I have a couple of models like so class Bill < ActiveRecord::Base has_many :bill_items belongs_to :store accepts_nested_attributes_for :bill_items end class BillItem <ActiveRecord::Base belongs_to :product belongs_to :bill validate :has_enough_stock def has_enough_stock stock_available = Inventory.product_is(self.product).store_is(self.bill.store).one.quantity errors.add(:quantity, "only #{stock_available} is available") if stock_available < self.quantity end end The above validation so obviously doesn't work because when I'm reading the bill_items from nested attributes inside the bill form, the attributes bill_item.bill_id or bill_item.bill are not available before being saved. So how do I go about doing something like that?