order

gistfile1.rb

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

  validates_presence_of     :user, :product, :product_quantity
  validates_numericality_of :product_quantity,
    :only_integer => true, :greater_than => 0, :less_than_or_equal_to => 10000, :allow_blank => true

  validate :enough_product_quantity,
# if does not work
    :if => lambda { |o| (!o.product_quantity.nil?) & (!o.product.nil?) & (!o.product.quantity_left.nil?) }

  def enough_product_quantity
    if product_quantity > product.quantity_left
      errors.add(:product_quantity, :not_enough_quantity_of_product)
    end
  end
end