This seems worthwhile in its own right, but also works around an Oracle
bug (in versions 10 -- 11.1) where the use of Unicode would reset the
date/time formats, causing ORA-01843 errors.
Thanks Trac users CarstenF for the report, jtiai for the initial patch,
and everyone who contributed to the discussion on the ticket.
Previously, saving a model instance to a non-related field (in
particular a FloatField) would silently convert the model to an Integer
(the pk) and save it. This is undesirable behaviour, and likely to cause
confusion so the validatio has been hardened.
Thanks to @PirosB3 for the patch and @jarshwah for the review.
