How to do calculations in the model?

Question

Ruby on Rails

There is an order model, it has a subordinate order position model.

class Order < ActiveRecord::Base
  has_many :order_items, :dependent => :destroy
  accepts_nested_attributes_for :order_items, :reject_if => lambda { |a| a[:inventory_id].blank? }, :allow_destroy => true
end

class OrderItem < ActiveRecord::Base
  belongs_to :order
  belongs_to :inventory
  # в схеме есть свойство quantity - количество товара в строке заказа 
end

There is a model Goods in warehouses.

class Inventory < ActiveRecord::Base
  belongs_to :product
  belongs_to :stock
  # в схеме есть свойство amount - количество товара 
end

I don’t understand how to take goods from Inventory when ordering, change when adding, modify the quantity of goods in the order.

Answer 1

inventory.update!(amount: inventory.amount - order_item.quantity)

A more specific decision depends on your product reservation process. One solution is to create a separate class that describes the reservation process

class OrderCheckoutService
  
  attr_accessor :order

  def initialize(order)
    @order = order
  end

  def checkout
    ActiveRecord::Base.transaction do
     # ваш код который переберет все позиции заказа, зарезервирует, сохранит, пошлет уведомления и т.п.
   end
  end

end

Answer 2

You need to read about such a wonderful thing as transactions.