subelements-lookup.yml

# Essentially, `with_subelements` calculates the Cartesian product of the two
# input lists. The special thing is: the second list is contained as a
# sub-element in the elements of the first list.

# Modified based on the following example:
# https://docs.ansible.com/ansible/latest/collections/ansible/builtin/subelements_lookup.html
#
# See the following link for how to replace `with_subelements` with `loop`:
# https://docs.ansible.com/ansible/latest/user_guide/playbooks_loops.html#with-subelements

- name: Demo subelements lookup.
  hosts: all
  gather_facts: no
  vars:
    users:
      - name: alice
        authorized_keys:
          - alice_first_authorized_key
          - alice_second_authorized_key
        mysql:
            password: mysql-password
            hosts:
              - "%"
              - "127.0.0.1"
              - "::1"
              - "localhost"
            privs:
              - "*.*:SELECT"
              - "DB1.*:ALL"
        groups:
          - wheel
      - name: bob
        authorized_keys:
          - bob_first_authorized_key
        mysql:
            password: other-mysql-password
            hosts:
              - "db1"
            privs:
              - "*.*:SELECT"
              - "DB2.*:ALL"

  tasks:
    - name: Set authorized ssh key, extracting just that data from 'users'
      debug:
        msg: "user = {{ item.0.name }}; key = {{ item.1 }}"
      with_subelements:
         - "{{ users }}"
         - authorized_keys

    - name: Setup MySQL users, given the mysql hosts and privs subkey lists
      vars:
        mysql_user:
          name: "{{ item.0.name }}"
          password: "{{ item.0.mysql.password }}"
          host: "{{ item.1 }}"
          priv: "{{ item.0.mysql.privs | join('/') }}"
      debug:
        var: mysql_user
      with_subelements:
        - "{{ users }}"
        - mysql.hosts

    - name: list groups for users that have them, don't error if groups key is missing
      debug:
        var: item
      loop: "{{ q('subelements', users, 'groups', {'skip_missing': True}) }}"

# See https://stackoverflow.com/a/41908853/630364
- name: Demo subelements lookup (with simple data).
  hosts: all
  gather_facts: no
  vars:
    families:
      - surname: Smith
        country: US
        children:
          - name: Mike
            age: 4
          - name: Kate
            age: 7
      - surname: Sanders
        country: UK
        children:
          - name: Pete
            age: 12
          - name: Sara
            age: 17
  tasks:
    - name: List children
      debug:
        msg: "Family={{ item.0.surname }} Child={{ item.1.name }} Age={{ item.1.age }}"
      with_subelements:
        - "{{ families }}"
        - children
        # Using `country` will result in the error "the key country should point to a list, got 'US'".
        # - country