Added a MongoEngine backend for Django sessions

docs/django.rst

@@ -27,3 +27,16 @@ file::
 The :mod:`~mongoengine.django.auth` module also contains a 
 :func:`~mongoengine.django.auth.get_user` helper function, that takes a user's
 :attr:`id` and returns a :class:`~mongoengine.django.auth.User` object.
+
+Sessions
+========
+Django allows the use of different backend stores for its sessions. MongoEngine
+provides a MongoDB-based session backend for Django, which allows you to use
+sessions in you Django application with just MongoDB. To enable the MongoEngine
+session backend, ensure that your settings module has
+``'django.contrib.sessions.middleware.SessionMiddleware'`` in the
+``MIDDLEWARE_CLASSES`` field  and ``'django.contrib.sessions'`` in your
+``INSTALLED_APPS``. From there, all you need to do is add the following line
+into you settings module::
+
+    SESSION_ENGINE = 'mongoengine.django.sessions'

mongoengine/django/sessions.py

@@ -0,0 +1,63 @@
+from django.contrib.sessions.backends.base import SessionBase, CreateError
+from django.core.exceptions import SuspiciousOperation
+from django.utils.encoding import force_unicode
+
+from mongoengine.document import Document
+from mongoengine import fields
+from mongoengine.queryset import OperationError
+
+from datetime import datetime
+
+
+class MongoSession(Document):
+    session_key = fields.StringField(primary_key=True, max_length=40)
+    session_data = fields.StringField()
+    expire_date = fields.DateTimeField()
+    
+    meta = {'collection': 'django_session', 'allow_inheritance': False}
+
+
+class SessionStore(SessionBase):
+    """A MongoEngine-based session store for Django.
+    """
+
+    def load(self):
+        try:
+            s = MongoSession.objects(session_key=self.session_key,
+                                     expire_date__gt=datetime.now())[0]
+            return self.decode(force_unicode(s.session_data))
+        except (IndexError, SuspiciousOperation):
+            self.create()
+            return {}
+
+    def exists(self, session_key):
+        return bool(MongoSession.objects(session_key=session_key).first())
+
+    def create(self):
+        while True:
+            self.session_key = self._get_new_session_key()
+            try:
+                self.save(must_create=True)
+            except CreateError:
+                continue
+            self.modified = True
+            self._session_cache = {}
+            return
+
+    def save(self, must_create=False):
+        s = MongoSession(session_key=self.session_key)
+        s.session_data = self.encode(self._get_session(no_load=must_create))
+        s.expire_date = self.get_expiry_date()
+        try:
+            s.save(force_insert=must_create, safe=True)
+        except OperationError:
+            if must_create:
+                raise CreateError
+            raise
+
+    def delete(self, session_key=None):
+        if session_key is None:
+            if self.session_key is None:
+                return
+            session_key = self.session_key
+        MongoSession.objects(session_key=session_key).delete()