django/docs/ref/models/relations.txt

.. _ref-models-relations:

=========================
Related objects reference
=========================

.. currentmodule:: django.db.models

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:

    * The "other side" of a ``ForeignKey`` relation. That is::
    
            class Reporter(models.Model):
                ...
                
            class Article(models.Model):
                reporter = models.ForeignKey(Reporter)
            
      In the above example, the methods below will be available on
      the manager ``reporter.article_set``.
      
    * Both sides of a ``ManyToManyField`` relation::
    
            class Topping(models.Model):
                ...
                
            class Pizza(models.Model):
                toppings = models.ManyToManyField(Topping)
                
      In this example, the methods below will be available both on
      ``topping.pizza_set`` and on ``pizza.toppings``.

.. method:: QuerySet.add(obj1, [obj2, ...])

    Adds the specified model objects to the related object set.

    Example::

        >>> b = Blog.objects.get(id=1)
        >>> e = Entry.objects.get(id=234)
        >>> b.entry_set.add(e) # Associates Entry e with Blog b.

.. method:: QuerySet.create(**kwargs)

    Creates a new object, saves it and puts it in the related object set.
    Returns the newly created object::

        >>> b = Blog.objects.get(id=1)
        >>> e = b.entry_set.create(
        ...     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.

    This is equivalent to (but much simpler than)::

        >>> b = Blog.objects.get(id=1)
        >>> 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 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``.

.. method:: QuerySet.remove(obj1, [obj2, ...])

    Removes the specified model objects from the related object set::
    
        >>> b = Blog.objects.get(id=1)
        >>> e = Entry.objects.get(id=234)
        >>> b.entry_set.remove(e) # Disassociates Entry e from Blog b.

    In order to prevent database inconsistency, this method only exists on
    ``ForeignKey`` objects where ``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`` ``ForeignKey`` doesn't have ``null=True``, this is invalid.

.. method:: QuerySet.clear()

    Removes all objects from the related object set::

        >>> b = Blog.objects.get(id=1)
        >>> b.entry_set.clear()

    Note this doesn't delete the related objects -- it just disassociates them.

    Just like ``remove()``, ``clear()`` is only available on ``ForeignKey``\s
    where ``null=True``.