dynamic_method.rb

module ActiveRecord
  class Base
    def self.has_one(something)
      define_method "#{something}=".to_sym do |new_value|
        instance_variable_set("@#{something}", new_value)
      end

      define_method something.to_sym do
        instance_variable_get("@#{something}")
      end
    end
  end
end

class Product < ActiveRecord::Base
  has_one :user
end

p1 = Product.new
p1.user = "123"
puts p1.user        #=> 123