How to override addTo* and RemoveFrom* GORM/Grails methods?

Question

I tried to override the dynamic method addTo* provided by Grails/GORM but it doesn't seem to work.

Here is the code :

class Match {
    static hasMany = [players: Player, matchPlayers: MatchPlayer]

    void addToPlayers(Player player) {
        if (players.add(player)) {
            MatchPlayer matchPlayer = new MatchPlayer(match: this, player: player)
            matchPlayers.add(matchPlayer)
        }
    }
}
ma = new Match().save()
ma.addToPlayers(player1)

The issue is that when calling addToPlayers I got the following exception:

java.lang.NullPointerException: Cannot invoke method add() on null object

So basically it seems that I have to initialize myself the collection 'players'.

Well, before doing that, I would like to have some insights on GORM mechanism :

1 - What is the default implementation for collections in GORM (I know that it is an implementation of java.util.Set but which one?)

2 - Is it the right thing to do (by overriding the addToPlayers method) ? (My only need is to create/remove an object MatchPlayer each time a player is added/removed in the match instance). If yes, why do I have an exception? Do you have a better design for this?

Thank you.

Answer 1

Your code is similar to the standard Grails approach, see the code for this in org.codehaus.groovy.grails.pluginsDomainClassGrailsPlugin.addRelationshipManagementMethods(). The initial Set implementation is either a HashSet if you don't specify the type of the collection, or a TreeSet if specify a SortedSet, e.g.

static hasMany = [players: Player, matchPlayers: MatchPlayer]
SortedSet players