title: RoR ActiveRecord::Base uniq undefined when trying to uniq.pluck(:column_name)
body: <p>so I have a very simple model called Movies.  I'm trying to add a class method which returns a tidy list of the ratings assigned to movies in the database.  It seems I'd want to call <code>Movie.uniq.pluck(:rating)</code></p>
  
  <p>So I've added the method like so:</p>
  
  <pre><code>class Movie &lt; ActiveRecord::Base
  
      def self.all_ratings
          self.uniq.pluck(:rating)
      end
  
  end
  </code></pre>
  
  <p>but it just doesn't work.  When it runs I get: <code>undefined method</code>uniq' for #`...  I've tried including ActiveRecord::Calculations but that doesn't seem to help either.  I also put a breakpoint in after 'def...' to inspect what methods self had and sure enough, uniq wasn't among them...</p>
  
  <p>I'm clearly doing something wrong, but I just don't quite get what it is.</p>
  
  <p>Anyone have any ideas?</p>
  
  <pre><code>ó_ò
  </code></pre>