web2py/web2py — pull request #352 by viniciusban (2014-01-17 20:26:53+00:00)
Title: Allow empty reference field in form without IS_EMPTY_OR() validator, avoiding constraint violation exception.

This situation occurs when you have a child table with a parent field and you don't want to see the dropdown in form and, for some reason, you don't validate your data using Web2py validators (IS_EMPTY_OR(IS_IN_DB()).