I'm still cutting my teeth on the Entity Framework (EF) and have just worked out why I have been receiving this kind of exception:

A relationship is being added or deleted from an AssociationSet 'FK_Shift_ShiftStatus'. With cardinality constraints, a corresponding 'Safety_Shift' must also be added or deleted.

Here is the code:

shift.Status = context.ShiftStatuses.First(x => x.Id == Status.ShiftStartedId);

EF Model:

Shift * <=> 1 ShiftStatus

Database:

Shift.ShiftStatusId has a Foreign Key Relationship with ShiftStatus.Id

After searching the Internet, I was none the wiser. I thought that maybe there was something theoretical that I was missing. In fact the only mistake I had made was that I didn't load the Shift Status reference before assigning it:

shift.StatusReference.Load();

shift.Status = context.ShiftStatuses.First(x => x.Id == Status.ShiftStartedId);