title: What is the correct way to set a custom primaryKey attribute in the latest build of ember-data?
body: <p>I've seen a few ways to do this but I'm curious what the "most correct" approach is using the latest build (master even) of ember-data rev 12</p>
  
  <p>Option #1 (would appear the most obvious way)</p>
  
  <pre><code>App.Post = DS.Model.extend({
      primaryKey: '_id',
      _id: DS.attr('string')
  });
  </code></pre>
  
  <p>Option #2 (map it using the adapter by type)</p>
  
  <pre><code>App.Adapter.map('App.Post', {
      primaryKey: '_id'
  });
  </code></pre>
  
  <p>Option #3 (hard code it in the serializer -assumes every model has the same custom pk)</p>
  
  <pre><code>App.MySerializer = DS.RESTSerializer.extend({
      primaryKey: function(type) {
          return '_id';
      }
  });
  </code></pre>