Ansible: How to merge two different sized lists of dicts

a task in ansible 2.10.8:

I would like to merge two different lists of dicts to get one complemented with missing items, but not add a dict to the first list.

List of dicts one:

user_users:
  - username: 'foo'
    primarygid: 42
  - username: 'bar'

List of dicts two:

user_users_skel:
  - username: foo
    uid: 600
    primarygid: 600
    shell: '/bin/bash'
    ssh_login: false
  - username: bar
    uid: 602
    primarygid: 602
    shell: '/bin/bash'
    ssh_login: true
  - username: blupp
    uid: 1000
    primarygid: 1000
    shell: '/bin/bash'
    ssh_login: true
    default_groups: 'admin'
    initial_password_hash: '$2349887rhfewklrjn'

Expected Output (also a list of dicts):

  - username: foo
    uid: 600
    primarygid: 42
    shell: '/bin/bash'
    ssh_login: false
  - username: bar
    uid: 602
    primarygid: 602
    shell: '/bin/bash'
    ssh_login: true

I used different approaches, like

    - set_fact:
        eff_users: "{{ user_users | ansible.builtin.combine(user_users_skel) }}"
    - ansible.builtin.debug:
        var: eff_users

with bogus output.

Solution

Merge the lists

  users_all: "{{ [user_users, user_users_skel] |
                 community.general.lists_mergeby('username') }}"

gives

  users_all:
  - primarygid: 602
    shell: /bin/bash
    ssh_login: true
    uid: 602
    username: bar
  - default_groups: admin
    initial_password_hash: $2349887rhfewklrjn
    primarygid: 1000
    shell: /bin/bash
    ssh_login: true
    uid: 1000
    username: blupp
  - primarygid: 600
    shell: /bin/bash
    ssh_login: false
    uid: 600
    username: foo

Get the list usernames

  usernames: "{{ user_users | map(attribute='username') }}"

gives

  usernames:
  - foo
  - bar

Select username in the list

  eff_users: "{{ users_all | selectattr('username', 'in', usernames) }}"

gives what you want

  eff_users:
  - primarygid: 602
    shell: /bin/bash
    ssh_login: true
    uid: 602
    username: bar
  - primarygid: 600
    shell: /bin/bash
    ssh_login: false
    uid: 600
    username: foo

^{Example of a complete playbook for testing
- hosts: all

vars:

user_users:
- username: foo
primarygid: 42
- username: bar

user_users_skel:
- username: foo
uid: 600
primarygid: 600
shell: /bin/bash
ssh_login: false
- username: bar
uid: 602
primarygid: 602
shell: /bin/bash
ssh_login: true
- username: blupp
uid: 1000
primarygid: 1000
shell: /bin/bash
ssh_login: true
default_groups: admin
initial_password_hash: $2349887rhfewklrjn

users_all: "{{ [user_users, user_users_skel] |
community.general.lists_mergeby('username') }}"
usernames: "{{ user_users | map(attribute='username') }}"
eff_users: "{{ users_all | selectattr('username', 'in', usernames) }}"

tasks:

- debug:
var: users_all
- debug:
var: usernames
- debug:
var: eff_users}