peewee get_or_create and then save: error binding - peewee

Question

Is there an easy way to update a field on a get of a get_or_create?
I have a class ItemCategory and I want to either create a new entry or get the already created entry and update a field (update_date).
What I do is:
item,created= ItemCategory.get_or_create(cat=cat_id,item=item_id)
if created == True:
print "created"
else:
item.update_date = datetime.now
item.somethingelse = 'aaa'
item.save()
This works for a while in my loop. But after 50-100 get/create it crashes:
peewee.InterfaceError: Error binding parameter 4 - probably unsupported type.
Maybe I should use upsert(), I tried but wasn't able to get anything working. Also it's not probably the best solution, since it makes a replace of the whole row instead of just a field.
I like peewee, it's very easy and fast to use, but I can't find many full examples and that's a pity

Answer 1

Newbie mistake
item.update_date = datetime.now()
I am not 100% sure this is the only answer though. I modified my code so many times that it might be also something else.
Regarding my question about create_or_update , I've done this:
try:
Item.create(...)
except IntegrityError:
Item.update(...)
peewee is really great, I wonder why no one ever asked for a create_or_update.