jq print value of an element where a key array is empty or a key is missing

Question

{
    "vehicle": [
        {
            "type": "car",
            "features": [
                {
                    "airbags": "yes",
                    "engine": [
                        {
                            "electric": "yes",
                            "horsepower": "1000"
                        }
                    ],
                    "number": "ABCD 1234"
                }
            ],
            "price": "100000"
        },
        {
            "type": "truck",
            "features": [],
            "price": "500000"
        },
        {
            "type": "train",
            "features": [
                {
                    "airbags": "no",
                    "engine": [
                        {
                            "horsepower": "10000"
                        }
                    ],
                    "number": "MNOPQRST"
                }
            ],
            "price": "100000"
        }
    ]
}

Using jq or jmespath, I want value of type where vehicle[].features[] is either an empty array or it is missing vehicle[].features[].engine[].electric element.

So this json should list me truck and train as truck is has an empty array vehicle[].features[] and train is missing vehicle[].features[].engine[].electric element.

Answer 1

Given your particular input where it seems features and engines have always at most one element, you should be able to do:

jq -r '.vehicle[] | select(.features[0].engine[0].electric != "yes").type'

Or:

jq -r '.vehicle[] | select(.features[0].engine[0] | has("electric") | not).type'

for the type of those vehicles whose first engine of the first features has not got an electric key.

Or if like me you know perl or another programming language already and would rather not learn yet another (and very specific) one like jqs:

perl -MJSON -l -0777 -ne '
  $j = from_json($_);
  for (@{$j->{vehicle}}) {
    print $_->{type} unless $_->{features}[0]->{engine}[0]->{electric} eq "yes"
  }' < file.js

(or unless defined($_->{features}[0]->{engine}[0]->{electric})).