title: Rails :counter_cache with custom query association
body: <p>I have a model like this:</p>
  
  <pre><code>class Vote
    belongs_to :content, counter_cache: true
  end
  
  class Content
    has_many :votes
    has_many :votes_up, -&gt; { where(positive: true) }, class_name: 'Vote'
    has_many :votes_down, -&gt; { where(positive: false) }, class_name: 'Vote'
  end
  </code></pre>
  
  <p>If I create a proper migration, when I execute <code>mycontent.votes_count</code>, it will not make a query.</p>
  
  <p>But what about <code>mycontent.votes_up.count</code> and <code>mycontent.votes_down.count</code>? Can I cache them in the same (simple) way?</p>