jq: parse result where some properties are empty arrays

Using jq, wish to parse results that may include array values

Confused. If data contains an empty array:

"connections": []

Thought that jq would be able to parse it with:

connections: .connections[]? | { id: .id }

For example, "apps" is an array, and each app is defined with the following:

• "actions" array
• "connections" array

Here result where apps has an app instance that contains connections as an empty array.

  "apps": [
    {
      "id": "1a902fb4-1b43-4df9-b9d7-9b54cd14e2ad",
      "actions": [
        {
          "id": "0dc38f41-49fe-4056-b035-850764733a44",
        }
      ],
      "connections": []
    },
  ]

Assume all apps have connections currently containing empty arrays.

Using jq, parsing apps without including connections (all currently contain empty arrays), no problem, result is parsed:

apps: .apps[]? | { id: .id \
  , actions: .actions[]? | { id: .id }
}

Using jq, parsing apps including empty connections, no results, nothing:

instances: .instances[]? | { id: .id \
  , actions: .actions[]? | { id: .id } \
  , connections: .connections[]? | { id: .id } \
}

What am I missing?

.connections[] will stream the elements of connection. When using inside an object construction filter, this will create one object per element in the stream.

If there are multiple streams involved, the cartesian product will be built.
If the array is empty, nothing will be streamed (and consequently, the product will be empty; see example at the bottom).

Here's an example:

$ jq -cn '{
  a: [ 1, 2 ],
  b: [ 3, 4 ]
}
| { x: (.a[] | . + 10), y: (.b[] | . + 20) }'
{"x":11,"y":23}
{"x":11,"y":24}
{"x":12,"y":23}
{"x":12,"y":24}

Note that ? only has an effect on missing values/properties. An empty array is not missing, it has a value: the empty array itself.

You probably want to use the map filter, e.g. to extract an array of ids from your connections: { connections: (.connections | map({id})) }.

Rewriting the example from above with map:

$ jq -cn '{
  a: [ 1, 2 ],
  b: [ 3, 4 ]
}
| { x: .a | map(. + 10), y: .b | map(. + 20) }'
{"x":[11,12],"y":[23,24]}

Empty cartesian product:

$ jq -cn '{
  a: [ 1, 2 ],
  b: [ 3, 4 ],
  c: []
}
| { x: (.a[] | . + 10), y: (.b[] | . + 20), z: (.c[] | . + 30) }'
$ # empty

$ jq -cn '{
  a: [ 1, 2 ],
  b: [ 3, 4 ],
  c: []
}
| { x: .a | map(. + 10), y: .b | map(. + 20), z: .c | map(. + 30) }'
{"x":[11,12],"y":[23,24],"z":[]}