title: Multiple Foreign Keys in loopback model relation
body: <p>In the loopback model relations section, you can specificity a foreignKey to be referenced in the other model. As far as I know, It will only take one field, but what about I have a composite primary key and want to be referenced in the other model? </p>
  
  <p>Can I do something like?</p>
  
  <pre><code>"foreignKey":["id1","id2"] 
  </code></pre>