Fluent NHibernate Generated.Insert does not work as expected

I want to auto generate a number for an开发者_StackOverflow中文版 entity from the database with the following mapping:

Map(m => m.No).Not.Nullable().ReadOnly().Unique().Generated.Insert();

Why is no value generated for the property "No" of type int?

This is not an identity column.

You have it marked ReadOnly()? So it's not going to insert or update the value.