Ansible run task once per database-name - ansible

Question

I'm using ansible to deploy several sites to the same server. Each site is a separate 'host' in the ansible hosts inventory, which works really well.
However, there are only two databases: production and testing.
How can I make sure my database-migration task only runs once per database?
I've read into the group_by, run_once and delegate_to features, but I'm not sure how to combine those.
The hosts look something like:
[production]
site1.example.com ansible_ssh_host=webserver.example.com
site2.example.com ansible_ssh_host=webserver.example.com
[beta]
beta-site1.example.com ansible_ssh_host=webserver.example.com
beta-site2.example.com ansible_ssh_host=webserver.example.com
[all:children]
production
beta
The current playbook looks like this:
---
- hosts: all
- tasks:
# ...
- name: "postgres: Create PostgreSQL database"
sudo: yes
sudo_user: postgres
postgresql_db: db="{{ DATABASES.default.NAME }}" state=present template=template0 encoding='UTF-8' lc_collate='en_US.UTF-8' lc_ctype='en_US.UTF-8'
tags: postgres
register: createdb
delegate_to: "{{ DATABASES.default.HOST|default(inventory_hostname) }}"
# ...
- name: "django-post: Create Django database tables (migrate)"
django_manage: command=migrate app_path={{ src_dir }} settings={{ item.settings }} virtualenv={{ venv_dir }}
with_items: django_projects
#run_once: true
tags:
- django-post
- django-db
- migrate

Answer 1

The best way I found was to restrict the execution of a task to the first host of a group. Therefore you need to add the groupname and the databases to a group_vars file like:
group_vars/production
---
dbtype=production
django_projects:
- name: project_1
settings: ...
- name: project_n
settings: ...
group_vars/beta
---
dbtype=beta
django_projects:
- name: project_1
settings: ...
- name: project_n
settings: ...
hosts
[production]
site1.example.com ansible_ssh_host=localhost ansible_connection=local
site2.example.com ansible_ssh_host=localhost ansible_connection=local
[beta]
beta-site1.example.com ansible_ssh_host=localhost ansible_connection=local
beta-site2.example.com ansible_ssh_host=localhost ansible_connection=local
[all:children]
production
beta
and limit the task execution to the first host that matches that group:
- name: "django-post: Create Django database tables (migrate)"
django_manage: command=migrate app_path={{ src_dir }} settings={{ item.settings }} virtualenv={{ venv_dir }}
with_items: django_projects
when: groups[dbtype][0] == inventory_hostname
tags:
- django-post
- django-db
- migrate

Answer 2

So, the below will illustrate why I say "once per group" is generally not supported. Naturally, I would love to see a cleaner out-of-the-box way, and do let me know if "once per group" isn't what you're after.
Hosts:
[production]
site1.example.com ansible_ssh_host=localhost ansible_connection=local
site2.example.com ansible_ssh_host=localhost ansible_connection=local
[beta]
beta-site1.example.com ansible_ssh_host=localhost ansible_connection=local
beta-site2.example.com ansible_ssh_host=localhost ansible_connection=local
[beta:vars]
dbhost=beta-site1.example.com
[production:vars]
dbhost=site1.example.com
[all:children]
production
beta
Example playbook which will do something on the dbhost once per group (the two real groups):
---
- hosts: all
tasks:
- name: "do this once per group"
sudo: yes
delegate_to: localhost
debug:
msg: "do something on {{hostvars[groups[item.key].0]['dbhost']}} for {{item}}"
register: create_db
run_once: yes
with_dict: groups
when: item.key not in ['all', 'ungrouped']