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documented RelatedManager calling save() behind the scenes, added section on direct replacement of related object set

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Matthew Rich 2013-09-06 11:37:08 -05:00
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docs/ref/models/relations.txt
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@ -36,89 +36,117 @@ Related objects reference
In this example, the methods below will be available both on In this example, the methods below will be available both on
``topping.pizza_set`` and on ``pizza.toppings``. ``topping.pizza_set`` and on ``pizza.toppings``.
These related managers have some extra methods: .. _related-manager-methods:
.. method:: add(obj1, [obj2, ...]) Related Manager Methods
-----------------------
Adds the specified model objects to the related object set. .. method:: add(obj1, [obj2, ...])
Example:: Adds the specified model objects to the related object set.
>>> b = Blog.objects.get(id=1) Example::
>>> e = Entry.objects.get(id=234)
>>> b.entry_set.add(e) # Associates Entry e with Blog b.
In the example above, ``e.save()`` is called to perform the update. >>> b = Blog.objects.get(id=1)
Using ``add()`` with a many-to-many relationship, however, will not >>> e = Entry.objects.get(id=234)
call any ``save()`` methods, but rather create the relationships >>> b.entry_set.add(e) # Associates Entry e with Blog b.
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) 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.
Creates a new object, saves it and puts it in the related object set. .. method:: create(**kwargs)
Returns the newly created object::
>>> b = Blog.objects.get(id=1) Creates a new object, saves it and puts it in the related object set.
>>> e = b.entry_set.create( Returns the newly created object::
... 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. >>> b = Blog.objects.get(id=1)
>>> e = b.entry_set.create(
... headline='Hello',
... body_text='Hi',
... pub_date=datetime.date(2005, 1, 1)
... )
This is equivalent to (but much simpler than):: # No need to call e.save() at this point -- it's already been saved.
>>> b = Blog.objects.get(id=1) This is equivalent to (but much simpler than)::
>>> 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 >>> b = Blog.objects.get(id=1)
that defines the relationship. In the above example, we don't pass the >>> e = Entry(
parameter ``blog`` to ``create()``. Django figures out that the new ... blog=b,
``Entry`` object's ``blog`` field should be set to ``b``. ... headline='Hello',
... body_text='Hi',
... pub_date=datetime.date(2005, 1, 1)
... )
>>> e.save(force_insert=True)
.. method:: remove(obj1, [obj2, ...]) 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``.
Removes the specified model objects from the related object set:: .. method:: remove(obj1, [obj2, ...])
>>> b = Blog.objects.get(id=1) Removes the specified model objects from the related object set::
>>> 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 >>> b = Blog.objects.get(id=1)
to perform the update. Using ``remove()`` with a many-to-many >>> e = Entry.objects.get(id=234)
relationship, however, will delete the relationships using >>> b.entry_set.remove(e) # Disassociates Entry e from Blog b.
: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 Similar to :meth:`add()`, ``e.save()`` is called in the example above
exists if ``null=True``. If the related field can't be set to ``None`` to perform the update. Using ``remove()`` with a many-to-many
(``NULL``), then an object can't be removed from a relation without relationship, however, will delete the relationships using
being added to another. In the above example, removing ``e`` from :meth:`QuerySet.delete()<django.db.models.query.QuerySet.delete>` which
``b.entry_set()`` is equivalent to doing ``e.blog = None``, and because means no model ``save()`` methods are called; listen to the
the ``blog`` :class:`~django.db.models.ForeignKey` doesn't have :data:`~django.db.models.signals.m2m_changed` signal if you wish to
``null=True``, this is invalid. execute custom code when a relationship is deleted.
.. method:: clear() 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.
Removes all objects from the related object set:: .. method:: clear()
>>> b = Blog.objects.get(id=1) Removes all objects from the related object set::
>>> b.entry_set.clear()
Note this doesn't delete the related objects -- it just disassociates >>> b = Blog.objects.get(id=1)
them. >>> b.entry_set.clear()
Just like ``remove()``, ``clear()`` is only available on Note this doesn't delete the related objects -- it just disassociates
:class:`~django.db.models.ForeignKey`\s where ``null=True``. them.
Just like ``remove()``, ``clear()`` is only available on
:class:`~django.db.models.ForeignKey`\s where ``null=True``.
.. 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.
.. _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.