title: grails hasOne but does not belong to
body: <p>I'm working on a project and have an <code>Invoice</code> domain class that currently <code>hasOne=[billingAddress:Address]</code>  When I try to launch my server, I get the following error:</p>
  
  <pre><code>hasOne property [Invoice.billingAddress] is not bidirectional. Specify the other side of the relationship!
  </code></pre>
  
  <p>I don't want to assign the other side of the relationship though... Invoices have a billing address but Addresses do not belong to Invoices.  Addresses belong to Users!!!</p>
  
  <p>What is the proper way to handle this situation?</p>