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/docs/ref/models/relations.txt

https://code.google.com/p/mango-py/
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  1=========================
  2Related objects reference
  3=========================
  4
  5.. currentmodule:: django.db.models.fields.related
  6
  7.. class:: RelatedManager
  8
  9    A "related manager" is a manager used in a one-to-many or many-to-many
 10    related context. This happens in two cases:
 11
 12        * The "other side" of a :class:`~django.db.models.ForeignKey` relation.
 13          That is::
 14
 15                class Reporter(models.Model):
 16                    ...
 17
 18                class Article(models.Model):
 19                    reporter = models.ForeignKey(Reporter)
 20
 21          In the above example, the methods below will be available on
 22          the manager ``reporter.article_set``.
 23
 24        * Both sides of a :class:`~django.db.models.ManyToManyField` relation::
 25
 26                class Topping(models.Model):
 27                    ...
 28
 29                class Pizza(models.Model):
 30                    toppings = models.ManyToManyField(Topping)
 31
 32          In this example, the methods below will be available both on
 33          ``topping.pizza_set`` and on ``pizza.toppings``.
 34
 35    These related managers have some extra methods:
 36
 37    .. method:: add(obj1, [obj2, ...])
 38
 39        Adds the specified model objects to the related object set.
 40
 41        Example::
 42
 43            >>> b = Blog.objects.get(id=1)
 44            >>> e = Entry.objects.get(id=234)
 45            >>> b.entry_set.add(e) # Associates Entry e with Blog b.
 46
 47    .. method:: create(**kwargs)
 48
 49        Creates a new object, saves it and puts it in the related object set.
 50        Returns the newly created object::
 51
 52            >>> b = Blog.objects.get(id=1)
 53            >>> e = b.entry_set.create(
 54            ...     headline='Hello',
 55            ...     body_text='Hi',
 56            ...     pub_date=datetime.date(2005, 1, 1)
 57            ... )
 58
 59            # No need to call e.save() at this point -- it's already been saved.
 60
 61        This is equivalent to (but much simpler than)::
 62
 63            >>> b = Blog.objects.get(id=1)
 64            >>> e = Entry(
 65            ...     blog=b,
 66            ...     headline='Hello',
 67            ...     body_text='Hi',
 68            ...     pub_date=datetime.date(2005, 1, 1)
 69            ... )
 70            >>> e.save(force_insert=True)
 71
 72        Note that there's no need to specify the keyword argument of the model
 73        that defines the relationship. In the above example, we don't pass the
 74        parameter ``blog`` to ``create()``. Django figures out that the new
 75        ``Entry`` object's ``blog`` field should be set to ``b``.
 76
 77    .. method:: remove(obj1, [obj2, ...])
 78
 79        Removes the specified model objects from the related object set::
 80
 81            >>> b = Blog.objects.get(id=1)
 82            >>> e = Entry.objects.get(id=234)
 83            >>> b.entry_set.remove(e) # Disassociates Entry e from Blog b.
 84
 85        In order to prevent database inconsistency, this method only exists on
 86        :class:`~django.db.models.ForeignKey` objects where ``null=True``. If
 87        the related field can't be set to ``None`` (``NULL``), then an object
 88        can't be removed from a relation without being added to another. In the
 89        above example, removing ``e`` from ``b.entry_set()`` is equivalent to
 90        doing ``e.blog = None``, and because the ``blog``
 91        :class:`~django.db.models.ForeignKey` doesn't have ``null=True``, this
 92        is invalid.
 93
 94    .. method:: clear()
 95
 96        Removes all objects from the related object set::
 97
 98            >>> b = Blog.objects.get(id=1)
 99            >>> b.entry_set.clear()
100
101        Note this doesn't delete the related objects -- it just disassociates
102        them.
103
104        Just like ``remove()``, ``clear()`` is only available on
105        :class:`~django.db.models.ForeignKey`\s where ``null=True``.