added BSONSerializer

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'))
+