Modify array elements while grouping by a specific key using jq - json

Question

Is it possible to modify/replace array elements while grouping by a specific key (.[].Parameter.Id) such that this array:
[{
"Id": 48,
"Parameter": {
"Id": 17
}
}, {
"Id": 196,
"Parameter": {
"Id": 17
}
}]
becomes this:
[
{
"p17": [48, 196]
}
]
Here is the source JSON file for a complete example:
[{
"Id": 78,
"PromotionType": 2,
"Amount": "100",
"UpperLimit": null,
"Variables": [{
"Id": 100,
"Parameter": {
"Id": 30
}
}]
}, {
"Id": 84,
"PromotionType": 2,
"Amount": null,
"UpperLimit": null,
"Variables": [{
"Id": 48,
"Parameter": {
"Id": 17
}
}, {
"Id": 196,
"Parameter": {
"Id": 17
}
}, {
"Id": 59,
"Parameter": {
"Id": 21
}
}, {
"Id": 60,
"Parameter": {
"Id": 21
}
}, {
"Id": 62,
"Parameter": {
"Id": 21
}
}]
}, {
"Id": 59,
"PromotionType": 2,
"Amount": "666.6",
"UpperLimit": null,
"Variables": [{
"Id": 96,
"Parameter": {
"Id": 8
}
}, {
"Id": 47,
"Parameter": {
"Id": 17
}
}]
}]
What I want to achieve is this:
[{
"Id": 78,
"PromotionType": 2,
"Amount": "100",
"UpperLimit": null,
"Variables": [{
"p30": [100]
}]
}, {
"Id": 84,
"PromotionType": 2,
"Amount": null,
"UpperLimit": null,
"Variables": [{
"p17": [48, 196]
}, {
"p21": [59, 60, 62]
}]
}, {
"Id": 59,
"PromotionType": 2,
"Amount": "666.6",
"UpperLimit": null,
"Variables": [{
"p8": [96]
}, {
"p17": [47]
}]
}]
I am reading through jq manual, jq cookbook and found some functions (e.g. with_entries, unique_by, inputs) that might help but could not figure out how to make it work.
Number of objects/inner objects are also not fixed. So I cannot simply replace using array indexes.
Any help would be appreciated.
Thanks,
Emre

Answer 1

jq solution:
jq 'map(.Variables
|= (group_by(.Parameter.Id)
| map(("p" + (.[0].Parameter.Id | tostring)) as $pid
| { ($pid) : map(.Id) }
)
)
)' input.json
The output:
[
{
"Id": 78,
"PromotionType": 2,
"Amount": "100",
"UpperLimit": null,
"Variables": [
{
"p30": [
100
]
}
]
},
{
"Id": 84,
"PromotionType": 2,
"Amount": null,
"UpperLimit": null,
"Variables": [
{
"p17": [
48,
196
]
},
{
"p21": [
59,
60,
62
]
}
]
},
{
"Id": 59,
"PromotionType": 2,
"Amount": "666.6",
"UpperLimit": null,
"Variables": [
{
"p8": [
96
]
},
{
"p17": [
47
]
}
]
}
]