foreach loop array promise not being returned

Question

I want to access a subcollection called 'followers' from a given id under the 'mobile_user', this contains the id's of the followers. I want to loop through each of the id's in the followers subcollection and query data from the 'mobile_user' collection . Notice that the id in the 'followers' subollection is the id of another user under 'mobile_user' which contains the document data i want.

I've tried messing around with promises with no luck, i'm able to do the foreach loop and just query the name as a test but the array gets populated correctly but the populated array with the names of the N users are never returned.

I need some assistance with my code, promises are driving me nuts I can't get the hang around them.

const getFollowers = (data, context) => {
        return new Promise((resolve, reject) => {

            let id = data.id
            const mobileUserRef = db.collection('mobile_user')

            return mobileUserRef.doc(id).collection('followers')
                .get()
                .then(function (doc) {
                    var result = []
                    doc.forEach(function (follower) {
                        mobileUserRef.doc(follower.id).get()
                            .then(function (followerdoc) {
                                result.push({
                                    name: followerdoc.data().name,
                                })
                                console.log(result)
                            })
                    })
                    return Promise.all(result);
                })
        });
    }

The expected result is an array with the data of every id under the followers sub, like this:

In this example only 2 user id's are present

[ { name: 'Luis' }, { name: 'Marb Rocha' } ]

do the return in the same position as console.log

Nabeel Khan
– Nabeel Khan

2019-04-22 17:54:31 +00:00
Commented Apr 22, 2019 at 17:54 — Nabeel Khan
– Nabeel Khan, Commented Apr 22, 2019 at 17:54
@NabeelKhan i'm still getting a socket hang up

Luis
– Luis

2019-04-22 18:11:15 +00:00
Commented Apr 22, 2019 at 18:11 — Luis
– Luis, Commented Apr 22, 2019 at 18:11

r3wt · Accepted Answer · 2019-04-22 18:41:28Z

1

nothing gets pushed to result by the time the return Promise.all(result); is called, therefore the promise resolves early with no data. you could utilize chaining to return the results once all the promises resolve, as shown below:

const getFollowers = (data, context) => {
    const results = [];

    const id = data.id
    const mobileUserRef = db.collection('mobile_user');

    //return the initial promise, and use chaining to *eventually* return the results
    return mobileUserRef.doc(id).collection('followers')
    .get()
    .then(doc=>{
        //use map to transform the doc results into an array of promises that call each user ref and load the doc
        const promises = doc.map(follower=>{
            return mobileUserRef.doc(follower.id).get()
            .then(followerdoc=>{
                //push items into the results
                results.push({
                    name: followerdoc.data().name,
                })
                return followerdoc;
            })
        })
        //execute all the promises, then return the results
        return Promise.all(promises).then(()=>{
            return results;//return the results, resolving our chain finally
        });
    })

}

edited Apr 22, 2019 at 18:41

answered Apr 22, 2019 at 18:35

r3wt

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4 Comments

Luis Over a year ago

thank you, this is helpful. Although when i try to run it i'm getting the error Unhandled error TypeError: doc.map is not a function

r3wt Over a year ago

@Louis that would seem to indicate that doc is not an Array, or the environment the code is running in doesn't have the Array.prototype.map method, which was added in es5 i believe, so it may not be available in all environments. To suggest a fix, i would need to know more about the environment.

Luis Over a year ago

thank you, the array was under doc.docs.map and I'm now getting the result i wanted. I love you

r3wt Over a year ago

@Louis no problem, glad you got it figured out

Renaud Tarnec · Accepted Answer · 2019-04-22 18:43:39Z

You need to add some promises to the array you pass to Promise.all(), so I think you should do as follows:

  const getFollowers = (data, context) => {
    let id = data.id;
    const mobileUserRef = db.collection('mobile_user');

    return mobileUserRef
      .doc(id)
      .collection('followers')
      .get()
      .then(querySnapshot => {
        var result = [];
        querySnapshot.forEach(follower => {
          result.push(mobileUserRef.doc(follower.id).get());
        });
        return Promise.all(result);
      })
      .then(results => {
        var finalResult = [];
        results.forEach(followerdoc => {
          finalResult.push({
            name: followerdoc.data().name
          });
        });
        return finalResult;
      });
  };

PS: not sure why you have a context parameter.

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