rails6 app: minitest failing with ActionView::Template::Error: uninitialized constant

Question

A bit of a puzzler - I've got an app using a very simple gem I wrote (UIAvatars) to generate a URL to an Avatar API. This demonstrably working in my Development rails app (the Avatar is displayed with no issues) but when I try to run my controller tests it causes errors like so:

Error:
InitiativesControllerTest#test_should_get_new:
ActionView::Template::Error: uninitialized constant ProfilesHelper::UIAvatars
    app/helpers/profiles_helper.rb:17:in `profile_image_link_for'
    app/views/layouts/_header.html.erb:62
    app/views/layouts/application.html.erb:23
    test/controllers/initiatives_controller_test.rb:15:in \`block in <class:InitiativesControllerTest>'

The method in profiles_helper is:

  def profile_image_link_for(profile, **options)

    image_tag(
      UIAvatars.generateURL(name: profile.full_name, size: "128", background: "random"), 
      options
      ).html_safe

  end

bundle says it is installed:

% bundle info ui_avatars
  * ui_avatars (0.0.1)
    Summary: Generate avatars with initials from names using https://ui-avatars.com
    Homepage: https://rubygems.org/gems/ui_avatars
    Path: /Users/thatsme/.rvm/gems/ruby-2.6.6/gems/ui_avatars-0.0.1

it's in my Gemfile in the default group

gem 'ui_avatars'

Any idea what could be causing this problem?

Answer 1

Answer is that .ruby-version had a different version of ruby (2.6.7) than I was actually using (2.6.6) and so ruby could not find the gem installed in the expected path when running minitest.

(why it still worked in rails is a question for another day).