)
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``.
.. method:: remove(obj1, [obj2, ...])
Removes the specified model objects from the related object set::
>>> b = Blog.objects.get(id=1)
>>> e = Entry.objects.get(id=234)
>>> b.entry_set.remove(e) # Disassociates Entry e from Blog b.
Similar to :meth:`add()`, ``e.save()`` is called in the example above
to perform the update. Using ``remove()`` with a many-to-many
relationship, however, will delete the relationships using
:meth:`QuerySet.delete()<django.db.models.query.QuerySet.delete>` which
means no model ``save()`` methods are called; listen to the
:data:`~django.db.models.signals.m2m_changed` signal if you wish to
execute custom code when a relationship is deleted.
For :class:`~django.db.models.ForeignKey` objects, this method only
exists if ``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`` :class:`~django.db.models.ForeignKey` doesn't have
``null=True``, this is invalid.
.. versionchanged 1.7::
For :class:`~django.db.models.ForeignKey` objects, this method accepts
a ``bulk`` argument to control how to perform the operation.
If ``True`` (the default), ``QuerySet.update()`` is used.
If ``bulk=False``, the ``save()`` method of each individual model
instance is called instead. This triggers the
:data:`~django.db.models.signals.pre_save` and
:data:`~django.db.models.signals.post_save` signals and comes at the
expense of performance.
.. method:: clear()
Removes all objects from the related object set::
>>> 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
:class:`~django.db.models.ForeignKey`\s where ``null=True`` and it also
accepts the ``bulk`` keyword argument.
.. note::
Note that ``add()``, ``create()``, ``remove()``, and ``clear()`` all
apply database changes immediately for all types of related fields. In
other words, there is no need to call ``save()`` on either end of the
relationship.
Also, if you are using :ref:`an intermediate model
<intermediary-manytomany>` for a many-to-many relationship, some of the
related manager's methods are disabled.
.. _direct-assignment:
Direct Assignment
-----------------
A related object set can be replaced in bulk with one operation by assigning a
new iterable of objects to it::
>>> new_list = [obj1, obj2, obj3]
>>> e.related_set = new_list
If the foreign key relationship has ``null=True``, then the related manager
will first call ``clear()`` to disassociate any existing objects in the related
set before adding the contents of ``new_list``. Otherwise the objects in
``new_list`` will be added to the existing related object set.
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