Error in apollo-server-express: Subscription field must return Async Iterable. Received: undefined

I’m trying to implement subscriptions with apollo-server-express. From graphql-playground I receive the following error…

{
  "error": {
    "message": "Subscription field must return Async Iterable. Received: undefined"
  }
}

Until now I had not had problems with queries and mutations. I also had no errors with subscriptions using apollo-server. But I do have them with apollo-server-express.

This is the code:

const {gql, ApolloError, PubSub} = require('apollo-server-express');

// Mongoose models
const NoteModel = require('../../mongoose/NoteModel');

// Instance PubSub (publications and subscriptions)
const pubsub = new PubSub();

// Subscriptions tags
const NOTE_ADDED = 'NOTE_ADDED';


// Type definitions define the "shape" of your data and specify
// which ways the data can be fetched from the GraphQL server.
const typeDefs = gql`
    type Note {
        _id: Int
        title: String
        description: String
    }

    input NoteInput {
        title: String
        description: String
    }

    # The "Query" type is the root of all GraphQL queries.
    # Using extend is particularly useful in avoiding a large 
    # list of fields on root Queries and Mutations
    # More info: https://www.apollographql.com/docs/graphql-tools/generate-schema#extend-types
    extend type Query {
        listNotes: [Note]
    }

    extend type Mutation {
        addNote(input: NoteInput) : Note!,
        updateNote(
            id: Int!,
            input: NoteInput
        ): Note!,
        deleteNote(id: Int!): Boolean!
    }

    extend type Subscription {
        noteAdded: [Note]
    }
`;


const resolvers = {
    Query: {
        listNotes: async (parent, args, context, info) => {
            
        }
    },
    Mutation: {
        addNote: (parent, args, context, info) => {
            pubsub.publish(NOTE_ADDED, { noteAdded: args.input });
            return NoteModel.create(args.input);
        },
        updateNote: (parent, args, context, info) => {

        },
        deleteNote: (parent, args, context, info) => {

        }
    },
    Subscription: {
        noteAdded: {
            subscribe: () => pubsub.asyncIterator([NOTE_ADDED]),
        },
    },
};


module.exports = {
    typeDefs,
    resolvers
};

[abernix]

👋 I’ll close this since this doesn’t appear to be a bug with Apollo Server, but rather a question about how to use it or one of its components.

Rather than asking it here in GitHub Issues — where efforts are focused on fixing bugs and adding new features — I’d ask that you take this question to the Apollo Server channel within the Apollo community on Spectrum.chat where there are community members who might be able to relate to a similar problem, or might be able to help you out more interactively. Thanks for your understanding!

[pavel-koryagin]

@abernix It is definitely a bug! And I did some investigation.

1. @LaurenceM10 wrote that it works with new ApolloServer({ typeDefs, resolvers }), but it does not with new ApolloServer({ modules })

2. In my case it does not work with new ApolloServer({ schema })

So, I found where the problem appears: http://center.koryagin.com/i/2019-08-04_00:29:39_6df691.png

And with this trivial monkey-patching everything is working, subscription messages arrive in browser.

const schema = buildFederatedSchema(federationSources);
schema._subscriptionType._fields.messageCreated.subscribe = 
  federationSources[11].resolvers.Subscription.messageCreated.subscribe;