99 lines
3.3 KiB
Plaintext
99 lines
3.3 KiB
Plaintext
.. _ref-models-relations:
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=========================
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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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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:
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* The "other side" of a ``ForeignKey`` relation. 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 ``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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.. 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 that
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defines the relationship. In the above example, we don't pass the parameter
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``blog`` to ``create()``. Django figures out that the new ``Entry`` object's
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``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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``ForeignKey`` objects where ``null=True``. If the related field can't be
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set to ``None`` (``NULL``), then an object can't be removed from a relation
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without being added to another. In the above example, removing ``e`` from
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``b.entry_set()`` is equivalent to doing ``e.blog = None``, and because the
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``blog`` ``ForeignKey`` doesn't have ``null=True``, this 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 them.
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Just like ``remove()``, ``clear()`` is only available on ``ForeignKey``\s
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where ``null=True``.
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