Delete all child records from record

Question

I have a teacher that has a many-to-many relationship with students . Now, in the Edit Teacher method I want to first remove all the students he has, and then add the students that are selected.

This is my code:

[HttpPost]
[ValidateAntiForgeryToken]
public ActionResult Edit([Bind(Include = "Id,Name")] Teacher teacher, FormCollection form)
{
    if (ModelState.IsValid)
    {
        teacher.Students.Clear();

        var students = form["Students"];
        foreach (string id in students.Split(','))
        {
                teacher.Students.Add(db.Students.Find(int.Parse(id)));
        }

        db.Entry(teacher).State = EntityState.Modified;
        db.saveChanges();
    }
}

Except the problem is, all the Teacher-Student relationships are not being removed. What am I doing wrong?

Answer 1

Clear() is not working as you think... EF is a bit pants when doing 'batch' delete... you have to iterate over them and remove one by one. eg You need something like...

foreach (var student in teacher.Students.ToList())
{
    db.Remove(student);
}

In this example you need ToList() to create a copy because most of the collection classes don't like it when you delete items from them during enumeration.

Edit: To get values / entities from database do something like

List<Students> students = (from stu in db.Students
                           where stu.TeacherId = teacher.Id
                           select stu).ToList();

Then iterate over these and remove as above, eg

foreach (var student in students)
{
    db.Remove(student);
}