Unable to make my many-to-one foreign key null

aopdeweegh · **Joined:** Mon Apr 18, 2011 7:45 am **Posts:** 2

Hi,

I am using a many-to-one relationship to model a linked list in my database. See subjoined code snippet.

Code:

public class Instruction
{
    ...
    [ManyToOne(Column = "NextInstructionId")]
    public virtual Instruction Next { get; set; }
    ....
}

Now, create a linked list of instruction works fine. The last instruction get a NULL value in the NextInstructionId column. But, when I remove the last instruction from my list, then the previous one becomes last. So, I set the Next property to null. But, on update, the database engine complains that the foreign key constraint is not met, because NextInstructionId is null.

Does anyone know what might happen here?

Thanks,
Alexander.

tkn · **Joined:** Mon Apr 18, 2011 10:51 am **Posts:** 2

Well you can not insert null into a ForeignKey column with a constraint that awaits a valid ForeignKey.

aopdeweegh · **Joined:** Mon Apr 18, 2011 7:45 am **Posts:** 2

tkn wrote:

Well you can not insert null into a ForeignKey column with a constraint that awaits a valid ForeignKey.

Sure, but it is possible to have a nullable foreign key field. Even more, when initially inserting a null value in the foreign key field, the database finds it ok. So, why does an update from a valid value to a null value does not work?

niks23 · **Joined:** Mon Apr 04, 2011 3:31 am **Posts:** 41

hi

Check this Two Link Related to your problem

Link 1

Link 2