Update NHibernate object with child collection

Question

I am trying to update an nHibernate object with a child collection using the .Update() method found on a hibernate session. The only thing that I can do with the current setup is add children, I can not modify them or remove them.

For clarification the objects and their mapping are as follows:

public class Parent {
   public virtual int Id { get; set; }
   public virtual string Name { get; set; }
   public virtual ISet<Child> Children { get; set; } = new HashSet<Child>();
}

public class ParentMap: ClassMap<Parent>
{
   public ParentMap()
   {
     Id(x => x.Id);
     Map(x => x.Name);
     HasMany(x => x.Children)
       .AsSet()
       .Inverse()
       .Cascade.AllDeleteOrphan();
   }
}

public class Child {
   public virtual int Id { get; set; }
   public virtual string Name { get; set; }
   public virtual Parent Parent { get; set; }
}

public class ChildMap: ClassMap<Child>
{
   public ChildMap()
   {
       Id(x => x.Id);
       Map(x => x.Name);
       References(x => x.Parent);        
   }
}

When I get changes from my UI layer and try to update the already existing object using:

using (var tx = _session.BeginTransaction())
     _session.Update(newParent);
     tx.Commit();
}

Here newParent is a transient object (obtained from the database in an earlier session andd shown in the UI) containing the same identifier as the object I would like to update, but with changes to the child collection. Somehow using this approach I can only add children, but not modify or remove them.

Where is my mistake?

Answer 1

Most likely what is happening to you, is that instead of modifying the set that NHibernate has instantiated in your Parent entity, you are replacing it all together by a new instance of HashSet .

When you save or get an entity from NHibernate, your Children ISet gets loaded with an instance of a PersistentGenericSet (that implements ISet ) which has the responsibility of helping with this change tracking for your collection.

In short, do not assign to the Children property. In fact, make the setter protected . Just Add() or Remove() or Clear() it as required.