title: Ember {embedded: 'always' } on model Vs Serializer
body: <p>I was reading through Ember docs and some examples on working with Embedded Object like JSON in Ember.</p>
  
  <p>I came across the <code>EmbeddedRecordsMixin</code> feature and saw that we can write code like below to tell it is embedded record.</p>
  
  <pre><code>import DS from 'ember-data';
  
  export default DS.RESTSerializer.extend(DS.EmbeddedRecordsMixin, {
    attrs: {
      author: { embedded: 'always' },
    }
  });
  </code></pre>
  
  <p>Qouting the below from Ember page</p>
  
  <p><em>Note that this use of { embedded: 'always' } is unrelated to the { embedded: 'always' } that is defined as an option on DS.attr as part of defining a model while working with the ActiveModelSerializer. Nevertheless, using { embedded: 'always' } as an option to DS.attr is not a valid way to setup embedded records.</em></p>
  
  <p>And i have also seen model written like this.</p>
  
  <pre><code>App.Child = DS.Model.extend({
    name: DS.attr('string'),
    toys: DS.hasMany('toy', {embedded: 'always'}),
  });
  </code></pre>
  
  <p>Where child object has toys object embedded.</p>
  
  <p>Going by the first example, can i write the child serailizer as below?</p>
  
  <pre><code>App.ChildSerializer = DS.RESTSerializer.extend(DS.EmbeddedRecordsMixin, {
    attrs: {
      toys: {embedded: 'always'}
    }
  });
  </code></pre>
  
  <p>Can someone help me understand the difference between these two <code>{embedded: 'always'}</code> and what to use when?</p>
  
  <p>Thanks</p>