Rails validation -- error is generated , but record is still saved

I have models order.rb , line_items.rb -- where line_item is belongs to order. I've added custom validations, as such an order should only have line_items all with same merchant_id

My question is : it seems the validation is working fine -- i am able to get the error whenever it is violated, but "line_item" records are still saved in any situation. Any idea?

Order.rb

validate_on_update :only_one_merchant

def only_one_merchant
        merchants = []
        for line_item in self.li开发者_开发问答ne_items
          merchants << line_item.product.merchant_id
        end
        merchants = merchants.uniq
        errors.add(:line_items, "You could only order from one same merchant at time")  if  merchants.length > 1
    end 

Order.rb adding line_item

  current_item = LineItem.new(:quantity => quantity)
  current_item.variant = variant
  current_item.price   = variant.price
  self.line_items << current_item

---

validate_on_update will check validation only when you update any record to validate the record in anycondition use validate

Chnage

validate_on_update :only_one_merchant

to

validate :only_one_merchant

& Check if it works or not

---

validate :only_one_merchant

def only_one_merchant
        merchants = []
        for line_item in self.line_items
          merchants << line_item.product.merchant_id
        end
        merchants = merchants.uniq
        errors.add(:line_items, "You could only order from one same merchant at time")  if  merchants.length > 1
    end 

It will solve your problem first these function call and then update