Trying to understand DS.Model

model.js

window.Models = EM.Namespace.create();

DS.RESTAdapter.map('Models.model', {
  primaryKey: 'idx'
})

Models.model = DS.Model.extend({
  init: function () {
    this.super();
    Em.run.next({
      this.set('idx', this.get('key1') + '|' + this.get('key2'));
    });
  },
  
  idx: DS.attr('string'), // key1 + "|" + key2
  key1: DS.attr('string'),
  key2: DS.attr('string')
})