Fixed #21430 -- Added a RuntimeWarning when unpickling Models and QuerySets from a different Django version.

Thanks FunkyBob for the suggestion, prasoon2211 for the initial patch,
and akaariai, loic, and charettes for helping in shaping the patch.

docs/ref/models/instances.txt

@@ -410,6 +410,28 @@ For more details, including how to delete objects in bulk, see
 If you want customized deletion behavior, you can override the ``delete()``
 method. See :ref:`overriding-model-methods` for more details.
 
+Pickling objects
+================
+
+When you :mod:`pickle` a model, its current state is pickled. When you unpickle
+it, it'll contain the model instance at the moment it was pickled, rather than
+the data that's currently in the database.
+
+.. admonition:: You can't share pickles between versions
+
+    Pickles of models are only valid for the version of Django that
+    was used to generate them. If you generate a pickle using Django
+    version N, there is no guarantee that pickle will be readable with
+    Django version N+1. Pickles should not be used as part of a long-term
+    archival strategy.
+
+    .. versionadded:: 1.8
+
+    Since pickle compatibility errors can be difficult to diagnose, such as
+    silently corrupted objects, a ``RuntimeWarning`` is raised when you try to
+    unpickle a model in a Django version that is different than the one in
+    which it was pickled.
+
 .. _model-instance-methods:
 
 Other model instance methods

docs/ref/models/querysets.txt

@@ -116,6 +116,13 @@ described here.
     Django version N+1. Pickles should not be used as part of a long-term
     archival strategy.
 
+    .. versionadded:: 1.8
+
+    Since pickle compatibility errors can be difficult to diagnose, such as
+    silently corrupted objects, a ``RuntimeWarning`` is raised when you try to
+    unpickle a queryset in a Django version that is different than the one in
+    which it was pickled.
+
 .. _queryset-api:
 
 QuerySet API

docs/releases/1.8.txt

@@ -176,6 +176,13 @@ Models
 * Django now logs at most 9000 queries in ``connections.queries``, in order
   to prevent excessive memory usage in long-running processes in debug mode.
 
+* Pickling models and querysets across different versions of Django isn't
+  officially supported (it may work, but there's no guarantee). An extra
+  variable that specifies the current Django version is now added to the
+  pickled state of models and querysets, and Django raises a ``RuntimeWarning``
+  when these objects are unpickled in a different version than the one in
+  which they were pickled.
+
 Signals
 ^^^^^^^