Firestore get sub documents from parent document

My database is organized this way:

-users (col)
    -user1 (doc)
        -some user data
        -printers (col)
            -printer1(doc)
            -printer2(doc)
    -user2 (doc)
        -some user data
        -printers (col)
            -printer1(doc)

What I want to do is get a reference of all users, for each user get some data and get all printers from this user. When finished getting all users data run a callback to do some things

What I've tried to do:

var users = []
db.get().then(function(users){
   users.forEach(function(user){

      var client = new Object();
      do some stuff..
      db.doc(user.id).collection("printers").get().then(function(printers){
          printers.forEach(function(printer){

              var clientPrinter = new Object();
              do some stuff..
              client.printers.push(clientPrinter);
          });
      });
   if(client.printers.length > 0){
       users.push(client);
   }
   });
   callback(users);
});

But this code is returning me an empty list, what I'm doing wrong here?