Ansible: Looping over a list of dicts and merging with another dict of lists

I'm trying to work out how to loop over a list of dicts and extract the name of each entry to merge with another variable.

jinja2 pseudo code which should give the desired result:

---
docker_applications:
  - name: foo
    zfs_opts:
      quota: 4G
      compression: false
  - name: bar
zfs_filesystems:
  - name: rust/ftp
  - name: ssd/one
    quota: 1T
  - name: ssd/two
  {% for i in docker_applications %}
  - name: "docker/{{ i.name }}"
    {% for k, v in i.zfs_opts %}
    {{ k }}: {{ v }}
    {% endfor %}
  {% endfor %}

Which would give:

---
zfs_filesystems:
  - name: rust/ftp
  - name: ssd/one
    quota: 1T
  - name: ssd/two
  - name: docker/foo
    quota: 4G
    compression: false
  - name: docker/bar

I know you can't use jinja2 templates in a variable files, so I am trying to do the merge in the inventory file but I'm at a loss on how I could achieve this.

- name: nas
  hosts: nas
  become: true
  become_user: root
  roles:
    - docker
    - zfs
  vars:
    zfs_filesystems: zfs_filesystems|do_something

Solution

Put the declaration into the vars

docker_fs: "{{ docker_applications|
               json_query('[].[{name: join(`/`, [`docker`, name])}, zfs_opts]')|
               map('combine')|list }}"

gives

docker_fs:
  - compression: false
    name: docker/foo
    quota: 4G
  - name: docker/bar

Then, concatenate the lists

    - set_fact:
        zfs_filesystems: "{{ zfs_filesystems + docker_fs }}"

gives

zfs_filesystems:
  - name: rust/ftp
  - name: ssd/one
    quota: 1T
  - name: ssd/two
  - compression: false
    name: docker/foo
    quota: 4G
  - name: docker/bar

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

vars:

docker_applications:
- name: foo
zfs_opts:
quota: 4G
compression: false
- name: bar

zfs_filesystems:
- name: rust/ftp
- name: ssd/one
quota: 1T
- name: ssd/two

docker_fs: "{{ docker_applications|
json_query('[].[{name: join(`/`, [`docker`, name])}, zfs_opts]')|
map('combine')|list }}"

tasks:

- debug:
var: docker_fs
- set_fact:
zfs_filesystems: "{{ zfs_filesystems + docker_fs }}"
- debug:
var: zfs_filesystems

The above solution puts the full value of the attribute zfs_opts into the dictionary. Optionally, select the attributes quota and compression. The declaration below
docker_fs: "{{ docker_applications|
json_query('[].{name: join(`/`, [`docker`, name]),
quota: zfs_opts.quota,
compression: zfs_opts.compression}') }}"

gives
docker_fs:
- compression: false
name: docker/foo
quota: 4G
- compression: null
name: docker/bar
quota: null

Optionally, get rid of the null values
docker_fs: "{{ docker_applications|
json_query('[].{name: join(`/`, [`docker`, name]),
quota: zfs_opts.quota,
compression: zfs_opts.compression}')|
map('dict2items')|
map('rejectattr', 'value', 'eq', None)|
map('items2dict')|
list }}"

gives
docker_fs:
- compression: false
name: docker/foo
quota: 4G
- name: docker/bar}