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.. _ref-models-relations:
=========================
Related objects reference
=========================
.. currentmodule:: django.db.models
This document describes extra methods available on managers when 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``.
.. method:: QuerySet.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:: QuerySet.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:: QuerySet.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:: QuerySet.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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