Hi gang (especially Malcolm, if you're listening),
I was recently trying to test out a few things with field subclassing,
and I made a PickleField just to see if I could. Basically, it pickles
arbitrary Python objects for storage into the database, then unpickles
them back into Python when accessed on a retrieved object. Everything
worked fine until I decided to prevent any kind of filterin on the
field, since there's no way to reliably compare pickled objects with
each other.
I thought it'd be easy, just writing PickleField.get_db_prep_lookup()
to look like this:
class PickleField(models.TextField):
# Unrelated methods here...
def get_db_prep_lookup(self, lookup_type, value):
raise TypeError("Can't make comparisons against pickled data.")
Unfortunately, this resulted in some very strange behavior that I
can't account for. When I try to query against a model that has that
field on it, there are two possible results, depending on *how* I use
the QuerySet. If I just use list(qs), it returns an empty list,
without raising my error. But if I try to iterate over it (using [x
for x in qs], for instance), it raises my error just fine. However,
when I raise a ValueError, it works fine in both cases.
I've posted a full interactive session[1] that illustrates the
problem. And yes, I do realize that in an interactive session, the
models don't get real tables in the database. That just helps
illustrate that even when the error does get suppressed, it still
somehow short-circuits the process, so that the query never actually
executes.
So, I guess I have two questions. Why does list(qs) suppress the
TypeError while [x for x in qs] doesn't? And why does ValueError work
even when TypeError doesn't?
-Gul
[1] http://dpaste.com/hold/49362/
--~--~---------~--~----~------------~-------~--~----~
You received this message because you are subscribed to the Google Groups
"Django users" group.
To post to this group, send email to django-users@googlegroups.com
To unsubscribe from this group, send email to [EMAIL PROTECTED]
For more options, visit this group at
http://groups.google.com/group/django-users?hl=en
-~----------~----~----~----~------~----~------~--~---
