title: Save only one field of an entity to another table
body: <p>How do I only save a certain field/property of an Entity for another entity?</p>
  <p>I have a <code>Key</code> entity with id and name properties.</p>
  <p>In my <code>Door</code> entity, I would like to accept a <code>Key</code> but only save in the database the <code>id</code></p>
  <p>Is that possible?</p>