Apollo subscription resolver variables are empty

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I have a apollo resovler to connect a user to a sesion:

connectUserToSession: async (_, args) => {
  const session = await prisma.session.update({
    where: {
      id: Number(args.sessionId)
    },
    data: {
      players: {
        connect: {
          id: Number(args.userId)
        }
      }
    },
    include: {
      players: true,
      host: true
    }
  })

  pubsub.publish('USER_JOINED_SESSION', { userJoinedSession: session })

  return session
},

When this resolves a user is successfully connected to a session and the publish is also fired.

But the userJoinedSession subscription doesn’t work as expected:

userJoinedSession: {
  subscribe: withFilter(
    () => pubsub.asyncIterator('USER_JOINED_SESSION'),
    (payload, variables) => {
      console.log('/// START SUBSCRIPTION ////')
      console.log(payload)
      console.log('///')
      console.log(variables)
      console.log('/// END SUBSCRIPTION ////')
    },
  ),
}

As you can see in the logs below the payload does have the info regarding the joined session, but the variables value is an empty object.

/// START SUBSCRIPTION ////
{
  userJoinedSession: {
    id: 671,
    name: 'Talk about dogs',
    code: '878614',
    userId: 690,
    players: [
      [Object], [Object], [Object],
      [Object], [Object], [Object],
      [Object], [Object], [Object],
      [Object], [Object], [Object],
      [Object], [Object], [Object],
      [Object], [Object], [Object],
      [Object], [Object], [Object],
      [Object], [Object], [Object],
      [Object], [Object], [Object],
      [Object], [Object], [Object],
      [Object], [Object], [Object]
    ],
    host: { id: 690, name: 'asdasdas', messageId: null, sessionId: 671 }
  }
}
///
{}
/// END SUBSCRIPTION ////
/// START SUBSCRIPTION ////
{
  userJoinedSession: {
    id: 671,
    name: 'Talk about dogs',
    code: '878614',
    userId: 690,
    players: [
      [Object], [Object], [Object],
      [Object], [Object], [Object],
      [Object], [Object], [Object],
      [Object], [Object], [Object],
      [Object], [Object], [Object],
      [Object], [Object], [Object],
      [Object], [Object], [Object],
      [Object], [Object], [Object],
      [Object], [Object], [Object],
      [Object], [Object], [Object],
      [Object], [Object], [Object]
    ],
    host: { id: 690, name: 'asdasdas', messageId: null, sessionId: 671 }
  }
}
///
{}
/// END SUBSCRIPTION ////

The Apollo docs state:

variables is an object containing all arguments the client provided when initiating their subscription.

So it looks like no arguments are provided when this subscription is initiated, but the docs don’t go in depth into how this should be done.

The goal of the code is to only show a notification to users who are connected to the session a new user joined.