Firebase v3 Query by Grandchild

Question

When registering new email/password type users, I need to make user that the displayName that they enter does not already exist in my Realtime Database before

Answer 1

Just thought I'd share my somewhat fleshed out solution. Call with myApp.displayNameExists('Joe').

var myApp = (function() {
    var pub = {};
    pub.displayNameExists = function(name) {
            var users = firebase.database().ref('users');
            var duplicate = users.orderByChild('displayName').equalTo(name);
            duplicate.once('value').then(function(snap) {
                if (snap.val()) {
                    console.log('found. ask for new display name');
                } else {
                    console.log('name unique.  ok to write this user to db');
                }
            }, function(error) {
                // The Promise was rejected.
                console.error(error);
            });
        }
        //API
    return pub;
}());

Answer 2

You'd use Firebase queries for that:

var users = firebase.database().ref('users');
var joes = users.orderByChild('displayName').equalTo('Joe');
joes.once('value', function(snapshot) {
  console.log('A Joe does '+(snapshot.exists()?'':'not ')+' exist')
});

Don't forget to define an index on users:

{
  "rules": {
    "users": {
      ".indexOn": "displayName"
    }
  }
}