title: JDO app engine: composite key
body: <p>Is this the right way to define a composite key for
  a class:</p>
  
  <pre><code>@PersistenceCapable
  class Item {
    @PrimaryKey
    long id;
    @PrimaryKey
    String sellerID;
    // ... other fields follow
  }
  </code></pre>
  
  <p>because I want the pair (id, sellerID) to be unique, not just id on its own.</p>
  
  <p>Thus in the app engine datastore I need an entity which incorporates both
  fields somehow into a key (for instance separating them with a dash and
  concatenating them) but I am not sure about how to go about instructing
  app engine to do so via JDO or even via the low-level API.</p>