diff --git a/dddp/__init__.py b/dddp/__init__.py
index 0047026..3a16a98 100644
--- a/dddp/__init__.py
+++ b/dddp/__init__.py
@@ -1,6 +1,7 @@
 """Django/PostgreSQL implementation of the Meteor DDP service."""
 from __future__ import unicode_literals
 import os.path
+import sys
 from pkg_resources import get_distribution, DistributionNotFound
 from gevent.local import local
 from dddp import alea
@@ -89,9 +90,16 @@ class RandomStreams(object):
         return self._streams[key]
 
 
+def serializer_factory():
+    """Make a new DDP serializer."""
+    from django.core.serializers import get_serializer
+    return get_serializer('ddp')()
+
+
 THREAD_LOCAL = ThreadLocal(
     alea_random=alea.Alea,
     random_streams=RandomStreams,
+    serializer=serializer_factory,
 )
 METEOR_ID_CHARS = u'23456789ABCDEFGHJKLMNPQRSTWXYZabcdefghijkmnopqrstuvwxyz'
 
diff --git a/dddp/api.py b/dddp/api.py
index 05a1447..a2bee16 100644
--- a/dddp/api.py
+++ b/dddp/api.py
@@ -191,6 +191,8 @@ class Collection(APIMixin):
         qs = self.get_queryset(qs)
         user_rels = self.user_rel
         if user_rels:
+            if user is None:
+                return qs.none()  # no user but we need one: return no objects.
             if isinstance(user_rels, basestring):
                 user_rels = [user_rels]
             user_filter = None
@@ -399,9 +401,9 @@ class DDP(APIMixin):
             this.error('Invalid publication name: %r' % name)
             return
         obj, created = Subscription.objects.get_or_create(
-            connection=this.ws.connection,
+            connection_id=this.ws.connection.pk,
             sub_id=id_,
-            user=this.request.user,
+            user_id=this.request.user.pk,
             defaults={
                 'publication': pub.name,
                 'publication_class': '%s.%s' % (
diff --git a/dddp/migrations/0005_auto_20150427_1209.py b/dddp/migrations/0005_auto_20150427_1209.py
new file mode 100644
index 0000000..c7adc86
--- /dev/null
+++ b/dddp/migrations/0005_auto_20150427_1209.py
@@ -0,0 +1,21 @@
+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import models, migrations
+from django.conf import settings
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('dddp', '0004_connection_server_addr'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='subscription',
+            name='user',
+            field=models.ForeignKey(blank=True, to=settings.AUTH_USER_MODEL, null=True),
+            preserve_default=True,
+        ),
+    ]
diff --git a/dddp/models.py b/dddp/models.py
index 2dc1cf9..8559ad7 100644
--- a/dddp/models.py
+++ b/dddp/models.py
@@ -136,7 +136,7 @@ class Subscription(models.Model, object):
     _publication_cache = {}
     connection = models.ForeignKey(Connection)
     sub_id = models.CharField(max_length=17)
-    user = models.ForeignKey(settings.AUTH_USER_MODEL)
+    user = models.ForeignKey(settings.AUTH_USER_MODEL, blank=True, null=True)
     publication = models.CharField(max_length=255)
     publication_class = models.CharField(max_length=255)
     params_ejson = models.TextField(default='{}')
diff --git a/dddp/msg.py b/dddp/msg.py
index 9e0a3e4..5cf55de 100644
--- a/dddp/msg.py
+++ b/dddp/msg.py
@@ -1,17 +1,28 @@
 """Django DDP utils for DDP messaging."""
+from copy import deepcopy
 from dddp import THREAD_LOCAL as this
-from django.core.serializers import get_serializer
-
-
-def serializer_factory():
-    """Make a new DDP serializer."""
-    return get_serializer('ddp')()
+from django.db.models.expressions import ExpressionNode
 
 
 def obj_change_as_msg(obj, msg):
     """Generate a DDP msg for obj with specified msg type."""
-    serializer = this.get('serializer', serializer_factory)
+    # check for F expressions
+    exps = [
+        name for name, val in vars(obj).items()
+        if isinstance(val, ExpressionNode)
+    ]
+    if exps:
+        # clone and update obj with values but only for the expression fields
+        obj = deepcopy(obj)
+        # Django 1.8 makes obj._meta public --> pylint: disable=W0212
+        for name, val in obj._meta.model.objects.values(*exps).get(pk=obj.pk):
+            setattr(obj, name, val)
+
+    # run serialization now that all fields are "concrete" (not F expressions)
+    serializer = this.serializer
     data = serializer.serialize([obj])[0]
+
+    # collection name is <app>.<model>
     name = data['model']
 
     # cast ID as string
diff --git a/dddp/websocket.py b/dddp/websocket.py
index 0afe891..2f25385 100644
--- a/dddp/websocket.py
+++ b/dddp/websocket.py
@@ -123,7 +123,7 @@ class DDPWebSocketApplication(geventwebsocket.WebSocketApplication):
         this.send_msg = self.send_msg
         this.reply = self.reply
         this.error = self.error
-        this.session_key = this.request.session.session_key
+        this.request.session.save()
 
         this.remote_addr = self.remote_addr = \
             '{0[REMOTE_ADDR]}:{0[REMOTE_PORT]}'.format(
@@ -280,7 +280,7 @@ class DDPWebSocketApplication(geventwebsocket.WebSocketApplication):
             this.version = version
             this.support = support
             self.connection = Connection.objects.create(
-                session_id=this.session_key,
+                session_id=this.request.session.session_key,
                 server_addr='%d:%s' % (
                     backend_pid,
                     self.ws.handler.socket.getsockname(),