NHibernate Remove and Add to ChildCollection ParentClass

Question

I have the following problem.

I have a parent class with a set of child objects.

public class Parent{ int _id; IList<Child> _childs = new List<Child>(); public IList<Child> Childs {get;} } public class Child{ int _id; string _name; Parent _parent; protected Child(){} public Child(Parent parent, string name){ _parent = parent; _name = name; } } 

Classes map to nhibernate for the database, where the tblChild.colName column has a unique index.

  // Parent <bag name="_childs" access="field" cascade="all-delete-orphan" inverse="true"> <key column="ParentId" /> <one-to-many class="Parent" /> </bag> // Child <many-to-one name="_parent" column="ParentId" cascade="none" access="field"> 

My problem: The following code throws an exception due to a unique index:

  Parent parent = new Parent(); Child child1 = new Child(parent, "Child1"); Child child2 = new Child(parent, "Child2"); Child child3 = new Child(parent, "Child3"); parent.Childs.Add(child1); parent.Childs.Add(child2); parent.Childs.Add(child3); parentDao.Save(parent); parentDao.FlushAndClear(); Child child4 = new Child(parent, "Child1"); // Duplicate Name parent.Childs.Remove(child1); parent.Childs.Add(child4); parentDao.Save(parent); parentDao.FlushAndClear(); 

The reason for the exception is that NHibernate first inserts child4, and then removes child1. Why does NHibernate do this? Can someone explain and can help me solve this problem?

Answer 1

The order of the SQL statements is predefined in NHibernate:

SQL statements are issued in the following order

• all insert entities, in the same order, the corresponding objects were saved using ISession.Save ()

• all entity updates

• all removal collections

• all collection item deletes, updates, and inserts

• all collection inserts

• all entity deletions, in the same order the corresponding objects were deleted using ISession.Delete ()

NHibernate thinks that the new instance of the child object is actually a new object. Therefore, he first inserts it, violating the database constraint. This means that you have two options:

1) Discard immediately after removal and before adding a child.

2) Change your design so that instead of Remove / Add you simply edit it. This seems more logical, because it looks like Child is an object that is identified by a name. It is not clear why you are actually adding and removing the same child:

 Child child = parent.GetChildByName("Child1"); child.DoSomething(); 

or like this:

 parent.DoSomethingWithChild("Child1"); 

PS I assume that your Child.Equals implementation uses a name, and in your mapping you have <one-to-many class="Child" /> , and not <one-to-many class="Parent" /> . This is probably just a typo.