Where is Logics?

case_1_model.rb

class User
  def buy_item(item)
    self.purchases.create(item: item)
  end
end

case_2_controller.rb

class PurchasesController
  def create
    item = Item.find(params[:item_id])
    current_user.purchases.create(item: item)
  end
end

case_3_dci.rb

class BuyItemContext
  def initialize(user, item)
    @user = user
    @item = item
    
    @user.extend(Customer)
  end
  
  def process
    @user.buy(item)
  end
end

module Customer
  def buy(item)
    self.purchases.create(item: item)
  end
end