NHibernate One-to-Many - why is it updating child with null foreign key?

I am troubleshooting code that is attempting to update a disconnected entity that has uninitialized references to child entities. The intent is to update only the properties on Parent without loading children.

HasMany(x => x.ChildEntities)
  .KeyColumn("ChildEntityId")
  .Table("ChildEntity")
  .Not.LazyLoad()
  .Inverse()
  .Cascade.All().AsBag();

When Session.Update(parent) is called, two update statements are executed. The first updates the parent object as expected.

update Parent set ... where ParentId = 12345

The second update confuses me...

update ChildEntity set ParentId = null where ParentId = 12345

Why is NHibernate issuing that second SQL statement? I realize that ChildEntities is uninitialized and that NHibernate is probably trying to enforce the state of Parent but I can't 开发者_JS百科seem to tweak the mapping to not make this second update. I've tried Merge, lazy loading, various cascade options, etc. without success. The only connected entity in the session when it tries to commit is Parent.

Note that I typically approach this by retrieving the entity with lazy loading enabled and then mapping from the disconnected object (DTO or entity) to the connected entity before letting NHibernate persist to the database. I want to understand why the above isn't working before I suggest an alternative approach.

This was annoying.

A quick search through the NHibernate source for "could not delete collection" showed up in a block that could only execute if !isInverse (AbstractCollectionPersister.cs). That drew my attention because the mapping code was explicitly setting Inverse on that collection.

If Inverse is false and the collection is empty, NH executes an update on the child table setting the foreign key to null where the foreign key equals the parent id.

Fluent is configured to auto-map all entities in a given namespace. The assumption was that anything with a manual mapping would be ignored by auto mapping. A quick check of the hbm.xml files produced by Fluent confirmed that Inverse was not being set. I added Parent to the list of entities that were explicitly excluded from Auto Mapping and everything started working.

.IgnoreBase<Parent>()