title: Rails 3: how to generate custom error message from failed validation
body: <p>I'm using</p>
  
  <pre><code>validates :feed_id, presence: true, uniqueness: true
  </code></pre>
  
  <p>How should I be generating a custom error message to specify that the user has already subscribed to this feed (the feed_id) field is a duplicate</p>
  
  <p>I know I can just do validate_uniqueness_of but it would clutter up the code unnecessarily. How do I pass a specific error message if uniqueness validation fails??</p>