title: Google datastore - querying on key values
body: <p>I have a EntityKind SuggestedInterest. 
  When I populate that with a key "GrpId" and property "suggestedint".</p>
  
  <p>Now, I need the "suggestedint" value for a requested "GrpId"</p>
  
  <p>So, I write the query as:</p>
  
  <pre><code>String findSuggestedInterest(String grpId)
  {
      DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();
      Filter filter = new FilterPredicate(Entity.KEY_RESERVED_PROPERTY,FilterOperator.EQUAL,grpId);
      Query q0 = new Query("SuggestedInterest").setFilter(filter);
      PreparedQuery pq0 = datastore.prepare(q0);
      Entity result = pq0.asSingleEntity();
      return result.getProperty("suggestedint").toString();       
  }
  </code></pre>
  
  <p>When I execute this code I get </p>
  
  <pre><code>java.lang.IllegalArgumentException: __key__ filter value must be a Key
  </code></pre>
  
  <p>The developer docs told to use Entity.KEY_RESERVED_PROPERTY to query on keys, but I guess I misunderstood. What is the correct way to query on key ?</p>