JSON_MODIFY does not work with variable parameter - sql

Question

I have a SQL query which modifies some JSON. I'm iterating through the data and modifies part of the JSON based on the iteration.
For that i need to pass a variable parameter into JSON_MODIFY, but for some reason, it doesn't work!
SET #json = JSON_MODIFY(#ProData, '$.' + #ProKey + '.hasAnswer', CAST(1 as BIT))
I also tried passing the whole expression as a variable:
DECLARE #hasAnswerPath VARCHAR(100);
SET #hasAnswerPath = '$.' + #ProKey + '.hasAnswer';
SET #json = JSON_MODIFY(#ProData, #hasAnswerPath, CAST(1 as BIT))
But it has the same output, the hasAnswer is added to the root of the JSON and not in the element specified by #ProKey.
This works just fine:
SET #json = JSON_MODIFY(#ProData, '$.SomeName1.hasAnswer', CAST(1 as BIT))
It's like the #ProKey is disregarded.
Complete query:
BEGIN TRAN
DECLARE #ProID as uniqueidentifier;
DECLARE #ProData as nvarchar(max);
DECLARE #ProKey as varchar(200);
DECLARE ProCursor CURSOR FOR
SELECT Id, [Data] FROM [dbo].[PRO]
OPEN ProCursor;
FETCH NEXT FROM ProCursor INTO #ProID, #ProData;
WHILE ##FETCH_STATUS = 0
BEGIN
DECLARE #json NVARCHAR(max);
DECLARE DataCursor CURSOR FOR
SELECT [key] FROM OPENJSON(#ProData) WHERE type = 5; --5 is object data
OPEN DataCursor;
FETCH NEXT FROM DataCursor INTO #ProKey;
WHILE ##FETCH_STATUS = 0
BEGIN
SET #json=JSON_MODIFY(#ProData, '$.' + #ProKey + '.hasAnswer', CAST(1 as BIT))
SET #json=JSON_MODIFY(#json,'$.' + #ProKey + '.questionType','intro')
FETCH NEXT FROM DataCursor INTO #ProKey;
END;
UPDATE [dbo].[PRO]
SET [Data] = #json
WHERE Id = #ProID
PRINT #json
CLOSE DataCursor;
DEALLOCATE DataCursor;
FETCH NEXT FROM ProCursor INTO #ProID, #ProData;
END
CLOSE ProCursor;
DEALLOCATE ProCursor;
ROLLBACK
Sample JSON:
{
"SomeName1": {
"header": "Some text",
"answer": {
"type": "specified",
"numberValue": 1.0
}
},
"SomeName2": {
"header": "Some text",
"answer": {
"type": "specified",
"numberValue": 4.0
}
},
"SomeName3": {
"header": "Some text",
"answer": {
"type": "specified",
"numberValue": 2.0
}
}
}
Expected result:
},
"SomeName1": {
"header": "Some text",
"answer": {
"type": "specified",
"numberValue": 1.0
}
"hasAnswer": true,
"questionType": "intro",
}
}
Actual result:
},
"SomeName1": {
"header": "Some text",
"answer": {
"type": "specified",
"numberValue": 1.0
}
}
},
"hasAnswer":true,
"questionType":"intro"
}
What am I doing wrong here?

Answer 1

You seem to be overwriting your variables unintentionally:
-- first iteration
SET #json=JSON_MODIFY(#ProData, '$.' + #ProKey + '.hasAnswer', CAST(1 as BIT))
-- #json contains the #prodata after modification to first key (#prodata itself is not changed)
SET #json=JSON_MODIFY(#json,'$.' + #ProKey + '.questionType','intro')
-- #json (its first key to be more precise) is modified further
But on next iteration, this line will revert the modified #json back to the original value of #prodata. Only the last key will retain the modifications:
-- second iteration
SET #json=JSON_MODIFY(#ProData, '$.' + #ProKey + '.hasAnswer', CAST(1 as BIT))
-- you just overwrote your modifications with the value inside #prodata
The solution is to re-arrange the code a little, may be initialize #json outside the loop.