```ruby
require 'spree_backend'
require 'spree_core'
require 'coffee_script'

module SpreeRelatedProducts
  class Engine < Rails::Engine
    engine_name 'spree_related_products'

    def self.activate

      Dir.glob(File.join(File.dirname(__FILE__), "../app/**/*_decorator*.rb")) do |c|
        Rails.configuration.cache_classes ? require(c) : load(c)
      end

    end

    initializer "spree.promo.register.promotion.calculators" do |app|
      app.config.spree.calculators.promotion_actions_create_adjustments << Spree::Calculator::RelatedProductDiscount
    end

    config.autoload_paths += %W(#{config.root}/lib #{config.root}/app/models/spree/calculator)
    config.to_prepare &method(:activate).to_proc

  end
end
```

Add require for sass/rails to fix font-awesome missing error
user
```ruby
require 'spree_backend'
require 'spree_core'
require 'coffee_script'
require 'sass/rails'

module SpreeRelatedProducts
  class Engine < Rails::Engine
    engine_name 'spree_related_products'

    def self.activate

      Dir.glob(File.join(File.dirname(__FILE__), "../app/**/*_decorator*.rb")) do |c|
        Rails.configuration.cache_classes ? require(c) : load(c)
      end

    end

    initializer "spree.promo.register.promotion.calculators" do |app|
      app.config.spree.calculators.promotion_actions_create_adjustments << Spree::Calculator::RelatedProductDiscount
    end

    config.autoload_paths += %W(#{config.root}/lib #{config.root}/app/models/spree/calculator)
    config.to_prepare &method(:activate).to_proc

  end
end
```
assistant