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.
Best Answer
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:
Inverse
callInverse
to work correctly)This is covered in the NHibernate documentation for one-to-many's, see the Very Important Note at the end.