title: Ruby on Rails get all comments
body: <p>I have a (polymorphic) object <code>Comment</code> (which will be used for <code>Vehicle</code> and <code>Review</code> objects). How can I get all <code>comments</code> for <code>User</code>'s <code>Vehicle</code>s: <code>@user.vehicles.comments</code>? It says that method <code>comments</code> is undefined for <code>ActiveRecord::Relation</code>. Any simple way to get it working? Is it many-to-many relation: many vehicles have many comments? Or am I wrong? <code>@user.vehicles.first.comments</code> works properly.  </p>
  
  <p>Relationships between objects (not full):</p>
  
  <pre><code>User 
  has_many Vehicles. 
  
  Vehicle 
  belongs_to User. 
  has_many Comments (as commentable). 
  
  Comment 
  belongs_to Commentable, :polymorphic =&gt; true
  </code></pre>