Add a new key-value to a json file using Ansible - json

Question

I'm using Ansible to automate some configuration steps for my application VM, but having difficult to insert a new key-value to an existing json file on the remote host.
Say I have this json file:
{
"foo": "bar"
}
And I want to insert a new key value pair to make the file become:
{
"foo": "bar",
"hello": "world"
}
Since json format is not line based, I'm excluding lineinfile module from my options. Also, I would prefer not to use any external modules. Google keeps giving me examples to show how to read json file, but nothing about change json values and write them back to file. Really appreciate your help please!

Answer 1

since the file is of json format, you could import the file to a variable, append the extra key:value pairs you want, and then write back to the filesystem.
here is a way to do it:
---
- hosts: localhost
connection: local
gather_facts: false
vars:
tasks:
- name: load var from file
include_vars:
file: /tmp/var.json
name: imported_var
- debug:
var: imported_var
- name: append more key/values
set_fact:
imported_var: "{{ imported_var | default([]) | combine({ 'hello': 'world' }) }}"
- debug:
var: imported_var
- name: write var to file
copy:
content: "{{ imported_var | to_nice_json }}"
dest: /tmp/final.json
UPDATE:
as OP updated, the code should work towards remote host, in this case we cant use included_vars or lookups. We could use the slurp module.
NEW code for remote hosts:
---
- hosts: greenhat
# connection: local
gather_facts: false
vars:
tasks:
- name: load var from file
slurp:
src: /tmp/var.json
register: imported_var
- debug:
msg: "{{ imported_var.content|b64decode|from_json }}"
- name: append more key/values
set_fact:
imported_var: "{{ imported_var.content|b64decode|from_json | default([]) | combine({ 'hello': 'world' }) }}"
- debug:
var: imported_var
- name: write var to file
copy:
content: "{{ imported_var | to_nice_json }}"
dest: /tmp/final.json
hope it helps

Answer 2

ilias-sp's solution is great!
In my case, it lacked the case when we may have to create a base json file.
So I had to add this task in the beginning of the play:
- name: Ensure json file exists
copy:
content: "{}"
dest: /tmp/var.json
force: false

Answer 3

For people who are OK with custom ansible modules: https://github.com/ParticleDecay/ansible-jsonpatch works great!
With this you can simply do:
- name: append key/values
json_patch:
src: /tmp/var.json
operations:
- op: add
path: "/hello"
value: "world"
pretty: yes
create: yes

Answer 4

- name: update log
copy:
content: "{{ log | to_nice_json}}"
dest: "{{ log_file }}"
vars:
log: "{{ (lookup('file', log_file) | from_json) + ([{'job': (build_id if build_id != '' else 'dev'), 'keystore': ks, 'timestamp': ansible_date_time.iso8601}]) }}"
log_file: log/log.json
build_id: "{{ lookup('ENV', 'BUILD_ID') }}"
tags: log