Grails hasMany Delete

Suppose the following:
class Group{
    String name
    static hasMany = [users:User]
}

class User{
    String name
}
This means, you have lots of Users for some Group, but also a User might have other Groups, so can't use a "belongsTo". To delete a Group is really easy:
def b = Group.get(0)
b.delete(flush:true)
On the other hand, you might want to delete a User, but not the Group:
def b = User.get(0)
b.delete(flush:true)
This will throw a lot of exceptions from Hibernate and JDBC. How are you supposed to do it? Well, first of all, you'll need a way to get a Group for a given User, but since "users" is transitive, you can't do something like Group.findByUser(user) so go ahead and add the following code to your Group class:
static Group findByUser(User user){
    def criteria = Group.createCriteria()
    def result = criteria.get {
        users {
            idEq(user.id)
        }
    }
    return result
}
Now, go to your UserController delete method and add the following
if (userInstance) {
    try {
        AlertGroup g = Group.findByUser(userInstance)
        if(g){
            g.users.remove(userInstance)
        }
        userInstance.delete(flush: true)
...
The clear call will empty the references in the current object so you don't get any trouble.