Compare 2 JSON-files and create a new key if values match - json

Question

I have 2 sets of JSON-files looking like below, data-A.json and data-B.json.
I need to somehow compare the key URL in data-A.json with the same key in data-B.json. Where there is a match take data from the key Position in data-A.json and write to new key PreviousPosition in data-B.json. If there is no matching URL, write a null value for this new key in data-B.json
Please see examples:
data-A.json
[
{
"Position": "1",
"TrackName": "One hit wonder",
"URL": "https://domain.local/xyz123"
},
{
"Position": "2",
"TrackName": "Random song",
"URL": "https://domain.local/123qwe"
},
{
"Position": "3",
"TrackName": "Dueling banjos",
"URL": "https://domain.local/asd456"
}
]
data-B.json
[
{
"Position": "1",
"TrackName": "Rocket",
"URL": "https://domain.local/nbs678"
},
{
"Position": "2",
"TrackName": "Dueling banjos",
"URL": "https://domain.local/asd456"
},
{
"Position": "3",
"TrackName": "One hit wonder",
"URL": "https://domain.local/xyz123"
}
]
(desired) data-B.json
[
{
"Position": "1",
"TrackName": "Rocket",
"URL": "https://domain.local/nbs678",
"PreviousPosition": null
},
{
"Position": "2",
"TrackName": "Dueling banjos",
"URL": "https://domain.local/asd456",
"PreviousPosition": "3"
},
{
"Position": "3",
"TrackName": "One hit wonder",
"URL": "https://domain.local/xyz123",
"PreviousPosition": "1"
}
]
I have done some mediocre attemps to solve this using jq with no luck. Also tried some PowerShell and Python but I just can't figure it out.
Any suggestions?

Answer 1

If a straightforward, two-line solution is what you're looking for, then jq is a good choice:
(INDEX($A[]; .URL) | map_values(.Position)) as $dict
| map( .PreviousPosition = $dict[ .URL ] )
This is perhaps more straightforward than it looks, as the expression in the first line is a commonly found idiom (namely INDEX(...) | map_values(...)) for creating a dictionary. In the first line, it is assumed that $A holds the JSON in data-A.json.
The second line just applies the lookup rule specified in the question.
The only tricky bit here is getting the command-line invocation right. The following will suffice:
jq --argfile A data-A.json -f program.jq data-B.json
where program.jq contains the above two-line program.