Error "can't serialize <model instance>" when using channels_redis as channel layer

When having following subscription code (MessageModel is a Django model class):

class Message(DjangoObjectType):
    class Meta:
        model = MessageModel

class MessageAddedSubscription(channels_graphql_ws.Subscription):
    Output = Message

    @staticmethod
    def publish(payload, info):
        return payload

and calling broadcast elsewhere like this:

new_message = MessageModel.objects.create(text='some text')
MessageAddedSubscription.broadcast(payload=new_message)

… application crashes because it’s unable to serialize the model instance to redis-backed channel layer:

An error occurred while resolving field Mutations.messageAdd
Traceback (most recent call last):
  File "/Users/xaralis/.venvs/obs/lib/python3.6/site-packages/graphql/execution/executor.py", line 311, in resolve_or_error
    return executor.execute(resolve_fn, source, info, **args)
  File "/Users/xaralis/.venvs/obs/lib/python3.6/site-packages/graphql/execution/executors/sync.py", line 7, in execute
    return fn(*args, **kwargs)
  File "/Users/xaralis/.venvs/obs/lib/python3.6/site-packages/graphene_django/debug/middleware.py", line 56, in resolve
    promise = next(root, info, **args)
  File "/Users/xaralis/.venvs/obs/lib/python3.6/site-packages/graphql/execution/middleware.py", line 57, in make_it_promise
    return Promise.resolve(next(*a, **b))
  File "/Users/xaralis/Workspace/obs/obs-server/obs/schema.py", line 30, in mutate
    MessageAddedSubscription.broadcast(payload=new_message)
  File "/Users/xaralis/.venvs/obs/lib/python3.6/site-packages/channels_graphql_ws/graphql_ws.py", line 182, in broadcast
    'payload': payload})
  File "/Users/xaralis/.venvs/obs/lib/python3.6/site-packages/asgiref/sync.py", line 64, in __call__
    return call_result.result()
  File "/usr/local/Cellar/python/3.6.5/Frameworks/Python.framework/Versions/3.6/lib/python3.6/concurrent/futures/_base.py", line 432, in result
    return self.__get_result()
  File "/usr/local/Cellar/python/3.6.5/Frameworks/Python.framework/Versions/3.6/lib/python3.6/concurrent/futures/_base.py", line 384, in __get_result
    raise self._exception
  File "/Users/xaralis/.venvs/obs/lib/python3.6/site-packages/asgiref/sync.py", line 78, in main_wrap
    result = await self.awaitable(*args, **kwargs)
  File "/Users/xaralis/.venvs/obs/lib/python3.6/site-packages/channels_redis/core.py", line 317, in group_send
    self._map_channel_to_connection(channel_names, message)
  File "/Users/xaralis/.venvs/obs/lib/python3.6/site-packages/channels_redis/core.py", line 368, in _map_channel_to_connection
    channel_to_message[channel] = self.serialize(message)
  File "/Users/xaralis/.venvs/obs/lib/python3.6/site-packages/channels_redis/core.py", line 386, in serialize
    value = msgpack.packb(message, use_bin_type=True)
  File "/Users/xaralis/.venvs/obs/lib/python3.6/site-packages/msgpack/__init__.py", line 47, in packb
    return Packer(**kwargs).pack(o)
  File "msgpack/_packer.pyx", line 284, in msgpack._packer.Packer.pack
  File "msgpack/_packer.pyx", line 290, in msgpack._packer.Packer.pack
  File "msgpack/_packer.pyx", line 287, in msgpack._packer.Packer.pack
  File "msgpack/_packer.pyx", line 234, in msgpack._packer.Packer._pack
  File "msgpack/_packer.pyx", line 281, in msgpack._packer.Packer._pack
TypeError: can't serialize <Message: Message object (39)>
Traceback (most recent call last):
  File "/Users/xaralis/.venvs/obs/lib/python3.6/site-packages/graphql/execution/executor.py", line 330, in complete_value_catching_error
    exe_context, return_type, field_asts, info, result)
  File "/Users/xaralis/.venvs/obs/lib/python3.6/site-packages/graphql/execution/executor.py", line 383, in complete_value
    raise GraphQLLocatedError(field_asts, original_error=result)
graphql.error.located_error.GraphQLLocatedError: can't serialize <Message: Message object (39)>

What is the recommended way to handle Django model instances? Thanks

[bewestphal]

I wanted to make the message object the same as one from a query so it’s easier to manipulate in the client.

This had me stuck for a while, so leaving this for anyone that comes here from google.

Testing on django==3.0.6 graphene-django==2.10.1 django-channels-graphql-ws==0.5.0

class ChatSubscription(channels_graphql_ws.Subscription):
    """Subscription triggers on a new chat message."""

    message = graphene.Field(Message)
  
   # ....the rest of your subscription class

   def publish(message: MessageModel, info: ResolveInfo, **arguments):
       return ChatSubscription(message=message)

   @classmethod
    def new_message(cls, message: MessageModel):
        cls.broadcast(group=message.chat_id, payload=message)

[xaralis]

@prokher You’re awesome!