Fluent NHibernate only cascade delete of association record - nhibernate

Question

We're using NHibernate for our membership system. A User can be a member of many Roles and a Role can have many users.
When a Role or a User is deleted, it should only cascade the delete of the association record ("RoleUsers" table).
Deleting a Role works as expected. However, deleting a User does not delete the association record and as such fails due to a foreign key constraint.
My mapping on the Role side:
HasManyToMany(r => r.Users)
.Access.CamelCaseField()
.Table("RoleUsers")
.ParentKeyColumn("RoleId")
.ChildKeyColumn("UserId")
.AsSet();
Mapping on the User side:
HasManyToMany(u => u.Roles)
.Access.CamelCaseField()
.Table("RoleUsers")
.ParentKeyColumn("UserId")
.ChildKeyColumn("RoleId")
.Inverse(); // we'll add user to role, not role to user
And the failing test:
[Test]
public void Deleting_user_should_not_delete_roles()
{
var user = new User("john#doe.com", "John", "Doe", "Secr3t");
var role = new Role("Admin");
role.AddUser(user);
object id;
using (var txn = Session.BeginTransaction())
{
id = Session.Save(user);
Session.Save(role);
txn.Commit();
}
Session.Clear();
var fromDb = Session.Get<User>(id);
using (var txn = Session.BeginTransaction())
{
Session.Delete(fromDb);
txn.Commit();
}
Session.Query<Role>().Count().ShouldEqual(1);
}
I've tried every combination of Cascade on the user mapping and it either fails or deletes the association record AND the role (not what I want).

Answer 1

Inverse and cascading are two different concepts. And of course, both are supported on <many-to-many> relation. See the documentation 6.8 (scroll down almost to 6.9)
http://nhibernate.info/doc/nh/en/index.html#collections-bidirectional
1) Firstly, we can remove the inverse setting of the User.Roles collection. This will straightens the behavior of the Users`s relations to Roles, and force their deletion before User itself is deleted.
2) Secondly. If the Roles collection of the User is marked as inverse,
HasManyToMany(u => u.Roles)
...
.Inverse(); // we'll add user to role, not role to user
deletion of any User, will never trigger deletion of the pair. That's because we are explicitly saying: the one and only one who care about the relation is the Role.
So if we would like to continue in your scenario:
.Inverse(); // we'll add user to role, not role to user
we should be consitent. "we'll remove user from roles, not roles from user":
[Test]
public void Deleting_user_should_not_delete_roles()
{
var user = new User("john#doe.com", "John", "Doe", "Secr3t");
var role = new Role("Admin");
role.AddUser(user);
object roleId;
object id;
using (var txn = Session.BeginTransaction())
{
id = Session.Save(user);
roleId = Session.Save(role);
txn.Commit();
}
Session.Clear();
// take both from DB
var userFromDb = Session.Get<User>(id);
var roleFromDb = Session.Get<Role>(roleId);
using (var txn = Session.BeginTransaction())
{
// HERE just remove the user from collection
roleFromDb.Users.Remove(userFromDb);
// all relations will be deleted
Session.Save(roleFromDb);
txn.Commit();
}
...
// assertions
// "John's" relation to Role "admin" is deleted
}
NOTE:
3) Cascade was not used in there, but could help to reduce Session.Save(user)...
EDIT: Extending the point 3)
Deletion of the user as Ben Foster noticed in a comment.
3) We should even allow the Role to manage its Users collection completely. Let's introduce the casdace="all" (casdace="all-delete-orhpan" if User without any Role should be deleted at all). Now we can add/update users only via Role object.
the mapping of the Role's Users collection should look like:
HasManyToMany(r => r.Users)
.Access.CamelCaseField()
.Table("RoleUsers")
.ParentKeyColumn("RoleId")
.ChildKeyColumn("UserId")
//.Cascade.All(); // just save or update instance of users
.Cascade.AllDeleteOrphan(); // will even delete User without any Role
.AsSet();
Having inverse and cascade we can adjust the test:
[Test]
public void Deleting_user_should_not_delete_roles()
{
var user = new User("john#doe.com", "John", "Doe", "Secr3t");
var role = new Role("Admin");
role.AddUser(user);
object roleId;
using (var txn = Session.BeginTransaction())
{
// I. no need to save user
roleId = Session.Save(role);
...
And later call this to get rid of a User
...
using (var txn = Session.BeginTransaction())
{
var user = Session.Get<User>(id);
var roles = user.Roles.ToList();
roles.ForEach(role => role.RemoveUser(user))
// II. not only relations, but even the User is deleted
// becuase updated roles triggered delete orhpan
// (no Session.Update() call there)
txn.Commit();
}