blah.rb

class SimpleAR
  def self.has_many(collection)
    define_method(collection) do
      var_name = "@#{collection}"
      instance_variable_set(var_name, []) unless instance_variables.include?(var_name)
      instance_variable_get var_name
    end
  end
end

class User < SimpleAR
  has_many :blog_posts
end