JSON/JQ: Merge 2 files on key-value with condition - json

Question

I have 2 JSON files. I would like to use jq to take the value of "capital" from File 2 and merge it with File 1 for each element where the same "name"-value pair occurs. Otherwise, the element from File 2 should not occur in the output. If there is no "name"-value pair for an element in File 1, it should have empty text for "capital."
File 1:
{
"countries":[
{
"name":"china",
"continent":"asia"
},
{
"name":"france",
"continent":"europe"
}
]
}
File 2:
{
"countries":[
{
"name":"china",
"capital":"beijing"
},
{
"name":"argentina",
"capital":"buenos aires"
}
]
}
Desired result:
{
"countries":[
{
"name":"china",
"continent":"asia",
"capital":"beijing"
},
{
"name":"france",
"continent":"europe",
"capital":""
}
]
}

Answer 1

You could first construct a dictionary from File2, and then perform the update, e.g. like so:
jq --argfile dict File2.json '
($dict.countries | map( {(.name): .capital}) | add) as $capitals
| .countries |= map( .capital = ($capitals[.name] // ""))
' File2.json
From a JSON-esque perspective, it would probably be better to use null for missing values; in that case, you could simplify the above by omitting // "".
Using INDEX/2
If your jq has INDEX/2, then the $capitals dictionary could be constructed using the expression:
INDEX($dict.countries[]; .name) | map_values(.capital)
Using INDEX makes the intention clearer, but if efficiency were a major concern, you'd probably be better off using reduce explicitly:
reduce $dict.countries[] as $c ({}; . + ($c | {(.name): .capital}))

Answer 2

One way:
$ jq --slurpfile file2 file2.json '
{ countries:
[ .countries[] |
. as $curr |
$curr + { capital: (($file2[0].countries[] | select(.name == $curr.name) | .capital) // "") }
]
}' file1.json
{
"countries": [
{
"name": "china",
"continent": "asia",
"capital": "beijing"
},
{
"name": "france",
"continent": "europe",
"capital": ""
}
]
}
An alternative:
$ jq -n '{ countries: ([inputs] | map(.countries) | flatten | group_by(.name) |
map(select(.[] | has("continent")) | add | .capital //= ""))
}' file[12].json