jq jq a lightweight and flexible command-line JSON processor.

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• Processing JSON using jq
• jq sur stackoverflow

members.json:

[
 {
  "member_id": 123,
  "loans":[
   {
    "date": "123",
    "media": [
     { "title": "foo" },
     { "title": "bar" }
    ]
   },
   {
    "date": "456",
    "media": [
     { "title": "foo" }
    ]
   }
  ]
 },
 {
  "member_id": 456,
  "loans":[
   {
    "date": "789",
    "media": [
     { "title": "foo"}
    ]
   }
  ]
 }
]

No need to use group_by → https://stackoverflow.com/questions/69423726/howto-count-occurrence-of-grouped-by-key-with-jq/69423860#69423860

jq '"foo" as $title | .[] | {
  id: .member_id,
  $title,
  count: [.loans[].media[] | select(.title == $title)] | length
}' members.json

{
  "id": 123,
  "title": "foo",
  "count": 2
}
{
  "id": 456,
  "title": "foo",
  "count": 1
}

jq  'map(
         (.member_id) as $m
        | .loans[].media[]
        | select(.title=="foo")
        | {id: $m, title: .title}
        )
    |group_by(.id)[]
    |.[0] + { count: length }
' members.json