Question

Right, I don’t know if I’m barking entirely up the wrong tree here – I’m finding JDO and the Google AppEngine a bit tricky to get the hang of. Anyway, here goes.

I have a class that contains another class as one of it’s internal variables (see player1)

  @PersistenceCapable(identityType = IdentityType.APPLICATION)
   public class JDOGame 
   {    
    @PrimaryKey 
    @Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY) 
    private Long id; 

    @Persistent
    private String map; 

    @Persistent
    private RPCDataPlayer player1;

   // getters, setters, constructors etc...
   }

The class RPCDataPlayer is Serializable and very basic….

public class RPCDataPlayer implements IsSerializable 
{
    public String name;
    public int id;

        // getters & setters & constructors oh my

        public int getId() { return id; }
}

So, my question is…how do I create a query where I can get all the JDOGames that contain an RPCDataPlayer with id = x?

I can’t do a query like…

SELECT FROM JDOGame.class.getName() WHERE player1.getId() == x

…so what techniques or suggestions do people have for this to work?

Thanks in advance.

Answer 1

The Google App Engine Database is not a relational database so you can not do joins. You can persist RPCDataPlayer as a table.

@PersistenceCapable(identityType = IdentityType.APPLICATION)
public class RPCDataPlayer {

    @PrimaryKey
    @Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
    private Key key;

    @Persistent(mappedBy = "player1")
    private List<JDOGame> jdoGames = new ArrayList<JDOGame>();

    @Persistent
    public String name;

    @Persistent
    public int id;

    public int getId() {
        return id;
    }

    public Key getKey() {
        return key;
    }

    public void setJdoGames(List<JDOGame> jdoGames) {
        this.jdoGames = jdoGames;
    }

    public List<JDOGame> getJdoGames() {
        return jdoGames;
    }

}

And then you can just query this like this.

SELECT FROM RPCDataPlayer.class.getName() WHERE id == x

Once you have and instance of RPCDataPlayer you can get get JDOGame by calling:

List<JDOGame> jdoGames = rpcDataPlayer.getJdoGames();