How to generate continuing indices for multiple objects in nested arrays that are in an array

Question

Given

[{
  "objects": [{
    "key": "value"
  },{
    "key": "value"
  }]
}, {
  "objects": [{
    "key": "value"
  }, {
    "key": "value"
  }]
}]

How do I generate

[{
  "objects": [{
    "id": 0,
    "key": "value"
  },{
    "id": 1,
    "key": "value"
  }]
}, {
  "objects": [{
    "id": 2,
    "key": "value"
  }, {
    "id": 3,
    "key": "value"
  }]
}]

Using jq?

I tried to use this one, but ids are all 0:

jq '[(-1) as $i | .[] | {objects: [.objects[] | {id: ($i + 1 as $i | $i), key}]}]'

Answer 1

The key to a simple solution here is to break the problem down into easy pieces. This can be accomplished by defining a helper function, addId/1. Once that is done, the rest is straightforward:

# starting at start, add {id: ID} to each object in the input array 
def addId(start):
  reduce .[] as $o
    ([];
     length as $l 
     | .[length] = ($o | (.id = start + $l)));

reduce .[] as $o
  ( {start: -1, answer: []};
    (.start + 1) as $next
    | .answer += [$o | (.objects |= addId($next))]
    | .start += ($o.objects | length) )
| .answer