Flutter Web Firebase - Get collection by sub-collection field

Question

I am building flutter web app linked with firestore. The database schema is as below:

My problem is I want to get the user document reference i.e (the company and user id) from just entering the users email address. Below is the code is my getUserRef() method:

Future<fs.DocumentReference> getUserRef (String userEmail) async {

String comp;
String id;
List<String> companies = await companyList();
    for (var i = 0; i < companies.length; i++) {
      store.collection('companies').doc(companies[i]).collection('users').get().
      then((val) {
        val.forEach((user){
          if(user.data()['Email'] == userEmail) {
            comp = user.data()['company'];
            id = user.id;
          }

        });

      });

    }
  fs.DocumentReference ref = store.collection('companies').doc(comp).collection('users').doc(id);

return ref;
 }

My companyList() method is working fine and retrieves a list of all the company doc ids. I can't figure out how to get this working and can't move on with the project without it.

Any help is appreciated.

Solution

It sounds like you're looking for a collection group query, which would be a single query that searches across the users of all companies at once.

It'd be a query like:

var query = Firestore.instance.collectionGroup("users").where("Email", isEqualTo: userEmail);
var querySnapshot = await query.getDocuments();
var userDocumentRef = querySnapshot.documents[0];

Also see:

• Understanding Collection Group Queries in Cloud Firestore