Move fields check to register so it happens on server start

channels/binding/base.py

@@ -53,8 +53,15 @@ class Binding(object):
     and tie that in as a consumer.
     """
 
+    # Model to serialize
+
     model = None
 
+    # Only model fields that are listed in fields should be send by default
+    # if you want to really send all fields, use fields = ['__all__']
+
+    fields = None
+
     @classmethod
     def register(cls):
         """
@@ -66,6 +73,9 @@ class Binding(object):
                 return
             else:
                 raise ValueError("You must set the model attribute on Binding %r!" % cls)
+        # If fields is not defined, raise an error
+        if cls.fields is None:
+            raise ValueError("You must set the fields attribute on Binding %r!" % cls)
         # Optionally resolve model strings
         if isinstance(cls.model, six.string_types):
             cls.model = apps.get_model(cls.model)

channels/binding/websockets.py

@@ -31,11 +31,6 @@ class WebsocketBinding(Binding):
 
     stream = None
 
-    # only model fields that are listed in fields should be send by default
-    # if you want to really send all fields, use fields = ['__all__']
-
-    fields = []
-
     # Outbound
     @classmethod
     def encode(cls, stream, payload):
@@ -55,10 +50,10 @@ class WebsocketBinding(Binding):
         Serializes model data into JSON-compatible types.
         """
         if self.fields == ['__all__']:
-            self.fields = None
-        elif not self.fields:
-            raise ValueError("You must set the fields attribute on Binding %r!" % self.__class__)
-        data = serializers.serialize('json', [instance], fields=self.fields)
+            fields = None
+        else:
+            fields = self.fields
+        data = serializers.serialize('json', [instance], fields=fields)
         return json.loads(data)[0]['fields']
 
     # Inbound