title: Rails associations: something has many, but only one per other entity
body: <p>I have three models. <code>User</code>, <code>Application</code> and <code>ApplicationVote</code></p>
  <p>The following are true:</p>
  <ol>
  <li>An <code>ApplicationVote</code> belongs to both a <code>User</code> and an <code>Application</code>.</li>
  <li>A <code>User</code> can cast a vote on an <code>Application</code>, thus creating an <code>ApplicationVote</code></li>
  <li>A user can vote on any number of applications, but can only have one vote per application.</li>
  </ol>
  <p>If there would be a way to say <code>User has_many ApplicationVotes but only one per Application</code>... what would it look like?</p>