title: ActiveModel::Serializer Caching - undefined method `fetch_or_store' for ThreadSafe::Cache
body: <p>I just upgraded my AMS gem to <code>0.10.0.rc1</code> in hopes of getting some better caching functionality. I am trying to cache some AppLabels and the serializer there looks like this:</p>
  
  <pre><code>class AppLabelSerializer &lt; ActiveModel::Serializer
    cache key: 'app_label', expires_in: 3.hours
    attributes :id, :key, :label, :label_plural
  
    def key
      object.app_label_dictionary.key
    end
  
  end
  </code></pre>
  
  <p>And then I hit one of the AppLabel end points with PostMan and I get this error:</p>
  
  <pre><code>NoMethodError (undefined method `fetch_or_store' for #&lt;ThreadSafe::Cache:0x007fcf70821378&gt;):
    app/controllers/v1/app_labels_controller.rb:22:in `auth_index'
  </code></pre>
  
  <p>I checked <code>app_labels_controller.rb</code> on line 22 and it is this method:</p>
  
  <pre><code>def auth_index
    render json: AppLabel.non_event
  end
  </code></pre>
  
  <p>Which is calling an AppLabel scope:</p>
  
  <pre><code>scope :non_event, -&gt; {joins(:app_label_dictionary).where(app_label_dictionary: {event_label: false} )}
  </code></pre>
  
  <p>I'm not familiar enough with AMS caching or Rails' Threadsafe elements to know what's going on here.</p>
  
  <p>Any thoughts?</p>