[Alerting UI] Add an option to being able to create a "sub-template" for a particular context variable.

[YulNaumenko]

It would be useful to have a possibility to define a specific template for some complex action variables like an object:

{
  "name": "Chris",
  "value": 10000,
  "taxed_value": 10000 - (10000 * 0.4),
  "in_ca": true
}

which will be added to the message as a text:

Hello {{name}}
You have just won {{value}} dollars!
{{#in_ca}}
Well, {{taxed_value}} dollars, after taxes.
{{/in_ca}}

[elasticmachine]

Pinging @elastic/kibana-alerting-services (Team:Alerting Services)

[pmuellr]

When would this be used? And how does this differ from the UI side default message capability? Or are you thinking this belongs in the server?

[pmuellr]

I think this issue is aimed at being able to create a "sub-template" for a particular context variable, or even property or element of a context variable, if that variable is an object or array.

We could support this kinda thing via "partials", in the mustache sense; some doc here: https://github.com/janl/mustache.js/#partials

We could then expose the partials in the "variables list", and when clicked, then insert the reference to the partial; eg, from the mustache below (stolen from the link above), we might list user as a variable (or perhaps > user), and it would insert the string {{> user}} in the action param edit field. And of course we'd have user available as a partial, however that happens.

Maybe that's too complicated tho ... and how would users even know what the partials expanded to?

It would be easier to allow a variable to insert not just a name surrounded by {{}}, but an entire string with mustache template bits in it.

For example, this template and partial:

base.mustache:
<h2>Names</h2>
{{#names}}
  {{> user}}
{{/names}}

user.mustache:
<strong>{{name}}</strong>

Can be thought of as a single, expanded template:

<h2>Names</h2>
{{#names}}
  <strong>{{name}}</strong>
{{/names}}