Django Channels: Exception inside application: No handler for message type app_consumer.notification_message

Question

I'm trying to send a notification to the user using websockets, on a particular event. I have the following model receiver which triggers a websocket method:

@receiver(pre_save, sender=settings.AUTH_USER_MODEL)
def create_auth_token(sender, instance=None, created=False, update_fields=None, **kwargs):
    if update_fields:
        if 'pan_number_verified' in update_fields and instance.pan_number_verified is True:
            notification = Notification.objects.create(message='Your PAN Number has been verified...Head on to '
                                                               'creating a campaign right away.', user=instance)
            channel_layer = get_channel_layer()
            async_to_sync(channel_layer.group_send)("notification_%s" % instance.id, {
                "type": "app_consumer.notification_message",
                "message": str(notification.id)
            })

Here is my app consumer:

class AppConsumer(WebsocketConsumer):
    def connect(self):
        self.room_name = self.scope['url_route']['kwargs']['user_id']
        self.room_group_name = 'notification_%s' % self.room_name
        async_to_sync(self.channel_layer.group_add)(
            self.room_group_name,
            self.channel_name
        )
        self.accept()

    def disconnect(self, close_code):
        async_to_sync(self.channel_layer.group_discard)(
            self.room_group_name,
            self.channel_name
        )

    def receive(self, text_data):
        text_data_json = json.loads(text_data)
        message = text_data_json['message']

        async_to_sync(self.channel_layer.group_send)(
            self.room_group_name,
            {
                'type': 'app_consumer.notification_message',
                'message': message
            }
        )

    def notification_message(self, event):
        message = event['message']

        self.send(text_data=json.dumps({
            'message': message
        }))

But I get the following error message:

Exception inside application: No handler for message type app_consumer.notification_message

File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/channels/consumer.py", line 59, in call [receive, self.channel_receive], self.dispatch

File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/channels/utils.py", line 51, in await_many_dispatch await dispatch(result)

File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/asgiref/sync.py", line 244, in call return await asyncio.wait_for(future, timeout=None)

File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/asyncio/tasks.py", line 414, in wait_for return await fut

File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/concurrent/futures/thread.py", line 57, in run result = self.fn(*self.args, **self.kwargs)

File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/channels/db.py", line 14, in thread_handler return super().thread_handler(loop, *args, **kwargs)

File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/asgiref/sync.py", line 277, in thread_handler return func(*args, **kwargs)

File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/channels/consumer.py", line 107, in dispatch raise ValueError("No handler for message type %s" % message["type"])

No handler for message type app_consumer.notification_message

Answer 1

The error you're seeing occurs because the message you're sending via channel_layer.group_send is missing a type attribute. In Django Channels, the message must include a type so that it knows how to handle the incoming message.

To fix this, you need to add a type to the message you're sending in the receive method. Here’s how you can modify your code:

def receive(self, text_data):
    text_data_json = json.loads(text_data)
    message = text_data_json["message"]
    async_to_sync(self.channel_layer.group_send)(
        self.room_group_name, {
            "type": "chat_message",  
            "message": message,
            "user": "user"
        }
    )

Then, you'll need to define a method to handle this message type. For example, you can add a chat_message method like this:

def chat_message(self, event):
    message = event["message"]
    user = event["user"]
    self.send(text_data=json.dumps({
        "message": message,
        "user": user
    }))

This will ensure that Django Channels can properly process and route the message based on the type you’ve specified (chat_message in this case).

Answer 2

You issue here is the name of your notification_message handler method.

Channels expects it to be named app_consumer_notification_message.

Channels replaces the . in message types with _ when looking for a message handling method.

(the prefix in the message type is not used for routing just the group name)