Can't save changes to child tables using LINQ to SQL that have foreign key constraints

Question

I am trying to teach myself LINQ to SQL and have decided to do a small MVC.NET project to get my feet wet. The problem is that I have fallen very early on with the following error.

I am making a golf application so I have setup the database and got my dbml classes made. The database has a course table and a hole table. The hole table references the courseId by its primary key and has a foreigh key constraint on it. So all pretty standard.

I have an MVC action on my controller that allows me to edit the course information (name, and hole info, for example, par and stroke index, etc.)

When I do the save after an edit I get the following error.

"An attempt was made to remove a relationship between a Course and a Hole. However, one of the relationship's foreign keys (Hole.CourseId) cannot be set to null."

I have debugged this and can't see any problems. Is there something I need to do on the database to allow me to edit tables with foreign key constraints?

Here is the action code:

    [HttpPost]
    [ValidateInput(true)]
    public ActionResult Edit(int id, FormCollection collection)
    {
        var courseViewModel = new CourseViewModel { Course = _repository.GetCourse(id) };

        if (courseViewModel.Course == null)
        {
            return View("NotFound", string.Format("Course {0} Not Found", id));
        }

        try
        {
            UpdateModel(courseViewModel);
            _repository.SubmitChanges();

            return RedirectToAction("Index", "Course");
        }
        catch (Exception ex)
        {
            ModelState.AddRuleViolations(courseViewModel.Course.GetRuleViolations());
            ModelState.AddModelError("", ex.Message);

            return View(courseViewModel);
        }
    }

Where _repository looks like this:

    namespace Web.Repository
    {
        public class MyRepository
        {
            private MyDataContext db = new MyDataContext();

            public void SubmitChanges()
            {
                db.SubmitChanges();
            }

            public Course GetCourse(int id)
            {
                return db.Courses.SingleOrDefault(i => i.CourseId == id);
            }
        }
    }

Answer 1

What does UpdateModel do? Assuming it makes changes to the Course member on your view model, you might want to consider a repository method like this:

public void SaveCourse(Course course)
{
  db.Courses.Attach(course);
  db.SubmitChanges();
}