Source code for: http://baylorrae.com/rails-link_to_with_notification-helper-method/

helper.rb

def link_to_with_notifications(*args, &block)

  # get notification count from hash
  notifications = args[2][:notifications] || 0

  # create the data- attribute unless notifications.zero?
  args[2]['data-notifications'] = notifications unless notifications.zero?

  # delete original notifications hash
  args[2].delete(:notifications)

  # run original link_to helper
  link_to(*args, &block)
end

usage.html.erb

<%= link_to_with_notifications 'Completed Orders',
  user_carts_path(user, view: 'completed'),
  class: 'button',
  notifications: user.shopping_carts.where('completed = ?', true).count %>