EF 6.1.1-无法更改关系，因为一个或多个外键属性不可为空

Question

I am getting this error when trying to remove relations between entities and creating new ones.

public partial class Course
{
    public int Course_ID { get; set; }
    /* CODE*/
    public virtual ICollection<Student> Students { get; set; }
}

public partial class Student
{
    public int Student_ID { get; set; }
    /* CODE*/
    public virtual ICollection<Course> Courses { get; set; }
    public virtual ICollection<Book> Books { get; set; }
}

public partial class Book
{
    public int Book_ID { get; set; }
    public int Student_ID { get; set; }
    /* CODE*/
    public virtual Student Student { get; set; }
}

The relations is as follows:

• Course * -- * Student (Student and Course have a many-to-many relation so the table Students_Courses was created during normalization - a student can engage multiple courses and a course has multiple students)
• Student 1--* Book (Student has 1-to-many relation with Book - a student has many books and a book belongs to one student)

I'm trying to update a Student Entity like this:

    public bool UpdateEntity(Entities.Student student)
    {
        using (var context = new Entities())
        {
            var record = context.Students
                                .Include("Courses")
                                .Include("Books")
                                .FirstOrDefault(c => c.Student_ID == student.Student_ID );

            // record.Courses.ToList().ForEach(s => record.Courses.Remove(s));
            // record.Books.ToList().ForEach(t => record.Books.Remove(t));

            record.Courses.Clear();
            record.Books.Clear();

            record.Courses= student.Courses;
            record.Books= student.Books;
            context.SaveChanges();

            return true;
        }
    }

A relation between a student and a course should be removed without removing the actual course. When updating a relation between a book and a student, the book should be deleted as well.

EDIT updated to make it clearer

Answer 1

When you want to remove items in a 1-many collection from the database you have to mark each item for delete, eg like so:

foreach (var book in record.Books)
{
    context.Books.Remove(book);
}
context.SaveChanges();

It's a nuisance sometimes. When you Clear a collection navigation property, EF errs on the side of caution and tries to only break the association without deleting the items, ie its tries to nullify the foreign keys. However, EF does know that the FK field is not nullable in the database, so it seems to me that EF should be able to figure out that in this case it's the user's intention to delete items, rather than to orphan them. But too bad, it doesn't.

I used to work with NHibernate and IIRC, NHibernate did delete child items in similar cases, even firing a one-shot delete ( delete from Child where Child.ParentId = ... ). EF fires a delete statement for each child separately.