Some guides to model m:n relationships in grails using lists(for sorting) or not

ManyToManyModelingWithGrails.groovy

//Domain classes... 

class Team {
    String name
    static hasMany = [memberships:Membership]
    static constraints = {}
} 

class Player {
    String name
    static hasMany = [memberships:Membership]
    static constraints = {}
}

//link class with attributes
class Membership {
    boolean active = true
    static belongsTo = [team:Team, player:Player]
    static constraints = {}
}

//Domain classes with lists to define some order on relationships

class Team {
    ...
    List squadMemberships
    static hasMany = [teamMemberships:Membership]
    ...
} 

class Player {
    ...
    List playerMemberships
    static hasMany = [playerMemberships:Membership]
    ...
}

/*
* Mapping name for Membership in Player and Team classes 
* must be different to avoid org.hibernate.MappingException
*/

//to persist:

def player = new Player(name:"Fernando Torres")
player.save()

def team = new Team(name:"Liverpool FC")
team.save()

def membership = new Membership(player:player, team:team, active:true)
player.addToMemberships(membership)
team.addToMemberships(membership)

//credits to http://codedumpblog.blogspot.com/2010/02/grails-many-to-many-with-lists.html