gistfile1.rb

class User
  include Buyer
  include BuyerWithShotgun
end

class Buyer
  private money
  def canPay(price)
     money > price
  end
end

class BuyerWithShotgun
  private money
  private haveShotgun
  def canPay(price)
    haveShotgun || money > price 
  end
end

class ShopContext
  def buy(user, shop, item)
    raise ArgumentError unless user.kind_of?(Buyer)
    raise ArgumentError unless catalog.kind_of?(Shop)
    if (user.canPay(item.price))
      # calling Buyer`s.add which pays honestly, & reduced money amount
      user.add(item, price)
        shop.lose(item)
      end
  end
end

class PoorMansShopContext
  def buy(user, shop, item)
    raise ArgumentError unless user.kind_of?(BuyerWithShotgun)
    raise ArgumentError unless shop.kind_of?(Shop)
    if (user.canPay(item.price))
      # calling BuyerWithShotgun`s.add which just shows shotgun & takes item for free
      user.add(item, price)
        shop.lose(item)
      end
   end
end

# context call
user = new User('Rob')
user.setBalance(0)
user.haveShotgun(true)

nosecurityShop = new PoorMansShop('grocery')
# will work as expected, shotgun-way
PoorMansShopContext.buy(user, nosecurityShop , 'water')

weaponShop = new PawnShop('weapon')
# will not work as expected, user still manages to buy
ShopContext.buy(user, weaponShop, 'bullets')