sortable_by.rb

module SortableBy
  module ClassMethods
    # defines a 'sorted_by' named scope, which accepts a column name and optional direction
    # example:  sortable_by :columns => %w(id name)
    #   usage:  SomeModel.sorted_by('name', 'asc')
    def sortable_by(options)
      class_eval "def self.sortable_columns; #{options[:columns].inspect}; end"
      
      named_scope :sorted_by, Proc.new { |column, direction|
        if sortable_columns.include?(column)
          direction = direction.downcase
          direction = 'asc' unless %w(asc desc).include?(direction)
          { :order => "#{column} #{direction}" }
        else
          {}
        end    
      }
    end
  end
  
  def self.included(base)
    base.extend ClassMethods
  end
end