title: How to Map Reference to Object with Composite Key ID
body: <p>Suppose I have the following:</p>
  
  <pre><code>class ObjectA implements Serializable {
      Foo foo
      String objectType
  
      static mapping = {
          version false
          id composite: ['foo', 'objectType']
          foo column: 'foo'
          objectType column: 'objecttype'
      }
  }
  </code></pre>
  
  <p>Now, I need to reference this object from another Domain.</p>
  
  <pre><code>class ObjectB {
      ObjectA objectA
  
      columns {
         objectA{
            column name: 'foo'
            column name: 'objecttype'
         }
      }
  }
  </code></pre>
  
  <p>When I try and load an ObjectB, I get the following:</p>
  
  <pre><code>Cannot treat multi-column property as a single-column property
  </code></pre>
  
  <p>How should I be mapping this?</p>