Below you can find an example of first_comma_set :
{% set first_comma_set = False %}
{
{% if title is defined %}
{% if first_comma_set %},{% else %}{% set first_comma_set = True %}{% endif %}
"title": "{{ title }}"
{% endif %}
{% if value is defined %}
{% if first_comma_set %},{% else %}{% set first_comma_set = True %}{% endif %}
"value": "{{ value }}"
{% endif %}
{% if currency is defined %}
{% if first_comma_set %},{% else %}{% set first_comma_set = True %}{% endif %}
"currency": "{{ currency }}"
{% endif %}
{% if user_id is defined %}
{% if first_comma_set %},{% else %}{% set first_comma_set = True %}{% endif %}
"user_id": {{ user_id }}
{% endif %}
{% if stage_id is defined %}
{% if first_comma_set %},{% else %}{% set first_comma_set = True %}{% endif %}
"stage_id": {{ stage_id }}
{% endif %}
}