Django sessions TTL support

docs/django.rst

@@ -45,6 +45,9 @@ into you settings module::
 
     SESSION_ENGINE = 'mongoengine.django.sessions'
 
+Django provides session cookie, which expires after ```SESSION_COOKIE_AGE``` seconds, but doesnt delete cookie at sessions backend, so ```'mongoengine.django.sessions'``` supports  `mongodb TTL
+<http://docs.mongodb.org/manual/tutorial/expire-data/>`_.
+
 .. versionadded:: 0.2.1
 
 Storage

mongoengine/django/sessions.py

@@ -31,9 +31,17 @@ class MongoSession(Document):
                                         else fields.DictField()
     expire_date = fields.DateTimeField()
 
-    meta = {'collection': MONGOENGINE_SESSION_COLLECTION,
+    meta = {
-            'db_alias': MONGOENGINE_SESSION_DB_ALIAS,
+        'collection': MONGOENGINE_SESSION_COLLECTION,
-            'allow_inheritance': False}
+        'db_alias': MONGOENGINE_SESSION_DB_ALIAS,
+        'allow_inheritance': False,
+        'indexes': [
+            {
+                'fields': ['expire_date'],
+                'expireAfterSeconds': settings.SESSION_COOKIE_AGE
+            }
+        ]
+    }
 
 
 class SessionStore(SessionBase):