rails更新时的唯一性验证

Question

I have the following validation rule on one of the models,

validates :reciept_num, :presence => true,
                        :numericality => { :only_integer => true },
                        :uniqueness => true,
                        :on => :update,
                        :if => "!status_id.nil?"`

Now, when I update the object using update_attributes method it gives me following error

reciept_num: has already been taken .

While updating the the object I'm not changing the reciept_num attribute? So, why does this validation fails on update?

If I'm not updating the value, it must be the old one and hence should pass validation. Am I missing something.

Answer 1

First off, validations don't run based on whether the attribute has changed or not (unless of course you ask for that explicitly). Everytime a record with a uniqueness validation saves and the validation can run (as defined by :on , :if , :unless options) it will check whether there are any instances other than itself with the value that is supposed to be unique.

Since you've got conditions on your validation, I imagine you could end up creating two instances with the same receipt num, but where both have a null status_id . Set the status_id column and the validation kicks into action and finds the other instances.

Another thing is that since your validation is on update only you could create multiple instances with the same receipt num, again trying to update the record would trigger the validation.

I'm only guessing at the precise scenarios though.