How to update a hierarchyid column - sql-server

Question

I have a table called items which has a parent/child relation that I am converting to a hierarchyid.
I have followed this tutorial to do so.
All the steps of the tutorial works, except the final update statement.
I get the error message:
Implicit conversion from data type hierarchyid to nvarchar(max) is not allowed. Use the CONVERT function to run this query.
But this makes no sense. The field I am updating is a hierarchyid, not an nvarchar(nax). So I don't see where an nvarchar(max) field is involved.
drop table #children
CREATE TABLE #Children
(
ID int,
TenantId int,
ParentID int,
Num int
);
GO
CREATE CLUSTERED INDEX tmpind ON #Children(TenantId, ParentID, ID);
GO
INSERT #Children (ID, TenantId,ParentID, Num)
SELECT ID, TenantId, ParentID,
ROW_NUMBER() OVER (PARTITION BY TenantId, ParentID ORDER BY ParentId)
FROM Items
GO
SELECT * FROM #Children ORDER BY TenantId, ParentID, Num
GO
WITH paths(path, ID, ParentId, TenantId)
AS (
-- This section provides the value for the root of the hierarchy
SELECT hierarchyid::GetRoot() AS OrgNode, ID, ParentId, TenantId
FROM #Children AS C
WHERE ParentId IS NULL
UNION ALL
-- This section provides values for all nodes except the root
SELECT
CAST(p.path.ToString() + CAST(C.Num AS varchar(30)) + '/' AS hierarchyid),
C.ID , C.ParentId, C.TenantId
FROM #Children AS C
JOIN paths AS p
ON C.ParentID = P.ID
)
-- This select statement runs just fine and shows expected data.
--Select i.Id as ItemId, p.path, p.path.ToString() as LogicalNode, p.Id, p.ParentId, p.TenantId from Paths P
--join Items I on p.Id = i.Id
--order by P.TenantId, P.path
--Note that I have tried using the convert function, but it still fails with the same error message.
UPDATE I Set OrgNode = Convert(hierarchyid, P.path)
FROM Items I
JOIN Paths AS P
ON I.ID = P.ID
GO
EDIT
Strangely, this DBFiddle works.

Answer 1

It looks like column OrgNode is not of type hierachyid. You could use ToString()
UPDATE I Set OrgNode = P.path.ToString()
FROM Items I
JOIN Paths AS P
ON I.ID = P.ID
or alter table Items and change column type.

Answer 2

It looks like you solved your problem, but I would suggest saving the conversion to hierarchyid to the end. Like this:
WITH paths(path, ID, ParentId, TenantId)
AS (
-- This section provides the value for the root of the hierarchy
SELECT cast('/' as varchar(max)) AS OrgNode, ID, ParentId, TenantId
FROM #Children AS C
WHERE ParentId IS NULL
UNION ALL
-- This section provides values for all nodes except the root
SELECT
CAST(concat(p.path.ToString(), C.Num, '/') AS varchar(max)),
C.ID , C.ParentId, C.TenantId
FROM #Children AS C
JOIN paths AS p
ON C.ParentID = P.ID
)
-- This select statement runs just fine and shows expected data.
--Select i.Id as ItemId, p.path, p.path.ToString() as LogicalNode, p.Id, p.ParentId, p.TenantId from Paths P
--join Items I on p.Id = i.Id
--order by P.TenantId, P.path
--Note that I have tried using the convert function, but it still fails with the same error message.
UPDATE I Set OrgNode = Convert(hierarchyid, P.path)
FROM Items I
JOIN Paths AS P
ON I.ID = P.ID
GO
Note, I also changed the + style of concatenation for the concat() function so you don't have to mess around with converting C.Num to a varchar.