How to re-order List inside Grails entity where order managed by indexColumn

Question

I have an entity like this

class Parent {
        
   //...
   List<Child> children
        
   static mapping = {
      //..
      children joinTable: [name:'ParentChild', key:'ParentID', column:'ChildID'],
         cascade: 'all', lazy: false, indexColumn: [name: 'OrderIndex', type: Byte]
                
      }
}

How can I re-order and save changes to my database?

I have tried the code below:

@Transactional
Parent reOrderChildren(long parentId, CustomChildrenComparator comparator) {
   Parent parent = Parent.findById(parentId)
   parent.children.sort(comparator)
   parent.save()
}

but the result is SqlException

Caused by: com.microsoft.sqlserver.jdbc.SQLServerException: Violation of PRIMARY KEY constraint 'PK_ParentChild'. Cannot insert duplicate key in object 'ParentChild'. The duplicate key value is (15, 1761).

But database does not contain any duplicates. As far as I understand hibernate try to insert new records in ParentChild table instead of update indexColumns of existing ones