NHibernate: Cascading saves to children does not insert

Question

I have a bidirectional association like this:

public class Parent
{
  public int ParentId {get; set;}
  ...other properties
  public IEnumerable<Child> Children {get; set;}
}

public class Child
{
  public int ChildId {get; set;}
  ...other properties
  public Parent Parent {get; set;}
}

The fluent mappings are as follows:

Parent mapping

Id(x => x.ParentId, "PARENT_ID").GeneratedBy.Identity();
.. other mappings
HasMany(x => x.Children).Cascade.All().KeyColumn("PARENT_ID");

Child mapping

Id(x => x.ChildId, "CHILD_ID").GeneratedBy.Identity();
.. other mappings
References(x => x.Parent).Column("PARENT_ID").Cascade.None();

When I execute code like this:

Parent parent = new Parent{ ..set some properties... };
parent.Children = new[] { new Child{ ..set some properties.. };
session.Save(parent);

I get a foreign key constraint violation because NHibernate is not setting the PARENT_ID column of the child record to the new ID when it attempts to insert the child.

Clearly I have requested cascading in the mapping for Parent. NHibernate is trying to save the child, but why is the ID not being set?

Answer 1

You need to make two changes.

1. You need .Inverse() on the HasMany(x => x.Children). See my answer explaining inverse for more information.
2. You also need to add child.Parent = parent; to the code that saves the entities.

In your case, the two relationships are conflicting with one another. parent.Children contains child, which means that NHibernate should persist Child.PARENT_ID as the parent's Id, but child.Parent is null, meaning that NHibernate should persist Child.PARENT_ID as null. Apparently child.Parent won. Actually, what's more likely is that they both won. NHibernate was probably executing two queries similar to these...

/* This is what child.Parent says we should save.
   This query will fail because PARENT_ID is NOT NULL. */
insert into Child (CHILD_ID, PARENT_ID) values (@childId, null);

/* This is what parent.Children says we should save. */
update Child set PARENT_ID = @parentId where CHILD_ID = @childId;

If you make the two changes I recommend above, NHibernate will be able to save this correctly, like so:

insert into Child (CHILD_ID, PARENT_ID) values (@childId, @parentId);