Cannot delete in EF4

I am trying to delete an "AttendeeEvent" from the database with EF4, however I am getting the following error:-

The operation failed: The relationship could not be changed because one or more of the foreign-key properties is non-nullable. When a change is made to a relationship, the related foreign-key property is set to a null value. If the foreign-key does not support null values, a new relationship must be defined, the foreign-key property must be assigned another non-null value, or the unrelated object must be deleted.

My code is as follows :-

        public void UnRegisterCurrentUserForEvent(int eventId)
    {
        Attendee attendee = GetOrCreateAttendeeForCurrentUser();
        AttendeeEvent av = at开发者_如何转开发tendee.AttendeeEvents.SingleOrDefault(x => x.EventID == eventId);

        if(av != null)
        {
            attendee.AttendeeEvents.Remove(av);
        }

        this.ObjectContext.SaveChanges();
    }

I tried to change the End 2 On Delete from the properties in the .edmx however when I set to cascade, I am getting an error:-

Error 1 Error 132: cannot have operation specified since its multiplicity is ''. Operations cannot be specified on ends with multiplicity ''

Can you guys help me out

Thanks for your help and time

---

You are only removing the AttendeeEvent from the collection of attendee events for an Attendee. So suppose you have an attendee A and event AV and you remove AV from A. What should happen inside the database? You haven't actually removed AV. You've only said that A should no longer be related to AV. So inside your database, the foreign key from AV to A is set to NULL, which is not allowed by your database model.

The fix is simple: replace the line where you remove the event from the attendee with the following line:

this.ObjectContext.AttendeeEvents.DeleteObject(av);