# Adds dynamic attributes for polymorphic associations based on the name of
# the class for the polymorphic record.  For example,
# 
#   class Tag < ActiveRecord::Base
#     belongs_to :taggable, :polymorphic => true
#   end
#   
#   class Article < ActiveRecord::Base
#     has_many :tags, :as => :taggable
#   end
#   
#   t = Tag.find(1)   # => #<Tag id: 1, taggable_id: 1, taggable_type: "Article">
#   t.taggable        # => #<Article id: 1>
#   t.article         # => #<Article id: 1>
module PolymorphicIdentity
  def self.included(base) #:nodoc:
    base.class_eval do
      alias_method_chain :method_missing, :polymorphic_identity
      alias_method_chain :respond_to?, :polymorphic_identity
    end
  end
  
  def method_missing_with_polymorphic_identity(method_id, *args, &block) #:nodoc:
    if association_name = find_polymorphic_association_name(method_id)
      send(association_name, *args, &block)
    else
      method_missing_without_polymorphic_identity(method_id, *args, &block)
    end
  end
  
  # True if a polymorphic association can be found whose foreign type is set
  # to the name of the method
  def respond_to_with_polymorphic_identity?(method, include_priv = false) #:nodoc:
    respond_to_without_polymorphic_identity?(method, include_priv) || !find_polymorphic_association_name(method).nil?
  end
  
  private
    # Finds the name of the polymorphic association whose foreign type is set to
    # the value specified.
    def find_polymorphic_association_name(foreign_type_value)
      foreign_type_value = foreign_type_value.to_s.camelize
      reflection = self.class.reflections.values.find do |reflection|
        reflection.options[:polymorphic] && self[reflection.options[:foreign_type]] == foreign_type_value
      end
      
      reflection ? reflection.name : nil
    end
end

ActiveRecord::Base.class_eval do
  include PolymorphicIdentity
end