[1.5.x] Fixed #19634 -- Added proper __hash__ methods.

Classes overriding __eq__ need a __hash__ such that equal objects have
the same hash.

Thanks akaariai for the report and regebro for the patch.

Backport of e76147a from master.

django/db/backends/__init__.py

@@ -47,7 +47,8 @@ class BaseDatabaseWrapper(object):
     def __ne__(self, other):
         return not self == other
 
-    __hash__ = object.__hash__
+    def __hash__(self):
+        return hash(self.alias)
 
     def _commit(self):
         if self.connection is not None:

django/db/models/fields/__init__.py

@@ -135,7 +135,8 @@ class Field(object):
             return self.creation_counter < other.creation_counter
         return NotImplemented
 
-    __hash__ = object.__hash__
+    def __hash__(self):
+        return hash(self.creation_counter)
 
     def __deepcopy__(self, memodict):
         # We don't have to deepcopy very much here, since most things are not

django/db/models/fields/files.py

@@ -30,7 +30,6 @@ class FieldFile(File):
         return not self.__eq__(other)
 
     def __hash__(self):
-        # Required because we defined a custom __eq__.
         return hash(self.name)
 
     # The standard File contains most of the necessary properties, but

django/dispatch/saferef.py

@@ -152,7 +152,8 @@ class BoundMethodWeakref(object):
 
     __repr__ = __str__
 
-    __hash__ = object.__hash__
+    def __hash__(self):
+        return hash(self.key)
 
     def __bool__( self ):
         """Whether we are still a valid reference"""

django/test/html.py

@@ -85,7 +85,8 @@ class Element(object):
             return False
         return True
 
-    __hash__ = object.__hash__
+    def __hash__(self):
+        return hash((self.name,) + tuple(a for a in self.attributes))
 
     def __ne__(self, element):
         return not self.__eq__(element)

django/utils/functional.py

@@ -150,7 +150,8 @@ def lazy(func, *resultclasses):
                 other = other.__cast()
             return self.__cast() < other
 
-        __hash__ = object.__hash__
+        def __hash__(self):
+            return hash(self.__cast())
 
         def __mod__(self, rhs):
             if self._delegate_bytes and not six.PY3:

tests/regressiontests/backends/tests.py

@@ -581,25 +581,27 @@ class ThreadTests(TestCase):
         Ensure that the connections are different for each thread.
         Refs #17258.
         """
-        connections_set = set()
+        # Map connections by id because connections with identical aliases
+        # have the same hash.
+        connections_dict = {}
         for conn in connections.all():
-            connections_set.add(conn)
+            connections_dict[id(conn)] = conn
         def runner():
             from django.db import connections
             for conn in connections.all():
                 # Allow thread sharing so the connection can be closed by the
                 # main thread.
                 conn.allow_thread_sharing = True
-                connections_set.add(conn)
+                connections_dict[id(conn)] = conn
         for x in range(2):
             t = threading.Thread(target=runner)
             t.start()
             t.join()
-        self.assertEqual(len(connections_set), 6)
+        self.assertEqual(len(connections_dict), 6)
         # Finish by closing the connections opened by the other threads (the
         # connection opened in the main thread will automatically be closed on
         # teardown).
-        for conn in connections_set:
+        for conn in connections_dict.values():
             if conn != connection:
                 conn.close()