A nice way to add asterisk* to "required" field in frontend used with https://github.com/amatsuda/html5_validators

label_required.rb

# config/initializers/label_required.rb
module ActionView
  module Helpers
    
    # For rails 4
    if ActionPack::VERSION::STRING >= '4'
      
      module Tags
        class Label
          def render_with_required(&block)
            inject_required_field

            render_without_required &block
          end
          alias_method_chain :render, :required
        end
      end
      
    else
      
      # For rails 3, but not tested
      class InstanceTag
        def to_label_tag_with_required(text = nil, options = {}, &block)
          if object.class.ancestors.include? ActiveModel::Validations
            options["required"] ||= object.class.attribute_required?(method_name)
          end
          to_label_tag_without_required text, options, &block
        end
        alias_method_chain :to_label_tag, :required
      end
      
    end
    
  end
end

style.scss

// For bootstrap
label.control-label[required='required']:before {
  content: "* ";
  color: red;
}
label:not(.control-label)[required='required']:after {
  content: " *";
  color: red;
}