Under accounting -> Vendors -> Bills:

How do I ensure that for a given vendor bill, if someone creates a new Bill in the system with the same reference ('ref') string it should raise an exception. Vendor bills should have unique reference. 

https://imgur.com/EWrnS0S

After Inheriting the account.move, how can I get the context with constrains method where if a user tries to create a new bill with a reference which already exists in the system, it should raise an exception and not make a duplicate?

Thank you for your time.