NHibernate不支持这个映射当你有一个非空的约束你的外键。如果你删除了这个约束，你会看到NHibernate插入了一个空的PostId的注释，然后用新的Post的Id更新它们。

你需要：

1. 移除非空约束和 逆调用


2. 保留约束，映射关系的另一面（使之成为一个双向关系，并允许 Inverse 正确地工作）

这在一对多的NHibernate文档，最后请参阅非常重要的注意事项。

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.

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.