title: :through relations in rails: query by attribute in related table
body: <p>I have the following Rails definition:</p>
  
  <pre><code>Class Book
    string title
    has_many readings
    has_many users, :through =&gt; :readings
  
  Class Reader
    int rating
    belongs_to :book
    belongs_to :user
  
  Class User
    has_many readings
    has_many books, through =&gt; :readings
  </code></pre>
  
  <p>No I want to query like this:</p>
  
  <p>Give me all readings ratings for user A that have book.title = "test"</p>
  
  <p>Could you help me? Thanks!</p>