title: rails scope and joins
body: <p>I have tried everything i thought would work for this and am turning up nothing. </p>
  
  <p>in rails 3, I need to find all users with a cd player in their car. A car has one user and one radio, and a user belongs to a car, and a radio has many cars.</p>
  
  <p>I am stumbling on how I would perform this search via a scope in the user model. </p>
  
  <pre><code>class User  
    belongs_to :car
  
  class Car  
    belongs_to radio
    has_one :user, :dependent =&gt; destroy
  
  class Radio  
    has_many :cars
  </code></pre>