SQL Server trigger(s) to maintain one IsPrimary/IsDefault row per FK - sql-server

Question

We have a few tables with an IsPrimary column, e.g. many members belonging to an account. The requirement is if an account has one or more members, one and only one of them must have IsPrimary = 1. We want to achieve this using triggers for both maximum assurance of data integrity and ease of use from applications. But due to the batch nature of triggers, I'm struggling to accomplish it and in the most efficient way.
So far I have an insert/delete trigger (see below) that handles inserting a new primary record or deleting the primary record. Where I got stuck is ensuring the first record inserted has IsPrimary=1 and then realizing there could be multiple modifications to the same account in the batch...
Does anyone have any experience or and example with something like this?
ALTER TRIGGER dbo.trg_PrimaryTest_InsertDelete
ON dbo.PrimaryTest
AFTER INSERT,DELETE
AS
BEGIN
-- SET NOCOUNT ON added to prevent extra result sets from
-- interfering with SELECT statements.
--SET NOCOUNT ON;
PRINT 'executing trigger'
--If inserting a primary, set all others to 0
UPDATE PrimaryTest
SET IsPrimary = 0
FROM inserted
INNER JOIN PrimaryTest ON inserted.fk_ID = PrimaryTest.fk_ID
WHERE inserted.IsPrimary = 1
AND PrimaryTest.pk_ID <> inserted.pk_ID
AND PrimaryTest.IsPrimary = 1
--If deleting the primary, set most recent remaining phone to 1
UPDATE PrimaryTest
SET IsPrimary = 1
WHERE PrimaryTest.pk_ID IN (
SELECT TOP 1 PrimaryTest.pk_ID
FROM deleted
INNER JOIN PrimaryTest ON deleted.fk_ID = PrimaryTest.fk_ID
WHERE deleted.IsPrimary = 1
ORDER BY PrimaryTest.CreatedDate DESC
)
PRINT 'trigger executed'
END
GO
Table ddl:
CREATE TABLE [dbo].[PrimaryTest](
[pk_ID] [uniqueidentifier] NOT NULL,
[Value] [nvarchar](50) NULL,
[CreatedDate] [datetime2](7) NOT NULL,
[IsPrimary] [bit] NOT NULL,
[fk_ID] [int] NOT NULL,
CONSTRAINT [PK_PrimaryTest] PRIMARY KEY CLUSTERED
(
[pk_ID] ASC
)
GO
EDIT:
I think this might work or is at least headed in the right direction. I think it may be reading more records than it needs to in the 2nd C.T.E. case though. (Note I added update)
ALTER TRIGGER dbo.trg_PrimaryTest_InsertDelete
ON dbo.PrimaryTest
AFTER INSERT, UPDATE, DELETE
AS
BEGIN
-- SET NOCOUNT ON added to prevent extra result sets from
-- interfering with SELECT statements.
--SET NOCOUNT ON;
PRINT 'executing trigger'
--If setting a new primary, set all others to 0
UPDATE PrimaryTest
SET IsPrimary = 0
FROM inserted
INNER JOIN PrimaryTest ON inserted.fk_ID = PrimaryTest.fk_ID
WHERE inserted.IsPrimary = 1
AND PrimaryTest.pk_ID <> inserted.pk_ID
AND PrimaryTest.IsPrimary = 1
--Set IsPrimary on any modified sets left without an primary
;WITH cte
AS
(
SELECT *, ROW_NUMBER() OVER(PARTITION BY fk_ID ORDER BY CreatedDate DESC) as RowNum
FROM PrimaryTest p1
WHERE fk_ID IN (SELECT FK_ID FROM inserted UNION SELECT FK_ID FROM deleted) --Only look at modified sets
AND NOT EXISTS (SELECT NULL FROM PrimaryTest p2 WHERE p2.fk_ID = p1.fk_ID AND p2.IsPrimary = 1) --Select rows in a set without an IsPrimary=1 record
)
UPDATE cte
SET IsPrimary = 1
WHERE RowNum = 1
PRINT 'trigger executed'
END
GO