"Transposing" objects in jq

Question

I'm unsure if "transpose" is the correct term here, but I'm looking to use jq to transpose a 2-dimensional object such as this:

[
    {
        "name": "A",
        "keys": ["k1", "k2", "k3"]
    },
    {
        "name": "B",
        "keys": ["k2", "k3", "k4"]
    }
]

I'd like to transform it to:

{
    "k1": ["A"],
    "k2": ["A", "B"],
    "k3": ["A", "B"],
    "k4": ["A"],
}

I can split out the object with .[] | {key: .keys[], name} to get a list of keys and names, or I could use .[] | {(.keys[]): [.name]} to get a collection of key–value pairs {"k1": ["A"]} and so on, but I'm unsure of the final concatenation step for either approach.

Are either of these approaches heading in the right direction? Is there a better way?

Answer 1

This should work:

map({ name, key: .keys[] })
    | group_by(.key)
    | map({ key: .[0].key, value: map(.name) })
    | from_entries

The basic approach is to convert each object to name/key pairs, regroup them by key, then map them out to entries of an object.

This produces the following output:

{
  "k1": [ "A" ],
  "k2": [ "A", "B" ],
  "k3": [ "A", "B" ],
  "k4": [ "B" ]
}

Answer 2

Here is a simple solution that may also be easier to understand. It is based on the idea that a dictionary (a JSON object) can be extended by adding details about additional (key -> value) pairs:

# input: a dictionary to be extended by key -> value 
# for each key in keys
def extend_dictionary(keys; value):
  reduce keys[] as $key (.; .[$key] += [value]);

reduce .[] as $o ({}; extend_dictionary($o.keys; $o.name) )


$ jq -c -f transpose-object.jq input.json
{"k1":["A"],"k2":["A","B"],"k3":["A","B"],"k4":["B"]}