On Mon, Oct 15, 2012 at 12:16 PM, Yo-Yo Ma <[email protected]> wrote:
> Problem:
>
> a = A.objects.get(...)
> form = AModelForm(data={...}, instance=a)
> if form.is_valid():
> a = form.save()
> else:
> a.calculate_foo_field()
> a.last_attempt = datetime.now()
> a.save() # Oops, now the instance has the bad data provided to the
> form
>
>
> Workarounds:
>
> # Get a fresh copy of ``a``
> a = A.objects.get(pk=a.pk)
> # Wasted query
> # Also, this won't work in the context of a ``select_for_update`` on
> the original instance
>
> # Use ``update`` instead
> A.objects.filter(pk=a.pk).update(last_attempt=datetime.now(), ...)
> # What about ``calculate_foo_field``?
> # Also has the ``select_for_update`` problem
>
>
> Solution:
>
> a.reset_state() # Resets the instance's field state to the point of
> creation (using data stored in a ``_state_reset_cache`` dict)
> a.reset_foo() # etc.
> a.latest_attempt = datetime.now()
> a.save()
>
> Problem: Uses more memory per instance
> Solution: Add ``QuerySet.cache_for_reset()`` allowing opt-in usage of the
> feature, treating ``reset_state`` as noop when instance doesn't have the
> state reset cache.
That seems like a heavy-weight solution for something that probably isn't
all that common. The same thing could be achieved by adding a method to
checkpoint the current state, or even clone the instance, which would only
incur the overhead when you know you need it. For example:
a = A.objects.get(...)
form = AModelForm(data={...}, instance=a.clone()) ## copy the state here
if form.is_valid():
a = form.save()
else:
a.calculate_foo_field()
a.last_attempt = datetime.now()
a.save()
This is easy enough to do piecemeal by adding .clone() methods to your
models, and trying to do it generically sends us even farther towards
Django models having all the bureaucracy of a C++ class with
copy-constructors, so I wouldn't advocate adding it to core. Hopefully it
addresses your use case though.
Here's an example of a similar snippet someone has written, though
obviously you want to keep the same PK instead of auto-generating a new
one: http://djangosnippets.org/snippets/904/
Best,
Alex Ogier
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