Fixed #19384 -- Documented the behavior of custom managers on abstract models.

This documents the behavior introduced by cc337a74, which is BACKWARDS
INCOMPATIBLE for any attempt to invoke a method on a manager using the
abstract class as the calling class (e.g., AbstractBase.objects.do_something())

Thanks to mhsparks for the report.

docs/releases/1.5.txt

@@ -341,6 +341,21 @@ Backwards incompatible changes in 1.5
     deprecation timeline for a given feature, its removal may appear as a
     backwards incompatible change.
 
+Managers on abstract models
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Abstract models are able to define a custom manager, and that manager
+:ref:`will be inherited by any concrete models extending the abstract model
+<custom-managers-and-inheritance>`. However, if you try to use the abstract
+model to call a method on the manager, an exception will now be raised.
+Previously, the call would have been permitted, but would have failed as soon
+as any database operation was attempted (usually with a "table does not exist"
+error from the database).
+
+If you have functionality on a manager that you have been invoking using
+the abstract class, you should migrate that logic to a Python
+``staticmethod`` or ``classmethod`` on the abstract class.
+
 Context in year archive class-based views
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 

docs/topics/db/managers.txt

@@ -274,6 +274,21 @@ it into the inheritance hierarchy *after* the defaults::
         # Default manager is CustomManager, but OtherManager is
         # also available via the "extra_manager" attribute.
 
+Note that while you can *define* a custom manager on the abstract model, you
+can't *invoke* any methods using the abstract model. That is::
+
+    ClassA.objects.do_something()
+
+is legal, but::
+
+    AbstractBase.objects.do_something()
+
+will raise an exception. This is because managers are intended to encapsulate
+logic for managing collections of objects. Since you can't have a collection of
+abstract objects, it doesn't make sense to be managing them. If you have
+functionality that applies to the abstract model, you should put that functionality
+in a ``staticmethod`` or ``classmethod`` on the abstract model.
+
 Implementation concerns
 -----------------------