Inner Firestore promise inside forEach, is executed after the outer promise

Question

I'm using node.js to get data from 2 different collections in Firestore.

This issue is similar to this, but there is no answer to this question: Nested Firebase Firestore forEach promise queries

In my case, I'm creating an instagram-like app, where I have a 'timeline' collection. Inside the timeline document, I have a user key. From that user key, I would like to perform another query from 'user' collection.

So logically here are the query steps:

1. Get the timeline data (in array) which contains a user key.
2. Using that user key, get a user data (single data)
3. Return a JSON response to the client.

The problem is, the JSON response is returned before getting the user data.

This is my code:

tlRoute.get((req,res)=>{

  //reading from firestore
  let afs = req.app.get('afs');

  var timelineArr = [];

  let timelineRef = afs.collection(`timeline/${req.params.key}/timeline`);
  timelineRef.get().then((snapshot) => {

    snapshot.forEach((doc) => {

      if(!doc.exists){
        console.log('No such document!');
      } else {

        //populating timelineData with doc.data()
        console.log('populating timelineData');
        let timelineData = doc.data();

        let userRef = afs.doc(`user/${doc.data().userKey}`);
        userRef.get().then((doc) => {

          //adding user details to timelineData
          console.log('adding user details to timelineData');
          timelineData.profileImageUrl = doc.data().profileImageUrl;
          timelineData.username = doc.data().username;
          timelineArr.push(timelineData);

        });
      }

    });
  })
  .catch(err => {
    console.log(err);
  });

  console.log('this is the final timelineArr', timelineArr);

  //returning response json data to client
  return res.json(timelineArr);

});

In console log, this is the log that I get:

this is the final timelineArr []
populating timelineData
populating timelineData
adding user details to timelineData
adding user details to timelineData

Any help would be appreciated.

Answer 1

I've tried to refactor your code to produce the same output, adding a few methods, each one with a simpler, isolated, testable purpose relating to a specific kind of object.

// return a promise that resolves to a user doc snapshot with the given key
function getUserWithKey(db, userKey) {
  return db.doc(`user/${userKey}`).get();
}

// return a promise that resolves to a timeline object augmented to include
// its doc id and its associated user's username
function timelineObject(db, timelineDocSnapshot) {
  let timelineObject = timelineDocSnapshot.data();
  timelineObject.postKey = timelineDocSnapshot.id;
  return getUserWithKey(db, timelineObject.userKey).then(userDocSnapshot => {
    timelineObject.username = userDocSnapshot.data().username;
    timelineObject.profileImageUrl = userDocSnapshot.data().profileImageUrl;
    return timelineObject;
  });
}

// return a promise that resolves to all of the timeline objects for a given key
function getTimeline(db, key) {
  let timelineRef = db.collection(`timeline/${key}/timeline`);
  return timelineRef.get().then(querySnapshot => {
    let promises = querySnapshot.docs.map(doc => timelineObject(db, doc));
    return Promise.all(promises);
  });
}

// get route for a timeline
tlRoute.get((req,res)=>{
  let db = req.app.get('db');
  let key = req.params.key;
  return getTimeline(db, key).then(timelineObjects => {
    return res.json(timelineObjects);
  });
})

This code could be further improved with the use of async / await syntax.

Answer 2

I decided to use a for to loop through the inner promise.

Here is my code:

tlRoute.get((req,res)=>{

  let db = req.app.get('db');
  let key = req.params.key;

  var timelineArr = [];

  getData(db, key, timelineArr).then(results => {

    for (let index = 0; index<results.length; index++) {
      timelineArr[index].profileImageUrl = results[index].data().profileImageUrl;
      timelineArr[index].username = results[index].data().username;
    }
    console.log(results.length);

    console.log(timelineArr);

    console.log('this is the final timelineArr');
    //returning response json data to client 
    return res.json(timelineArr);

  });

});

function getData(db, key, timelineArr){
  let timelineRef = db.collection(`timeline/${key}/timeline`);
  return timelineRef.get().then((snapshot) => {

    console.log('snapshot length: ', snapshot.length);

    var promiseArr = [];

    snapshot.forEach((doc) => {

      if(!doc.exists){
        console.log('No such document!');
      } else {

        //populating timelineData with doc.data()
        console.log('populating timelineData');
        let timelineData = doc.data();
        timelineData.postKey = doc.id;
        timelineArr.push(timelineData);

        console.log('userKey: ', doc.data().userKey);
        let userRef = db.doc(`user/${doc.data().userKey}`);
        promiseArr.push(userRef.get());

      }
    });
    return Promise.all(promiseArr);

  })
  .catch(err => {
    console.log(err);
  });
}

Answer 3

Sooo Firebase uses callbacks or promises(the “.then((snapshot) => {})” thingy) which runs after the data has been retrieved from Firestore. What you are doing is returning the timelineArr before the callback-method has runned, and thus before it has been populated with the data from Firestore!

One solution to this is to move the return statement inside the callback-method and make the whole function asynchronous. It will be something like this:

var timelineArr = [];
async function RetrieveData() {
    let timelineRef = afs.collection(`timeline/${req.params.key}/timeline`);
    await timelineRef.get().then((snapshot) => {

    snapshot.forEach((doc) => {

      if(!doc.exists){
        console.log('No such document!');
      } else {

        //populating timelineData with doc.data()
        console.log('populating timelineData');
        let timelineData = doc.data();

        let userRef = afs.doc(`user/${doc.data().userKey}`);
        userRef.get().then((doc) => {

          //adding user details to timelineData
          console.log('adding user details to timelineData');
          timelineData.profileImageUrl = doc.data().profileImageUrl;
          timelineData.username = doc.data().username;
          timelineArr.push(timelineData);

        });
      }

    });

      //returning response json data to client
      return res.json(timelineArr);

    }).catch(err => {
      console.log(err);
    });
}

RetrieveData()

console.log('this is the final timelineArr', timelineArr);

Good luck!

Best regards, Eskils.