helm using argument in template to check if value exists in Values.yaml - kubernetes-helm

Question

The problem is following: I want to check if field in Values.yaml exists based on argument given to the template in _helpers.tpl:
{{- define "example-name" -}}
{{- $objectRef := index . 0 -}}
{{- if .Values.custom -}}
{{- if .Values.custom.$objectRef -}}
{{- if .Values.custom.$objectRef.annotations -}}
{{- include "some-library" (tuple .Values.custom.$objectRef.annotations) | indent 4 }}
{{- end }}
{{- end }}
{{- end }}
{{- end }}
Then in my deployment for example:
{{- template "example-name" "someField" }}
I want the result to be following:
{{- define "example-name" -}}
{{- $objectRef := index . 0 -}}
{{- if .Values.custom -}}
{{- if .Values.custom.someField-}}
{{- if .Values.custom.someField.annotations -}}
{{- include "some-library" (tuple .Values.custom.someField.annotations) | indent 4 }}
{{- end }}
{{- end }}
{{- end }}
{{- end }}
But the only thing I get is following error: bad character U+0024 '$'
I want to use template multiple times with various arguments. I haven't seen anyone dealing with the same problem before.
Any ideas?

Answer 1

The standard template function you're looking for is index. In its simplest form, index $map $key does a dynamic lookup of the $key (can be any expression) in the provided $map (can be any expression). It can also do integer-index lookups in arrays (Go slices) and nested lookups if you need to.
The other problem that you'll run into is that $map.undefinedKey (or index $map "undefinedKey"), assuming undefinedKey is not present in $map, is valid but evaluates to Go nil. So you can't do further lookups in that. The workaround to this I typically use is to use the Helm (Sprig) default function to use an empty dict if a value is not present.
That would give you a template like:
{{- define "example-name" -}}
{{- $top := index . 0 -}}
{{- $objectRef := index . 1 -}}
{{- $custom := $top.Values.custom | default dict -}}
{{- $object := index $custom $objectRef | default dict -}}
{{- with $object.annotations -}}
{{- include "some-library" (list $top .) | indent 4 }}
{{- end }}
{{- end }}
This is called with a list of two values, the top-level Helm object and a reference to a key in .Values.custom
{{- include "example-name" (list . "someField") -}}
The template extracts the two values from the list parameter. It then traverses the values structure one level at a time, at each level defaulting to an empty dictionary. So for example if there is no .Values.custom then $custom is set to an empty dictionary, which allows index $custom $objectRef to execute successfully (and return nil, but not abort). At the bottom level we use the with template function to check to see if come value is truthy, and if so, temporarily bind . to its value. When we make the inner call, we already have the top-level Helm object in a variable, and we can assemble a list of $top and the non-empty annotation structure . as the single template parameter.