graphql-subscriptions: Subscription field must return Async Iterable. Received: undefined

We are trying to get the resolver for a subscription working in Apollo Server. The subscription works when it is a top-level field (i.e. directly under Subscription at the schema root).

However, if the subscription is contained within another type, I always get the error Subscription field must return Async Iterable. Received: undefined on the client’s websocket connection – the server’s resolver is never executed.

i.e. this schema works:

type Subscription {
  postAdded: Post
}

but this one does not:

type Subscription {
  post: PostSubscription
}

type PostSubscription {
  postAdded: Post
}

My resolver for the second case looks like this, but I’ve tried a bunch of different variations with no success:

Subscription: {
  post: () => ({
    PostSubscription: {}
  })
},
PostSubscription: {
  postAdded: {
    subscribe: () => pubSub.asyncIterator(['postAdded'])
  }
}

Maybe relevant package versions:

"apollo-server-express": "1.3.6",
"apollo-server-module-graphiql": "1.3.4",
"graphql-subscriptions": "0.5.8",
"graphql": "0.13.2",

P.s.: Maybe unrelated, but just in case, this reminds me of https://github.com/yarax/swagger-to-graphql/commit/fa1000accd046b5db9f5d973e2e120c3f658f3b6#diff-6d186b954a58d5bb740f73d84fe39073L54

Clone of: https://stackoverflow.com/questions/51862571/apollo-server-subscription-nested-within-a-type

About this issue

Original URL
State: closed
Created 6 years ago
Reactions: 12
Comments: 36 (10 by maintainers)

Most upvoted comments

Hi @builtbyproxy

No, I have my resolvers (and schema) separated by small files. In the case of resolvers, they are javascript files, with a final export default resolvers;, and that’s the line I had forgotten.

I’ll show the code just in case:

import { pubSub, names } from '~/src/config/subscriptions';
import { EzfacilityClientBooking } from '~/src/repositories';

const resolvers = {
  Subscription: {
    clientBookingsChanged: {
      subscribe: () => {
        const asyncIterator = pubSub.asyncIterator([names.CLIENT_BOOKINGS_CHANGED]);

        // Fetch immediately the data from EZ
        const clientBooking = new EzfacilityClientBooking();
        clientBooking.getAll();

        return asyncIterator;
      }
    }
  }
};

export default resolvers;

abelosorio on Nov 12, 2018

@iamcristos more less check have you export subscription and you don’t use nested types for subscription 😃 read that thread carefully 😉

agborkowski on Apr 3, 2020

I somehow miss the answer of 4F2E4A2E’s initial question. Is there a support of subscriptions in subfields (non root subscriptions)? Do I overlook something?

SDLWebDev on Feb 19, 2019

Upon staring at these resolvers - it’s not clear to me why this is actually supposed to work. foo needs to be an object with a resolve and a subscribe field - instead right now it’s returning a { fooObj: true }.

I tried to fix up your resolvers into something that I think approximates what you want. Here it is: https://github.com/grantwwu/graphql-nested-subscriptions/compare/grantwu_fixed~...grantwwu:grantwu_fixed

(Note that I also made some formatting changes, as well as updating your dependencies when I was ruling that out as a bug).

grantwwu on Nov 13, 2018

I have some time now and I’m a bit confused - @builtbyproxy @abelosorio were you working on the same issue as OP? @RedShift1 are you still having issues?

grantwwu on Nov 13, 2018

@abelosorio by final export are you talking about:

schema {
	query: Query
	mutation: Mutation
	subscription: Subscription
}

or something else?

builtbyproxy on Nov 11, 2018