Checking uniqueness contraint during form validation in App Engine

Question

I am using Flask and WTforms in App Engine, trying to implement uniqueness contraint on one of the field. The question is big, please be patient and I have been stuck here from

Answer 1

Edit: didn't answer your question properly the first time.

Separate instances of the Form object will be instantiated for the GET and POST requests, so you can't save the old_name to self.

You'll need to pass the old_name to the browser in the form, and have the browser submit the old_name back in the POST request.

The easyish way to do this is to create a hidden form field that the user doesn't see, but will get submitted by the POST request. I'm not too familiar with WTForms but I assume you can initialize the old_name field value in your the GET request handler.