Fixed #18556 -- Allowed RelatedManager.add() to execute 1 query where possible.

Thanks Loic Bistuer for review.

docs/ref/models/relations.txt

@@ -36,7 +36,7 @@ Related objects reference
       In this example, the methods below will be available both on
       ``topping.pizza_set`` and on ``pizza.toppings``.
 
-    .. method:: add(*objs)
+    .. method:: add(*objs, bulk=True)
 
         Adds the specified model objects to the related object set.
 
@@ -48,7 +48,13 @@ Related objects reference
 
         In the example above, in the case of a
         :class:`~django.db.models.ForeignKey` relationship,
-        ``e.save()`` is called by the related manager to perform the update.
+        :meth:`QuerySet.update() <django.db.models.query.QuerySet.update>`
+        is used to perform the update. This requires the objects to already be
+        saved.
+
+        You can use the ``bulk=False`` argument to instead have the related
+        manager perform the update by calling ``e.save()``.
+
         Using ``add()`` with a many-to-many relationship, however, will not
         call any ``save()`` methods, but rather create the relationships
         using :meth:`QuerySet.bulk_create()
@@ -56,6 +62,12 @@ Related objects reference
         some custom logic when a relationship is created, listen to the
         :data:`~django.db.models.signals.m2m_changed` signal.
 
+        .. versionchanged:: 1.9
+
+            The ``bulk`` parameter was added. In order versions, foreign key
+            updates were always done using ``save()``. Use ``bulk=False`` if
+            you require the old behavior.
+
     .. method:: create(**kwargs)
 
         Creates a new object, saves it and puts it in the related object set.
@@ -135,7 +147,7 @@ Related objects reference
         :class:`~django.db.models.ForeignKey`\s where ``null=True`` and it also
         accepts the ``bulk`` keyword argument.
 
-    .. method:: set(objs, clear=False)
+    .. method:: set(objs, bulk=True, clear=False)
 
         .. versionadded:: 1.9
 
@@ -150,6 +162,8 @@ Related objects reference
         If ``clear=True``, the ``clear()`` method is called instead and the
         whole set is added at once.
 
+        The ``bulk`` argument is passed on to :meth:`add`.
+
         Note that since ``set()`` is a compound operation, it is subject to
         race conditions. For instance, new objects may be added to the database
         in between the call to ``clear()`` and the call to ``add()``.