title: Mongoid cache doesn't seem to work
body: <p>I am trying to work with mongoid cahce.</p>
  
  <p>First off I am not sure where to put my criteria right now it's sitting in a class variable ( feels like the wrong spot )</p>
  
  <pre><code>class YearDates
    include Mongoid::Document
    @@yeardates_cache =  YearDates.where(year: DateHelper.today.year).cache
  
    def self.current_year
      @@yeardates_cache.first
    end
  </code></pre>
  
  <p>Now i expected to only hit the mongo once when i call YearDates.current_year</p>
  
  <p>However ever time I call the current_year I see in the logs:</p>
  
  <pre><code>MOPED: 127.0.0.1:27017 QUERY        database=unit_test collection=year_dates selector={"$query"=&gt;{"year"=&gt;2013}, "$orderby"=&gt;{:_id=&gt;1}} flags=[] limit=-1 skip=0 batch_size=nil fields=nil 
  </code></pre>
  
  <p>Any ideas?</p>