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=========================
Related objects reference
=========================
.. 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``.
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