module Orderable
  def self.included(mod)
    class << mod
      include Orderable::ClassMethods
    end

    mod.class_eval do
      # orderable scope takes an array of sorts of the form [(key | [key, dir])*]
      # and sorts the collection by the given keys
      scope :orderable, lambda{|orders|
        self.order(self.orders_string(orders)) if !orders.blank?
      }
    end
  end

  module ClassMethods
    def order_by(*cols)
      cols.each{|col| orderable_by << col.to_s}
    end

    def orderable_by
      @orderable_by ||= []
    end

    def order_string(col, dir)
      col.to_s + (( dir && dir.to_s =~ /^d(?:esc)?$/i ) ? " desc" : "")
    end

    def selected_orders(orders)
      if orders
        orders.split(%r{ |\+}).
          map{|order| order.split("/") if order }.
          reject{|col,dir| !orderable_by.include?(col.to_s)}
      else
        []
      end
    end

    def orders_string(orders)
      selected_orders(orders).
        map{|(col,dir)| order_string(col,dir)}.join(",")
    end
  end
end