Fluent NHibernate One-To-Many Cascade Delete if Many-Side is not referenced anywhere else

Question

I have a working solution but would be interested to know if there is a way to achieve this through fluent mapping..

For simplicity, I will use a illustrative example:

class Tag {
    string name;
    IList<Book> books;
}

class Book {
    string title;
    Tag primaryTag;
}

There is a business case, where Books are deleted and right now, I query the db to check if any other book references the current tag as primary. If not, I delete the book and after that, I delete the tag because it is not used anywhere else. If the tag is stil used, I only delete the book.

Now it's your turn... do you know a way to achieve this using mappings? I tried the following:

BookMap : ClassMap<Book> {
    ...
    References(x => x.primaryTag)
    .Cascade.All()  //the collection in TagMap is set to "inverse"
}

But not surprisingly, it throws a foreign key constraint error when the tag is used in other books.

Regards, Martin

Answer 1

There's no way to do that. NHIbernate is mimicking what you can do in Sql Server config with cascade-deletes. There's no way to go up to a parent and delete "orphans" without using triggers in Sql Server.

There's a way to mimic triggers in NHibernate using "Interceptors" - a way to listen for CRUD on specific Entities and then perform actions. But really it's an anti-pattern since you may as well add the same code to the method that removes the Tag (rather in some hidden/obscure approach like the following, which is usefull for cross-cutting concerns such as Auditing).

This is a really nice article on how to do it (but there's loads out there just google "NHibernate Interceptors").

I'd make sure to use Session.Delete(entity) to ensure deleted entities are removed from Session (for sanity) rather than