django/docs/ref/models/relations.txt

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Related objects reference
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.. currentmodule:: django.db.models.fields.related

.. class:: RelatedManager

    A "related manager" is a on managers used in a one-to-many or many-to-many
    related context. This happens in two cases:

        * The "other side" of a ``ForeignKey`` relation. That is::

                class Reporter(models.Model):
                    ...

                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 ``ManyToManyField`` relation::

                class Topping(models.Model):
                    ...

                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``.

    These related managers have some extra methods:

    .. 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.

    .. 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.

        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.

    .. 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
        ``ForeignKey``\s where ``null=True``.