使用函数生成器将值返回到GraphQL Mutation中的outputFields

Question

I am having problems using a function generator inside a GraphQL mutation .

The function generator is being used to clean up the promise chain inside the mutation. Before refactoring the code was working fine, and at present the console.logs are showing the newly created writes to the database so as far as I can tell the issue is getting the final return value out of the function generator and setting it as the return value of the mutateAndGetPayload function.

Error

Cannot set property 'clientMutationId' of undefined

I have been at this for a while now and would appreciate any advice or help.

How do I get the return value of the function generator into the scope of the enclosing function so I can return it from that function?

---

Below is the mutation. The database writes work fine, and the output works when it gets the right payload (it is not working, which is my main issue). The only issue is getting the newUser to the resolve function in the output field. I have omitted some of the code to simplify the issue.

const addEmployeeMutation = mutationWithClientMutationId({
  name: 'AddEmployee',
  inputFields: {
    /** this section works fine **/
  },
  outputFields: {
    attendee: {
      type: AttendeeType,
      resolve: r.compose(
          /** this section works fine **/
          r.prop('id')
        )
    }
  },
  mutateAndGetPayload: (payload, {userId}) => {
    const getUser = (userId) => {
      return authorize(userId)
        .then(can => /** works fine **/ )
        .then(user => gen.next(user))
    }    
    const createHash = (plainPassword) => {
      return genSalt(10)
        .then((salt, err) => { /** works fine **/ })
        .then((hashed_password, err) => gen.next(hashed_password))
    }   
    const createUser = (payload, hashed_password, user) => {
      return new UserModel( /** works fine **/ ).save()
        .then(newUser => gen.next(newUser))
    }    
    const createProfile = (payload, newUserId) => {
      return new ProfileModel({ /** works fine **/ })
        .then(profile => gen.next(profile))
        .catch(err => {throw new GraphQLError(`Error: ${err}`)})
    }  
    let gen = generator()
    gen.next()    
    function* generator() {   
      const user = yield getUser(userId)
      console.log('USER >>>>>>>>>>>>>>>>>>', user)
      const hash = yield createHash(payload.password)
      console.log('HASH >>>>>>>>>>>>>>>>>>', hash)   
      const newUser = yield createUser(payload, hash, user)
      console.log('NEW USER >>>>>>>>>>>>>>>>>> ', newUser)    
      const newProfile = yield createProfile(payload, newUser.id)
      console.log('NEW PROFILE >>>>>>>>>>>>>>>>>> ', newProfile)    
      return newUser
    }
    return newUser // <- How do I get a value to here?
}

Answer 1

Async Await

Thanks to some help from @Bergi I changed to using async / await .

I am posting my answer here in case someone else goes down this road and needs a hand.

The answer turns out to be a quick fix.

Change the .then() in each of the promises that are doing the database writes to return the needed object. And then moved them into a separate utility module.

In the mutation itself I use the following code to run through the async writes and return the required payload to the output field:

mutateAndGetPayload: (payload, {userId}) => {
  async function generateUser() {
    const user = await getUser(userId)
    const hash = await createHash(payload.password)
    const newUser = await createUser(payload, hash, user)
    await createProfile(payload, newUser.id)
    return newUser
  }
  return generateUser()
}

---

edit

An even neater solution, thanks again to @Bergi. I am leaving the original solution as it provides some clarity as to how async / await is being used, and demonstrates how it is possible to refactor code to improve terseness. Again, someone may find this useful.

mutateAndGetPayload: async function(payload, {userId}) {
  const user = await getUser(userId)
  const hash = await createHash(payload.password)
  const newUser = await createUser(payload, hash, user)
  await createProfile(payload, newUser.id)
  return newUser
}

---

Explanation

As the use of a function generator in the OP was asynchronous and hence non-blocking, the mutateAndGetPayload function was returning undefined before the generator had time to complete. By using async / await (in both versions) the mutateAndGetPayload function was blocked from completing until add the await values had resolved in the order in which they were specified.

Answer 2

Ramda JS

Another approach that makes use of the ramda functional programming library. With some help from q to manage the promises.

Using ramda.js involves currying the utility functions so that they are waiting for the data to come in the right order.

mutateAndGetPayload: (payload, {userId}) =>
  r.compose(
    then(
      r.compose(
        then(r.last),
        r.converge(
          r.unapply(q.all),
          [
            createProfile(payload),
            createUserRole(payload),
            r.identity
          ]
        )
      )
    ),
    r.converge(
      r.compose(
        then(createUser(payload)),
        r.unapply(q.all)
      ),
      [
        r.identity,
        then(_ => createHash(payload))
      ]
    ),
    getUser
  )(userId)