title: Validate uniqueness of multiple columns
body: <p>Is there a rails-way way to validate that an actual record is unique and not just a column?  For example, a friendship model / table should not be able to have multiple identical records like:</p>
  
  <pre><code>user_id: 10 | friend_id: 20
  user_id: 10 | friend_id: 20
  </code></pre>