NHibernate - how to delete an item from many-to-many relationship?

Question

I've following mapping for two tables having a Many-to-Many relationship between them. How do I delete an entry from the mapping table, which is 'ProjectUser' in my case?

public ProjectMap()
{
    Id(x => x.Id);
    Map(x => x.ProjectName);
    Map(x => x.Description);
    References<User>(x => x.Owner);
    HasManyToMany(x => x.Users)
        .Cascade.SaveUpdate()
        .Table("ProjectUser")
        .Not.LazyLoad();
}


public UserMap()
{
    Id(x => x.Id);
    Map(x => x.FirstName);
    Map(x => x.LastName);
    Map(x => x.UserName);
    HasManyToMany(x => x.Projects)
        .Cascade.SaveUpdate()
        .Inverse()
        .Table("ProjectUser")
        .Not.LazyLoad();
}

EDIT: changed Cascade to SaveUpdate as suggested in answers. Here is the code I use to commit data to SQLite database.

using (var trans = session.BeginTransaction())
{
    var existingUsers = project.Users.ToList();
    foreach (var item in existingUsers)
    {
        if (selectedUsers.Count(x => x.Id == item.Id) == 0)
            project.Users.RemoveAt(project.Users.IndexOf(item));
    }
    session.SaveOrUpdate(project); // This fixed the issue
    session.Flush();
    foreach (var item in selectedUsers)
    {
        if (project.Users.Count(x => x.Id == item.Id) == 0)
        {
            project.AddUser(session.Get<User>(item.Id));
        }
    }
    session.SaveOrUpdate(project);
    session.Flush();
    trans.Commit();
}

// Add user code in Project class
public virtual void AddUser(User userToAdd)
{
    if (this.Users == null)
        this.Users = new List<User>();
    userToAdd.Projects.Add(this);
    this.Users.Add(userToAdd);
}

Whenever I try to save/update I'm getting the following error:

a different object with the same identifier value was already associated with the session: 10, of entity: Models.Project

EDIT2: Should use session.SaveOrUpdate(project) and session.Flush() to avoid error stated above.

Cole W Cole W · Accepted Answer · 2012-02-16T18:56:34

If you only want to remove the ProjectUser entry and not actually delete the entity on the other side you need to change from Cascade.All() to Cascade.SaveUpdate().

Currently if you removed a user from a project and saved the project it would delete the ProjectUser entry and the User object.

NHibernate - how to delete an item from many-to-many relationship?

3 Answers