title: Active Model Serializers data attributes from associations are not loading
body: <p>I am trying to load the association as follows:</p>
  
  <p>This is my controller which is trying to render the json.</p>
  
  <pre><code>#app/controllers/store/products_controller.rb
    def customize
      @option_types = OptionType.all
      respond_to do |format|
        format.json { render :json =&gt; @option_types}
      end
    end
  </code></pre>
  
  <p>This is the serializer for above object</p>
  
  <pre><code>#app/serializers/option_type_serializer.rb
  class OptionTypeSerializer &lt; ActiveModel::Serializer
    attributes :id, :key, :customizable, :name
    has_many :option_values
  end
  </code></pre>
  
  <p>option_value <code>belong_to</code> option_type</p>
  
  <pre><code>#app/serializers/option_value_serializer.rb
  class OptionValueSerializer &lt; ActiveModel::Serializer
    attributes :id, :key, :option_type_id, :percentage_price_impact, :fixed_price_impact, :absolute_price_impact, :name
  end
  </code></pre>
  
  <p>This is generating the following JSON. Although a number of other params are specified in the option_value_serializer.rb, only id and type appear in the generated JSON</p>
  
  <pre><code>// http://0.0.0.0:3000/products/810/customize.json
  {
    "data": [
      {
        "id": "1",
        "type": "option-types",
        "attributes": {
          "key": "fabric",
          "customizable": true,
          "name": "FABRIC"
        },
        "relationships": {
          "option-values": {
            "data": [
              {
                "id": "1",
                "type": "option-values"
              },
              {
                "id": "2",
                "type": "option-values"
              }
            ]
          }
        }
      },
      {
        "id": "2",
        "type": "option-types",
        "attributes": {
          "key": "cuff-type",
          "customizable": false,
          "name": "CUFF TYPE"
        },
        "relationships": {
          "option-values": {
            "data": [
  
            ]
          }
        }
      },
      {
        "id": "3",
        "type": "option-types",
        "attributes": {
          "key": "vents",
          "customizable": false,
          "name": "VENTS"
        },
        "relationships": {
          "option-values": {
            "data": [
  
            ]
          }
        }
      }
    ]
  }
  </code></pre>