Reshape a array to object with jq - json

Question

I am learning advance concepts of jq. And I made a tiny json with array with some films of Charles Chaplin...well this array in json:
[
{
"title": "The Great Dictator",
"year": 1940,
"country": "USA",
"genre": "political satire"
},
{
"title": "Modern Times ",
"year": 1936,
"country": "USA",
"genre": "comedy"
},
{
"title": "The Gold Rush",
"year": 1925,
"country": "USA",
"genre": "comedy"
},
{
"title": "The Kid",
"year": 1921,
"country": "USA",
"genre": "drama"
}
]
And I want to convert or reshape into a object with the genres as the keys and the list of the films as array (comedy is only has two element in the array):
{
"comedy": [
{
"title": "Modern Times ",
"year": 1936,
"country": "USA"
},
{
"title": "The Gold Rush",
"year": 1925,
"country": "USA"
}
],
"political satire": [
{
"title": "The Great Dictator",
"year": 1940,
"country": "USA"
}
],
"drama": [
{
"title": "The Kid",
"year": 1921,
"country": "USA"
}
]
}
But I can't do it. I trying the first step to create a object with genre and foo string as var, but it fails: cat c.json | jq '{.[] | (.genre): "foo" ]}'

Answer 1

It can be done in three lines:
[group_by(.genre)[]
| {(.[0].genre): map_values(del(.genre))}]
| add
aggregate_by/3
The relevant generic abstraction here is:
def aggregate_by(s; f; g):
reduce s as $x (null; .[$x|f] += [$x|g]);
This allows the solution to be written directly as:
aggregate_by(.[]; .genre; del(.genre))

Answer 2

I found:
$ cat c.json | jq '
group_by(.genre)
| map({"genre": .[0].genre,
"film": map(. | del(.genre))})
| [ .[] | {(.genre): .film}]
| add'
{
"comedy": [
{
"title": "Modern Times ",
"year": 1936,
"country": "USA"
},
{
"title": "The Gold Rush",
"year": 1925,
"country": "USA"
}
],
"drama": [
{
"title": "The Kid",
"year": 1921,
"country": "USA"
}
],
"political satire": [
{
"title": "The Great Dictator",
"year": 1940,
"country": "USA"
}
]
}
Maybe it is not the best, because I think there are a lot of steps...but it runs.

Answer 3

You can use a modified version of Jeff Mercado's answer on the page you linked to.
jq 'reduce .[] as $i ({}; .[$i.genre] += [$i])'
That groups the objects as you want but leaves the genre key-value pair. You can delete them like so.
jq 'reduce .[] as $i ({}; .[$i.genre] += [$i|del(.genre)])'
Really, this is just a concrete version of peak's "generic abstraction".