django/docs/ref/models/relations.txt

=========================
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, on_delete=models.CASCADE)

      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(*objs, bulk=True)

        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,
        :meth:`QuerySet.update() <django.db.models.query.QuerySet.update>`
        is used to perform the update. This requires the objects to already be
        saved.

        You can use the ``bulk=False`` argument to instead have the related
        manager perform the update by calling ``e.save()``.

        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(*objs)

        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.

        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.

    .. method:: set(objs, bulk=True, clear=False)

        Replace the set of related objects::

            >>> new_list = [obj1, obj2, obj3]
            >>> e.related_set.set(new_list)

        This method accepts a ``clear`` argument to control how to perform the
        operation. If ``False`` (the default), the elements missing from the
        new set are removed using ``remove()`` and only the new ones are added.
        If ``clear=True``, the ``clear()`` method is called instead and the
        whole set is added at once.

        The ``bulk`` argument is passed on to :meth:`add`.

        Note that since ``set()`` is a compound operation, it is subject to
        race conditions. For instance, new objects may be added to the database
        in between the call to ``clear()`` and the call to ``add()``.

    .. note::

       Note that ``add()``, ``create()``, ``remove()``, ``clear()``, and
       ``set()`` 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, then the
       ``add()``, ``create()``, ``remove()``, and ``set()`` methods are
       disabled.

       If you use :meth:`~django.db.models.query.QuerySet.prefetch_related`,
       the ``add()``, ``remove()``, ``clear()``, and ``set()`` methods clear
       the prefetched cache.