Question

I’m having some trouble with NHibernate, while trying to remove an object describing a relation between rows in two different tables – a many to many relation.

I’ve got Users, CourseInstances and UserRoles, where Users are mapped to certain course instances. Users can be a part of many course instances, and course instances have many users attending.

All this is mapped by Fluent.

User: HasMany(x => x.UserRoles).KeyColumn("UserId");
CourseInstance: HasMany(x => x.UserRoles);
UserRole:
    References(x => x.User).Column("UserId");
    References(x => x.CourseInstance).Column("InstanceId");

The above describes how they are mapped together, its a fairly simple mapping, though each have some additional information but nothing related to each other.

I try to run the following code:

using (var session = factory.OpenSession())
{
    var user = session.Get<NData.User>(userId);
    if (user == null)
        throw new FaultException(new FaultReason("No user with that id found."));
    var instance = session.Get<NData.CourseInstance>(courseInstanceId);
    if (instance == null)
        throw new FaultException(new FaultReason("No course instance with that id found."));
    var userrole = session.CreateQuery(string.Format("from UserRole where User.Id = {0} and CourseInstance.Id = {1} and Role.Role = {2}", userId, courseInstanceId, role)).UniqueResult<NData.UserRole>();
    if (userrole == null)
        throw new FaultException(new FaultReason("That user is not present in that course instance with that role."));
    instance.UserRoles.Remove(userrole);
    user.UserRoles.Remove(userrole);
    session.Delete(userrole);
    session.Update(user);
    session.Update(instance);
    session.Flush();
}

However, Flush throws an exception:

could not delete collection rows: [Giraffe.WebService.NHibernate.Data.User.UserRoles#8][SQL: UPDATE UserRoles SET UserId = null WHERE UserId = @p0 AND Id = @p1]

Why is it trying to UPDATE the role first?

Answer 1

Because you’re doing

user.UserRoles.Remove(userrole);

user is a managed entity that NHibernate will update, just as you’re seeing.