Fluent NHibernate one to many uni-directional mapping

Question

I have Post and Comment classes, and they have a one to many relationship where Post has a list of Comments. How can I map this as a uni-directional relationship with Fluent NHibernate, since a comment does not need to know its parent Post? Currently, this is my mapping for Comment:

Id(x => x.Id);
Map(x => x.Body);
References(x => x.User);

and for Post:

Id(x => x.Id);
Map(x => x.Title);
HasMany(x => x.Comments)
 .Inverse()
 .WithKeyColumn("PostId")
 .Cascade.AllDeleteOrphan();

This doesnt work because when I try to save a Post instance with a newly added Comment, the foreign key on Comment (PostId) is left NULL. And of course a comment cannot be saved with a NULL PostId. I've tried removing the .Inverse() clause, but that doesn't work either.

Answer 1

NHibernate doesn't support this mapping when you have a not-null constraint on your foreign key. If you remove that constraint, you'll see that NHibernate inserts the Comments with a null PostId, then updates them with the Id of the new Post.

You either need to:

1. Remove the not-null constraint and the Inverse call
2. Keep the constraint, and map the other side of the relationship (making this a bi-directional relationship, and allowing Inverse to work correctly)

This is covered in the NHibernate documentation for one-to-many's, see the Very Important Note at the end.