Fixed model.__eq__ and __hash__ for no pk value cases

The __eq__ method now considers two instances without primary key value
equal only when they have same id(). The __hash__ method raises
TypeError for no primary key case.

Fixed #18864, fixed #18250

Thanks to Tim Graham for docs review.

2 files changed, 27 insertions, 0 deletions

diff --git a/docs/ref/models/instances.txt b/docs/ref/models/instances.txt
index 6295eb0407..da657a9a01 100644
--- a/docs/ref/models/instances.txt
+++ b/docs/ref/models/instances.txt
@@ -521,6 +521,25 @@ For example::
   In previous versions only instances of the exact same class and same
   primary key value were considered equal.
 
+``__hash__``
+------------
+
+.. method:: Model.__hash__()
+
+The ``__hash__`` method is based on the instance's primary key value. It
+is effectively hash(obj.pk). If the instance doesn't have a primary key
+value then a ``TypeError`` will be raised (otherwise the ``__hash__``
+method would return different values before and after the instance is
+saved, but changing the ``__hash__`` value of an instance `is forbidden
+in Python`_).
+
+.. versionchanged:: 1.7
+
+  In previous versions instance's without primary key value were
+  hashable.
+
+.. _is forbidden in Python: http://docs.python.org/reference/datamodel.html#object.__hash__
+
 ``get_absolute_url``
 --------------------
 
diff --git a/docs/releases/1.7.txt b/docs/releases/1.7.txt
index 774d9d3161..6480a2505f 100644
--- a/docs/releases/1.7.txt
+++ b/docs/releases/1.7.txt
@@ -199,6 +199,14 @@ Miscellaneous
   equal when primary keys match. Previously only instances of exact same
   class were considered equal on primary key match.
 
+* The :meth:`django.db.models.Model.__eq__` method has changed such that
+  two ``Model`` instances without primary key values won't be considered
+  equal (unless they are the same instance).
+  
+* The :meth:`django.db.models.Model.__hash__` will now raise ``TypeError``
+  when called on an instance without a primary key value. This is done to
+  avoid mutable ``__hash__`` values in containers.
+
 Features deprecated in 1.7
 ==========================