Discourage SolidusStarterFrontend from being run as an engine

Rakefile

@@ -1,6 +1,8 @@
 # frozen_string_literal: true
 
 require 'solidus_dev_support/rake_tasks'
+
+ENV['SOLIDUS_STARTER_FRONTEND_ALLOW_AS_ENGINE'] = 'true'
 SolidusDevSupport::RakeTasks.install
 
 task default: 'extension:specs'

docs/development.md

@@ -91,6 +91,19 @@ Note that `solidus_starter_frontend` has conditional references to
 `solidus_auth_devise`, so it's important to place `solidus_starter_frontend`
 before `solidus_auth_devise`.
 
+#### 3) Enable `SOLIDUS_STARTER_FRONTEND_ALLOW_AS_ENGINE` environment variable
+
+We're strongly encouraging people to use `solidus_starter_frontend` as a
+generator. Thus, if you want to run it as an engine, you'll need to set an
+environment variable explicitly stating so.
+
+One way to do this is to set the environment variable within an initializer:
+
+```sh
+# config/initializers/solidus_starter_frontend.rb
+ENV['SOLIDUS_STARTER_FRONTEND_ALLOW_AS_ENGINE'] = 'true'
+```
+
 #### 3) Install Solidus
 
 ```sh
@@ -109,6 +122,12 @@ bundle exec rails generate solidus:auth:install --auto-run-migrations
 You will need to run `bundle exec rails g solidus_starter_frontend:install` to add the
 frontend assets to the existing vendored Solidus manifest files.
 
+#### 5) Run the server
+
+```sh
+rails server
+```
+
 ### Updating the changelog
 Before and after releases the changelog should be updated to reflect the 
 up-to-date status of the project:

lib/solidus_starter_frontend/engine.rb

@@ -15,6 +15,8 @@ class Engine < Rails::Engine
     end
 
     config.to_prepare do
+      raise 'Error: SolidusStarterFrontend is not allowed to run as an engine' unless ENV['SOLIDUS_STARTER_FRONTEND_ALLOW_AS_ENGINE']
+
       if defined?(Spree::Auth::Engine)
         [
           Spree::UserConfirmationsController,

spec/spec_helper.rb

@@ -2,6 +2,7 @@
 
 # Configure Rails Environment
 ENV['RAILS_ENV'] = 'test'
+ENV['SOLIDUS_STARTER_FRONTEND_ALLOW_AS_ENGINE'] = 'true'
 
 # Run Coverage report
 require 'solidus_dev_support/rspec/coverage'