associations_fun.rb

require 'active_support/core_ext/string'

class MyFakeActiveRecord
  def pluralized_table
    self.class.name.underscore.downcase.pluralize
  end

  def self.has_many(association)
    define_method(association) do
      "INNER JOIN #{association} ON #{association}.id = #{pluralized_table}.id"
    end
  end

  def self.belongs_to(association)
    define_method(association) do
      "INNER JOIN #{association.to_s.pluralize} ON #{association}.id = #{pluralized_table}.id"
    end
  end
end

class SeaFoodShop < MyFakeActiveRecord
  has_many :octopi
end

class Octopus < MyFakeActiveRecord
  belongs_to :sea_food_shop
end

sea_food_shop = SeaFoodShop.new
puts sea_food_shop.octopi

octopus = Octopus.new
puts octopus.sea_food_shop

__END__
INNER JOIN octopi ON octopi.id = sea_food_shops.id
INNER JOIN sea_food_shops ON sea_food_shop.id = octopi.id