title: YAML syntax for unique columns in Propel
body: <p>As far as I know to make a column unique the structure below should be used:</p>
  
  <pre><code>database:
    table_name:
      column_name: { ..., index: unique }
  </code></pre>
  
  <p>I want to make multiple columns unique at once. How am I going to do this?<br />
  Thanks in advance.</p>