Objectify/GAE - a concrete denormalization issue

Question

I'm using Google App Engine and Objectify 3.1 and slowly learning about denormalization and designing entities based on their usage, but I'm still struggling with some aspects.

I'm currently building a system in which a User entity can participate in a Game entity and need to be able to find all games a user participates in. The way I see it, there's two basic solutions:

Solution 1)

Store a list of user keys (participantKeys) on the game and find the games a member participates in like this:

List<Key<User>> userList = new ArrayList<Key<User>>();
userList.add(new Key<User>(User.class, myUserId));
Collection<Game> games = ofy().query(Game.class).filter("participantKeys in" , userList).list();

Solution 2)

In addition to storing a participant list on the game entity, also store a list of games the user has participated in on the user entity and find the games like this:

User myUser = userDao.getUser(myUserId);
Collection<Game> games = user.getParticipatedGameKeys();

Solution 1 would become pretty slow once there's a lot of games in the system.

Solution 2 would make finding the games faster, but I'll need to constantly keep the list updated as users join and leave games.

The list of games would also become large once a user has been using the system for a long time. I only want to return all games the user is currently participating in, so that would require traversing the list and excluding "historical" games.

Am I missing a more elegant solution? Neither of the above seem very attractive.

Any and all help is greatly appreciated!

Edit:

I decided to try something like Mikl suggested after thinking about alternatives for a long time.. so it's good to hear a solution that's very pretty much exactly like it :-)

I have created a GameParticipation entity which contains a link to the game, a link to the user and all other information that I need to be able to get.

Every time a game is joined, I update the GameParticipation entity to reflect the current state of the game. When a game is left, I delete the entity. Also, when a game is changed, I update all related GameParticipation entities.

I've done a little performance testing and it seems to work reasonably well!

Mikl Mikl · Accepted Answer · 2012-06-28T08:08:58

you can also have an another entity I don't know how you can call it (UserGame?) , but in this entity you will store the game key, the user key and also some information you want to access to, for instance the user name, the game name and so on. Then when the user enter a game you will create this entity.

With this entity you can easily retrieve the games a member participates in and also all the users that partipate to a game.

The inconvenient with this method is that, if a user property or a game property change you need to update also the information you stored in the USerGame entity like the userName.

I don't know if it's a good solution but it should work.

Objectify/GAE - a concrete denormalization issue

1 Answers