writeable_mixin.rb

module Writeable
  module Mixin
    def self.included(base)
      base.extend ClassMethods
    end

    module ClassMethods
      def retrieve_connection
        @pool ||= connection_handler.connection_pool_list.select do |p|
          p.spec.config[:writeable] == true
        end.first
        @pool.try(:connection) || begin
                                   establish_connection(:writeable)
                                   retrieve_connection
                                 end
      end
    end
  end
end

Rails 4.2