ModelForms don't work well with prefetch_related

[anonymous]

Following the example at ​https://docs.djangoproject.com/en/1.5/ref/models/querysets/#prefetch-related, ModelForms are a pain to use if you want to eagerly load many pizza forms. It is more or less worth it to skip writing forms, since figuring out how to write a manual view that can populate 15 pizza forms with 3 queries is easy, whereas writing a form that can do it for fewer than 33 is hard. Manually setting choices on the form can reduce this down to 17, 1 for the list of pizzas, 1 inner join for each pizza, and 1 master list for all toppings, plus 1 superfluous inner join for each pizza. Removing that last bit seems hard and is certainly undocumented.

[Simon Charette]

#18597 is a InlineFormSet related issue, we should really eat our own dog food in ModelForms.

[david08uc@…]

One line patch that avoids reduces sql multiplication for prefetched forms

[david08uc@…]

I added a patch, which I made to the trunk version.
I ran the test suite, but with many skips and expected failures (and I believe the relavant code path was not exercised). I don't have the knowledge or time to fix that.

However, I believe this patch is a good idea, because the current behavior resorts to a model utility method that always clones the queryset of a many-to-many manager. This is overkill when the goal is to populate a list of pk's, especially when the result is frozen as a list anyway.

[Tim Graham]

This would a test that uses assertNumQueries() to show that the query count is reduced.

[Jim Bailey]

Hello,

I have also run into this issue, so I fixed it on this branch:
​https://github.com/dgym/django/tree/ticket_20849

The commit includes a unit test that now passes.

The fix only changes the behaviour if there is prefetched data, as the above patch would hurt performance in the non-prefetched case.

[Jim Bailey]

I have made the following pull request:
​https://github.com/django/django/pull/1840

[loic84]

Looks good to me.

[Anssi Kääriäinen <akaariai@…>]

In 539e3693d4712b249a95cfad8cfdeecdad1777a6:

Fixed #20849 -- ModelForms do not work well with prefetch_related.

model_to_dict() (used when rendering forms) queries the database
to get the list of primary keys for ManyToMany fields. This is
unnecessary if the field queryset has been prefetched, all the
keys are already in memory and can be obtained with a simple
iteration.