django之关联field 描述子

"""
Accessors for related objects.

When a field defines a relation between two models, each model class provides
an attribute to access related instances of the other model class (unless the
reverse accessor has been disabled with related_name='+').

Accessors are implemented as descriptors in order to customize access and
assignment. This module defines the descriptor classes.

Forward accessors follow foreign keys. Reverse accessors trace them back. For
example, with the following models::

class Parent(Model):
        pass

class Child(Model):
        parent = ForeignKey(Parent, related_name='children')

``child.parent`` is a forward many-to-one relation. ``parent.children`` is a
reverse many-to-one relation.

There are three types of relations (many-to-one, one-to-one, and many-to-many)
and two directions (forward and reverse) for a total of six combinations.

1. Related instance on the forward side of a many-to-one or one-to-one
   relation: ``ForwardManyToOneDescriptor``.

Uniqueness of foreign key values is irrelevant to accessing the related
   instance, making the many-to-one and one-to-one cases identical as far as
   the descriptor is concerned. The constraint is checked upstream (unicity
   validation in forms) or downstream (unique indexes in the database).

If you're looking for ``ForwardOneToOneDescriptor``, use
   ``ForwardManyToOneDescriptor`` instead.

2. Related instance on the reverse side of a one-to-one relation:
   ``ReverseOneToOneDescriptor``.

One-to-one relations are asymmetrical, despite the apparent symmetry of the
   name, because they're implemented in the database with a foreign key from
   one table to another. As a consequence ``ReverseOneToOneDescriptor`` is
   slightly different from ``ForwardManyToOneDescriptor``.

3. Related objects manager for related instances on the reverse side of a
   many-to-one relation: ``ReverseManyToOneDescriptor``.

Unlike the previous two classes, this one provides access to a collection
   of objects. It returns a manager rather than an instance.

4. Related objects manager for related instances on the forward or reverse
   sides of a many-to-many relation: ``ManyToManyDescriptor``.

Many-to-many relations are symmetrical. The syntax of Django models
   requires declaring them on one side but that's an implementation detail.
   They could be declared on the other side without any change in behavior.
   Therefore the forward and reverse descriptors can be the same.

If you're looking for ``ForwardManyToManyDescriptor`` or
   ``ReverseManyToManyDescriptor``, use ``ManyToManyDescriptor`` instead.
"""