How to change parent in GORM? - grails

Question

I do not have too much expierence in Grails, so maybe I do not understand hasMany and belongsTo relations in GORM.
Say, I have 2 classes Parent.groovy and Child.groovy
class Parent {
String name
List childen = new ArrayList()
static hasMany = [children: Child]
}
class Child {
String name
static belongsTo = [parent: Parent]
}
Person person1 = new Person(name: "Person1")
Child child1 = new Child(name: "child1")
Child child2 = new Child(name: "child2")
person1.addToChildren(child1).save(flush: true)
person1.addToChildren(child2).save(flush: true)
Person person2 = new Person(name: "Person2").save(flush: true)
Now I want to change a parent of a child
child1.parent = parent2 // no effect
child1.save(flush: true)
In controller it is possible
Child child1 = Child.get(1)
bindData(child1, [parent: [id: 2]])
child1.save(flush: true)
But now there is null in movie1.children, in DB I can see that parent_id has changed to 2
Note: In Active Record (Rails) it is easy
child1.parent_id = 2
Maybe I do not need to use such relation if I want to change parent?
Maybe there is another way to do it?

Answer 1

After some investigation I understood why there is a null in collection.
At the very beginning there are such values in Child table
id | name | parent_id | parent_idx
1 | child1 | 1 | 0
2 | child2 | 1 | 1
After
bindData(child1, [parent: [id: 2]])
We have
id | name | parent_id | parent_idx
1 | child1 | 2 | 0
2 | child2 | 1 | 1
So now parent2 has child1 with parent_idx 0 - it is ok. But parent1 has child2 with parent_idx = 1 (without 0). So we changed parent_id value without changing parent_idx.
I wish there was a possibility to change index in collection too.
So the conclusion is:
I we want to change parent we should not use List collection, so we will not have idx column and no problems