Retain null in nested JMESPath query with Ansible

I make an API call in ansible and analyse the response. A repro:

---
- hosts: localhost
  gather_facts: false

  vars:
    response: {
        "results": [
          {
            "item": "server1",
            "json": {
              "data": null,         # <----- here's the problem
              "ok": true
            },
            "status": 200
          },
          {
            "item": "server2",
            "json": {
              "data": [
                {
                  "id": 7,
                  "name": "Production5"
                }
              ],
              "ok": true
            },
            "status": 200
          }
        ]
      }

  tasks:
    - debug:
        msg: "{{ response.results | community.general.json_query(_query) }}"
      vars:
        _query: "[].json.data[*].id | []"

Which yields:

TASK [debug] *********************************************
ok: [localhost] => {
    "msg": [
        7
    ]
}

But I actually need:

TASK [debug] *********************************************
ok: [localhost] => {
    "msg": [
        null,                    # <-----
        7
    ]
}

I want to keep the null so the input and output arrays have the same length, as this is important in future tasks.

I tried converting the null without success, e.g.: ([].json.data || [])[*].id and not_null([].json.data, '[]')[*].id.

Solution

The filter json_query is not capable of doing this, I'm afraid. Try Jinja instead

  result: |
    {% filter from_yaml %}
    {% for i in response.results|map(attribute='json.data') %}
    - {{ i.0.id|default('null') }}
    {% endfor %}
    {% endfilter %}

gives what you want

  result:
    - null
    - 7

Optionally, use json_query, create the lists of values, and map the value. The below expression gives the same result

  result: "{{ response.results|
              json_query('[].[json.data[0].id, item]')|
              map('first') }}"

^{Example of a complete playbook for testing

- hosts: localhost

vars:

response:
results:
- item: server1
json:
data: null
ok: true
status: 200
- item: server2
json:
data:
- {id: 7, name: Production5}
ok: true
status: 200

result1: |
{% filter from_yaml %}
{% for i in response.results|map(attribute='json.data') %}
- {{ i.0.id|default('null') }}
{% endfor %}
{% endfilter %}

result2: "{{ response.results|
json_query('[].[json.data[0].id, item]')|
map('first') }}"

tasks:

- debug:
var: result1
- debug:
var: result2}