rails - validate product ordered has the available quantity

I have two related validations on separate models. I can validate one way but not another. The post validation works: if I try to change the quantity available as the seller, and people have already ordered some of it, it (correctly) won't let me change the quantity below the amount ordered already.

The order validation does not work: if I try changing the order amount to more than is available, it lets me. It checks only the current sum (I guess before the save), and does not notice the buyer trying to sneak out more than what's available. When I try and change the order back to less than what is available, it won't let me, checking the current sum ordered (before save).

I also tried validating the quantity of the order + the remaining available stock (available -sum ordered), but have the same problem.

How would I get the validation to check what the quantity would be after the save, and then not save it should the value be invalid? It would have to work also for editing an order

class Post < ActiveRecord::Base
    belongs_to :product
    belongs_to :event
    has_many :orders, :d开发者_如何学运维ependent => :destroy

    attr_accessible :quantity, :deadline, :product_id, :event_id

    validate :quantity_gt_ordered

    def quantity_gt_ordered
       self.errors.add(:quantity, " - People have already ordered more than this") unless self.quantity >= self.sum_orders
    end

    def sum_orders
        self.orders.sum(:quantity)
    end

class Order < ActiveRecord::Base
      belongs_to :user
      belongs_to :post

    attr_accessible :order_price, :post_id, :user_id, :quantity

    validate :quantity_is_available

    def quantity_is_available
       self.errors.add(:quantity, " - Please order only what's available") unless self.sum_post_orders <= self.post.quantity
    end

    def sum_post_orders
      Order.where(:post => self.post).sum(:quantity)
    end

You should just be able to compare to the available quantity minus the ordered quantity:

available_quantity = self.post.quantity - self.sum_post_order
if quantity > available_quantity
  self.errors.add(:quantity, " - Please order only what's available")
end

And also make sure your sum_post_orders doesn't include the current order:

Order.where("id != ?", self.id).where(:post => self.post).sum(:quantity)