Firebase Query Collection And Merge Subcollection Data

Question

I'm trying to figure out a way to get a collection of documents along with their subcollection data.

For example, my tree is...

-locations
 -{locationId}
   -historic
     -april
       -airTemp: 12.4
       -displayOrder: 4
     -august
       -airTemp: 14.5
       -displayOrder: 9
     -december
       -airTemp: 13.5
       -displayOrder: 12
     etc..

...where locationId is a document and historic is the subcollection with monthly documents in it.

I know how to get the top level collection items and store it into an array but I want to add their subcollection data (i.e. the jan, feb, etc.) into each document as well.

  var locations = []

  let ref = db.collection('locations')
  db.collection('locations').get()
  .then(snapshot => {
      snapshot.forEach(doc => {
          let location = doc.data()
          location.id = doc.id
          location.name = doc.name

          // get the historic subcollection data here

      })
  })

How can I get the combined data (collection and subcollection) from each object and then push it into an array?

bounty is wrong structure each month should be its own object

Answer 1

There is no way, with the mobile/web client libraries, to get, in one query, the data of a Firestore document and the data of its sub-collection(s) documents.

There is even no way to get the list of the sub-collections of a document (with the mobile/web client libraries), see https://firebase.google.com/docs/firestore/query-data/get-data#list_subcollections_of_a_document

So you need to know the name of the sub-collections in order to query them, which is your case (name = 'historic').

You could do as follows. First you get all the parent documents and at the same time you use Promise.all() to query, in parallel, all the 'historic' sub-collections.

    var db = firebase.firestore();

    var promises = []
    db.collection('locations').get()
        .then(snapshot => {
            snapshot.forEach(doc => {
                let location = doc.data()
                location.id = doc.id
                location.name = doc.data().name
                console.log(location);
                promises.push(doc.ref.collection('historic').get());
            })
            return Promise.all(promises);
        })
        .then(results => {
            results.forEach(querySnapshot => {
                querySnapshot.forEach(function (doc) {
                    console.log(doc.id, " => ", doc.data());
                });
            });
        });

Note that the order of the results array is exactly the same than the order of the promises array.

Also note that to get the value of a document item (e.g. name) you need to do doc.data().name and not doc.name.