Use jq to count elements based on group by

I have a list of movies like this:

[
   {
      "title":"X",
      "genres":[
         {
            "tag":"Horror"
         },
         {
            "tag":"Thriller"
         },
         {
            "tag":"Mystery"
         }
      ]
   },
   {
      "title":"Zero Dark Thirty",
      "genres":[
         {
            "tag":"Thriller"
         },
         {
            "tag":"Drama"
         },
         {
            "tag":"History"
         },
         {
            "tag":"War"
         }
      ]
   }
]

I want to query all unique genres and count the number of movies, where the output looks like this:

{
   "Horror":1,
   "Thriller":2,
   "Mystery":1,
   "Drama":1,
   "History":1,
   "War":1
}

Is this possible with jq?

>Solution :

Yes, it is.

1. Extract all genres into array
2. Group genres
3. Map to a key-value pair (key = any element of the group, we’ll take the first; value = count of elements in the group)
4. Build object from key-value pairs

map(.genres[].tag)
| group_by(.)
| map({ key:first, value:length })
| from_entries

Output:

{
  "Drama": 1,
  "History": 1,
  "Horror": 1,
  "Mystery": 1,
  "Thriller": 2,
  "War": 1
}

Alternatively, use a reduce based approach and simply increase a counter:

reduce .[].genres[].tag as $genre ({}; .[$genre] += 1)

This is likely more efficient than building an array and grouping.