Allow self-referential foreign keys to be null from an alter-table I am trying to establish a hierarchical relationship between rows in an existing table.  There are two fields of interest: a not-nullable primary key field id, and a second nullable field parentId.

Both these fields are pre-existing, and the normal case is that parentId will be null. This seems to be a normal use case, and in fact we have some old tables using the same pattern.

I am trying to use fluentMigrator to add a foreign key constraint after the fact on these rows:

Create.ForeignKey("C", "ParentId", "C", "CId").OnUpdate(Rule.None);


It is failing with the error:


  The ALTER TABLE statement conflicted with the FOREIGN KEY SAME TABLE constraint "FK_C_ParentId". The conflict occurred in database "R", table "dbo.C", column 'CId'.


This is not a problem with illegal values, as I did a bulk update on the test system, and set all C.ParentId = null, and am still getting the same error.

Thoughts?
If there are no illegal values in C.ParentID, then I am not clear what the problem might be.  Is this Microsoft SQL Server?

I created a test case as follows:

Create Table dbo.Hier
(Id  INT IDENTITY Primary KEy,
 ParID INT NULL,
 Other varchar(10));
GO
 INSERT INTO dbo.Hier (ParID, Other) VALUES (NULL, 'Able');
 INSERT INTO dbo.Hier (ParID, Other) VALUES (1, 'Baker');
 INSERT INTO dbo.Hier (ParID, Other) VALUES (NULL, 'Charlie');
GO
ALTER TABLE dbo.Hier ADD 
      CONSTRAINT FK_Parent
      FOREIGN    KEY (ParID)
      REFERENCES Hier(ID);
GO
INSERT INTO dbo.Hier (ParID, Other) VALUES (3, 'Delta');
SELECT * FROM dbo.Hier;
GO
DROP TABLE dbo.Hier;
GO


Seems to work fine. If you are on a different SQL Server it might have different behavior.  Or fluentMigrator may have a problem.