title: How to define composite primary key in SilverStripe ORM/Dataobject
body: <p>SilverStripe's DataObject gives us the following to start with:</p>
  
  <p>ID - primary key</p>
  
  <p>But how do you define a composite key (primary key composed of 2 or more columns)?  I've searched the documentation and can not find this information anywhere.</p>