Add mappedBy attribute to bidirectional relationships

[David Coles]

Relationships that should be accessible from both sides should include a mappedBy attribute (see: http://code.google.com/appengine/docs/java/datastore/jdo/relationships.html ).

In particular, most of our one-to-many relations will need to be bidirectional to allow traversal from either object.

[Matt Giuca (mgiuca)]

Prageeth raises the point that we put the "mappedBy" attribute on the singleton variable, not the collection, whereas in the Google examples, it is on the collection.

For example, in http://bazaar.launchpad.net/~mugle-dev/mugle/trunk/revision/28 we have got:

Achievement.java:
@Persistent(mappedBy="achievements")
private Game game;

Game.java:
@Persistent
private Set<Achievement> achievements;

It probably should be the other way around. I'm not sure if it makes a difference. Also I'm not sure why you can't put the mappedBy on a many-to-many relation. Perhaps mappedBy only works if the thing you are mapping (the value of mappedBy="") is a singleton.

[Prageeth Silva (prageethsilva)]

I've set all the "mappedBy" attributes on the singleton. For example:

Achievement.java:
@Persistent
private Game game;

Game.java:
@Persistent(mappedBy="game")
private Set<Achievement> achievements;

In my opinion this makes more sense; for every entry in the set "achievements", make sure it's field "game" is linked to "this" instance of the Game class.

[Matt Giuca (mgiuca)]

I think you mean you've set them all on the collection.

I don't think mappedBy means "for every entry in the set "achievements", make sure it's field "game" is linked to "this" instance of the Game class." Rather the other way around:

For every Achievement, if the 'game' is set to a particular Game object, make sure its 'achievements' set contains this Achievement." Am I correct about this?