title: 'Add Foreign Key properties' for a One to Zero-to-one relationship
body: <p>I have an entity model which defines a One to Zero-to-one association between two entities. i.e.</p>
  
  <pre><code>A  [0..1]..[1]  B
  A has one reference to B. B can have a reference to A.
  </code></pre>
  
  <p>In defining this association, I want to have the ID of 'B' within 'A'. e.g. a B_ID property. For some reason, the option to do so - 'Add foreign key properties to the 'A' entity' - is disabled. I don't know why this is, and I'm unable to figure out how to do it manually. </p>
  
  <p>If anyone could help, or give me a reason for this, I'd be grateful. </p>