SCSS code like this: foo{ __bar{} } in results CSS will be like this: foo_bar{} =|= Create filename.rb in config/initializers folder with gist contents.

sass-rails-bem.rb

require 'sprockets/railtie'
Rails.application.assets.register_postprocessor 'text/css', :sass_bem_class_builder do |context, data|
  data.gsub(/([a-zA-Z0-9\-]+)[ ]([_]+)/i){ $1 + $2 }
end