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105 lines
3.7 KiB
Text
105 lines
3.7 KiB
Text
=========================
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Related objects reference
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=========================
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.. currentmodule:: django.db.models.fields.related
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.. class:: RelatedManager
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A "related manager" is a manager used in a one-to-many or many-to-many
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related context. This happens in two cases:
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* The "other side" of a :class:`~django.db.models.ForeignKey` relation.
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That is::
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class Reporter(models.Model):
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...
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class Article(models.Model):
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reporter = models.ForeignKey(Reporter)
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In the above example, the methods below will be available on
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the manager ``reporter.article_set``.
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* Both sides of a :class:`~django.db.models.ManyToManyField` relation::
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class Topping(models.Model):
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...
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class Pizza(models.Model):
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toppings = models.ManyToManyField(Topping)
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In this example, the methods below will be available both on
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``topping.pizza_set`` and on ``pizza.toppings``.
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These related managers have some extra methods:
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.. method:: add(obj1, [obj2, ...])
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Adds the specified model objects to the related object set.
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Example::
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>>> b = Blog.objects.get(id=1)
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>>> e = Entry.objects.get(id=234)
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>>> b.entry_set.add(e) # Associates Entry e with Blog b.
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.. method:: create(**kwargs)
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Creates a new object, saves it and puts it in the related object set.
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Returns the newly created object::
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>>> b = Blog.objects.get(id=1)
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>>> e = b.entry_set.create(
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... headline='Hello',
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... body_text='Hi',
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... pub_date=datetime.date(2005, 1, 1)
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... )
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# No need to call e.save() at this point -- it's already been saved.
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This is equivalent to (but much simpler than)::
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>>> b = Blog.objects.get(id=1)
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>>> e = Entry(
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... blog=b,
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... headline='Hello',
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... body_text='Hi',
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... pub_date=datetime.date(2005, 1, 1)
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... )
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>>> e.save(force_insert=True)
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Note that there's no need to specify the keyword argument of the model
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that defines the relationship. In the above example, we don't pass the
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parameter ``blog`` to ``create()``. Django figures out that the new
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``Entry`` object's ``blog`` field should be set to ``b``.
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.. method:: remove(obj1, [obj2, ...])
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Removes the specified model objects from the related object set::
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>>> b = Blog.objects.get(id=1)
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>>> e = Entry.objects.get(id=234)
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>>> b.entry_set.remove(e) # Disassociates Entry e from Blog b.
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In order to prevent database inconsistency, this method only exists on
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:class:`~django.db.models.ForeignKey` objects where ``null=True``. If
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the related field can't be set to ``None`` (``NULL``), then an object
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can't be removed from a relation without being added to another. In the
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above example, removing ``e`` from ``b.entry_set()`` is equivalent to
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doing ``e.blog = None``, and because the ``blog``
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:class:`~django.db.models.ForeignKey` doesn't have ``null=True``, this
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is invalid.
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.. method:: clear()
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Removes all objects from the related object set::
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>>> b = Blog.objects.get(id=1)
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>>> b.entry_set.clear()
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Note this doesn't delete the related objects -- it just disassociates
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them.
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Just like ``remove()``, ``clear()`` is only available on
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:class:`~django.db.models.ForeignKey`\s where ``null=True``.
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