How to template Helm to insert templating language from other application?

I have a configMap that has a piece of alerting that looks like this:

alert_output:
  outputs:
    - type: webhook
      enabled: true
      template: |
          {{- if ne (print .PolicyType) "Heartbeat" -}}
          {"text": "New Alert\n Type: {{.PolicyType}}\n Details: {{.Description}}"
          {{- end -}}

How can I template this in Helm so that the text inside the outputs.template can be customizable via values file like this?

alert_output:
  outputs:
    - type: webhook
      enabled: true
      template: |
          {{- if ne (print .PolicyType) "Heartbeat" -}}
          {"text": {{ .Values.custom_alert_text }}
          {{- end -}}

valus.yaml

custom_alert_test: "New Alert\n Type: {{.PolicyType}}\n Details: {{.Description}}"

Of course the original {{}} are interpreted as being the Helm templates instead of the application templating language.

Solution

You need to do two things to make this work.

In the template itself, you need to cause Helm (or more properly the Go text/template engine) to output a {{ rather than interpret it as template markup. I've found the easiest way to do this is to write a template block that emits the curly braces:

{{/* A template that evaluates to the literal string "{{" */}}
{{"{{"}}- if ne ... -}}

The curly braces are not interpreted in the values.yaml file so you do not need to make a change there. However, parsing the YAML file will convert the \n escape sequence into a newline character and consume any quoting that might have been there. Helm includes a toJson function that will render anything to valid JSON, and should do the required quoting and escaping for you.

In your example there also needs to be one more } at the end of the line matching the { JSON object syntax.

Putting these things together, you should get something like:

alert_output:
  outputs:
    - type: webhook
      enabled: true
      template: |
          {{"{{"}}- if ne (print .PolicyType) "Heartbeat" -}}
          {"text": {{ toJson .Values.custom_alert_text }}}
          {{"{{"}}- end -}}

You could also assemble the entire object in template language and then serialize that; which would look something like

template: |
  {{"{{"}}- if ne (print .PolicyType) "Heartbeat" -}}
  {{ dict "text" .Values.custom_alert_text | toJson }}
  {{"{{"}}- end -}}