Set multiple data with Ember and make it persist on Firebase

I am trying to set data from two models (that has hasMany & belongsTo relationship) and save them to firebase.

'list' data ends up being saved to firebase but not user data.

I think I'm doing something wrong at step 3. I'd appreciate your help!

import Ember from 'ember';

export default Ember.Route.extend({
    model: function() {
        return this.store.find('list');
    },
actions: {
    createList: function() {
        var newListTitle = this.controllerFor('lists').get('newListTitle');
        var username = this.get('session.user.displayName');
        alert(this.get('session.user.displayName'));

        if (Ember.isBlank(newListTitle)) { return false; }

    //1
        var list = this.store.createRecord('list', {
            title: newListTitle,
            user: username,
        });

    //2
        //this.controllerFor('lists').set('newListTitle', '');

        this.controllerFor('lists').setProperties({
            title: newListTitle,
            user: username,
        });
        alert('username');

        var _this = this;

    //3
    list.get('users').then(function(users) {
        users.addObject(user);
            list.save().then(function() {
                user.save().then(function(list) {
            _this.transitionTo('lists.show', list); //4
                });
            });
    });

/*
//this was my old code when I only had one property to set
        list.save().then(function(list) {
            _this.transitionTo('lists.show', list); //4
        });
*/

        }
    }
});