title: QSqlRelationalTableModel additional related columns
body: <p>I have a <code>QSqlRelationalTableModel</code> setup with a <code>QSqlRelation</code> to another table working great. Now, I would like to lookup additional columns for that row on the related table. How do I accomplish this?</p>
  
  <p>Example tables:</p>
  
  <pre><code>city ('id', 'name', 'state_id')
  state ('id', 'name', 'capital')
  </code></pre>
  
  <p>what I have so far:</p>
  
  <pre><code>model = QtSql.QSqlRelationalTableModel()
  model.setTable('city')
  model.setRelation(2, QtSql.QSqlRelation("state", "id", "name"))
  model.select()
  </code></pre>
  
  <p>Which would give me a row like:</p>
  
  <pre><code>| 1 | 'San Francisco' | 'California' |
  </code></pre>
  
  <p>Now I would like to lookup the capital in the state model, from the city table. How do I do this? My understanding of <code>setRelation</code> is that it takes the id from the main_table at column X and replaces it with the mapped column, but I don't want to alter the state table. I've looked into the <code>QSqlRecord</code> and <code>QSqlRelation</code>, but I seem to be missing something.</p>
  
  <p>Thanks!</p>