Firebase realtime database read operation - Only get part of document returned possible?

Question

I realise this question is marked as a duplicate - but I think this question is more comprehensive and easier to understand, and therefore more likely to help others. (^__^)

I have read about reading data from Firebase realtime database. https://firebase.google.com/docs/database/admin/retrieve-data

What I have not found is any documentation on how to control what data gets returned from the read query.

Consider the following structure:

-users
  -EiYu7OWooToo6noo
    alias: "Kermit"
    createdAt: 1538330532000
    firstName: "Kermit"
    surName: "Frogman" 
    pictureURL: "/imgs/kermit.png"
    phonePrimary: "555-1234"
    email: "kermit@themuppetshow.com"
    -myBooks
      -A1234567890
        title: "Frogs 101"
      ...

Is there a way to tell Firebase realtime database to only return some of the document data?

This code returns the entire document:

var ref = db.ref(`users/${uid}`);

ref.once("value", (snapshot) => {
  console.log(snapshot.val());
}, function (errorObject) {
  console.log("The read failed: " + errorObject.code);
});

Would it be possible to only get a partial document, for example:

    {
      alias: "Kermit"
      firstName: "Kermit"
    }

My reasoning is that this would be a huge performance gain when reading large amount of data. But is it possible?

console.log(snapshot.val().alias + " " + snapshot.val().firstName); — Christophe Chenel, Nov 12 '18 at 18:06
Yes, that would filter the result but the entire document would still be returned from the query? I edited my question to be less ambiguous. Or did you mean I can specify what parts of the document I want returned from the query? — Kermit, Nov 13 '18 at 08:09
If you target this var ref = db.ref(`users/${uid}/alias`); you will get only the alias — Christophe Chenel, Nov 13 '18 at 13:43
True, thanks! That would mean two requests, right? 1. ref = db.ref(users/${uid}/alias); 2. ref = db.ref(users/${uid}/firstName); — Kermit, Nov 13 '18 at 16:49
yes but i'm not sure you will gain in performance with that. — Christophe Chenel, Nov 13 '18 at 16:58
Many thanks! I will consider restructuring my data to better meet my needs! — Kermit, Nov 13 '18 at 18:00

score 3 · Accepted Answer · answered Nov 12 '18 at 18:26

You can't select a subset of child nodes for a query. Every query at a node always fetches the entire contents of the node - all the children and their children.

If you want only a subset of children, you have to perform different queries for each one. Alternatively, you can organize your data in such a way that commonly grouped fields are collected together at a particular node.

Firebase realtime database read operation - Only get part of document returned possible?

1 Answers1