when_assigning.rb

# Usage:
# 
# class Comment < ActiveRecord::Base
#   when_assigning :subject do |value|
#     value.strip!
#     super value
#   end
# end
#
module ActiveRecord::WhenAssigning
  def when_reading(field, &block)
    chain_attribute field, &block
  end
  
  def when_writing(field, &block)
    when_assigning field, &block
  end
  
  def when_assigning(field, &block)
    assignment_field = field.to_s.ends_with?('=') ? field : :"#{field}="
    chain_attribute assignment_field, &block
  end
  
  def chain_attribute(method, &block)
    mod = Module.new do
      define_method method, &block
    end
    class_eval do
      include mod
    end
  end
end