Collecting variables from multiple hosts in a playbook

I have a playbook that collects a list of information from multiple hosts, and I want to collect those lists together into a single variable. So for example, my output might look like this:

ok: [ALBSWA052] => {
    "myvar": [
        "ALBSWA052 exists",
        "ALBSWA052 is secondary"
    ]
}
ok: [ALBSWA051] => {
    "myvar": [
        "ALBSWA051 exists",
        "ALBSWA051 is primary"
    ]
}

and I'd like to combine them into a list like

    "myvar": [
        "ALBSWA052 exists",
        "ALBSWA052 is secondary"
        "ALBSWA051 exists",
        "ALBSWA051 is primary"
    ]

Is there an elegant way to do this? The only way I can think of is to write it into a file and read it back in.

• For example,

myvar1: "{{ ansible_play_hosts | map('extract', hostvars, 'myvar')
                               | flatten }}"

gives the expected result

myvar1:
  - ALBSWA051 exists
  - ALBSWA051 is secondary
  - ALBSWA052 exists
  - ALBSWA052 is secondary

• If the variable myvar is missing in any host the variable myvar1 will be undefined

myvar1: VARIABLE IS NOT DEFINED!

In this case, use json_query. For example,

myvar2: "{{ hostvars | json_query('*.myvar') | flatten }}"

gives, if my_var is missing in ALBSWA052

myvar2:
  - ALBSWA051 exists
  - ALBSWA051 is secondary

• The case gets complicated if there are more hosts in the inventory. For example,

shell> cat hosts
ALBSWA051
ALBSWA052
ALBSWA053

If myvar is defined in ALBSWA053 the former declaration will give

myvar2:
  - ALBSWA051 exists
  - ALBSWA051 is secondary
  - ALBSWA053 exists
  - ALBSWA053 is secondary

In this case, select the hostvars

myvar3: "{{ hostvars | dict2items
                     | selectattr('key', 'in', ansible_play_hosts)
                     | json_query('[].value.myvar')
                     | flatten }}"

gives

myvar3:
  - ALBSWA051 exists
  - ALBSWA051 is secondary

Of course, if myvar is defined in both ALBSWA051 and ALBSWA052 you get the expected result

myvar3:
  - ALBSWA051 exists
  - ALBSWA051 is secondary
  - ALBSWA052 exists
  - ALBSWA052 is secondary

shell> cat pb.yml
- hosts: ALBSWA051,ALBSWA052
  gather_facts: false

  vars:

    myvar1: "{{ ansible_play_hosts | map('extract', hostvars, 'myvar')
                                   | flatten }}"
    myvar2: "{{ hostvars | json_query('*.myvar') | flatten }}"
    myvar3: "{{ hostvars | dict2items
                         | selectattr('key', 'in', ansible_play_hosts)
                         | json_query('[].value.myvar')
                         | flatten }}"

  tasks:

    - debug:
        var: myvar1
      run_once: true

    - debug:
        var: myvar2
      run_once: true

    - debug:
        var: myvar3
      run_once: true

shell> tree .
.
├── ansible.cfg
├── hosts
├── host_vars
│   ├── ALBSWA051.yml
│   ├── ALBSWA052.yml
│   └── ALBSWA053.yml
└── pb.yml

shell> cat host_vars/ALBSWA051.yml 
myvar: [ALBSWA051 exists, ALBSWA051 is secondary]

shell> cat host_vars/ALBSWA052.yml 
myvar: [ALBSWA052 exists, ALBSWA052 is secondary]

shell> cat host_vars/ALBSWA053.yml 
myvar: [ALBSWA053 exists, ALBSWA053 is secondary]