added BSONSerializer

docs/django.rst

@@ -90,10 +90,15 @@ session backend, ensure that your settings module has
 into your settings module::
 
     SESSION_ENGINE = 'mongoengine.django.sessions'
+    SESSION_SERIALIZER = 'mongoengine.django.sessions.BSONSerializer'
 
 Django provides session cookie, which expires after ```SESSION_COOKIE_AGE``` seconds, but doesn't delete cookie at sessions backend, so ``'mongoengine.django.sessions'`` supports  `mongodb TTL
 <http://docs.mongodb.org/manual/tutorial/expire-data/>`_.
 
+.. note:: ``SESSION_SERIALIZER`` is only necessary in Django 1.6 as the default
+   serializer is based around JSON and doesn't know how to convert
+   ``bson.objectid.ObjectId`` instances to strings.
+
 .. versionadded:: 0.2.1
 
 Storage

mongoengine/django/sessions.py

@@ -1,3 +1,4 @@
+from bson import json_util
 from django.conf import settings
 from django.contrib.sessions.backends.base import SessionBase, CreateError
 from django.core.exceptions import SuspiciousOperation
@@ -103,3 +104,15 @@ class SessionStore(SessionBase):
                 return
             session_key = self.session_key
         MongoSession.objects(session_key=session_key).delete()
+
+
+class BSONSerializer(object):
+    """
+    Serializer that can handle BSON types (eg ObjectId).
+    """
+    def dumps(self, obj):
+        return json_util.dumps(obj, separators=(',', ':')).encode('ascii')
+
+    def loads(self, data):
+        return json_util.loads(data.decode('ascii'))
+