Add min_num and validate_min on formsets

[Gabriel Farrell]

Formsets have a max_num for limiting the number of forms, but no way to ensure a minimum.

Use case: I want an address formset that displays all addresses that have been created and no extras, unless there are no addresses, then one empty form should be displayed. I would set extra=0 and min_num=1.

Patch included is just a beginning. I may have missed a function or two and I haven't touched docs or tests.

[charliebubblegum]

double +1. This was very hard for me to work around, and would be a very logical addition.

[anonymous]

Replying to charliebubblegum:

double +1. This was very hard for me to work around, and would be a very logical addition.

agreed. will like to hear inputs from devs.

[Mathijs de Bruin]

Bump! It would make a lot of sense to have this kind of functionality built into Django, and the effort would not be gigantic.

[michal@…]

Added min_num argument to formset_factory, updated docs

[michal@…]

Removed maxDiff = None from test case

[Claude Paroz]

#17642 has a patch to apply this feature to admin inlines.

[wim@…]

Hi, probably it is a good idea to add min_num to modelformset_factory and inlineformset_factory as well.

[Till Backhaus]

Hi AnkitBagria, I just started working on this too. I'm in Utrecht right now, I'm also in #django-sprint. My nick is tback.

[Till Backhaus]

I think I fixed it. ​https://github.com/django/django/pull/784

[Till Backhaus]

Sorry, it's not fixed yet. BaseModelFormsets are still broken. The current state of my work is here: ​https://github.com/tback/django/tree/formset-min-num

[Rogério Yokomizo]

Please, check it out: ​https://github.com/django/django/pull/1208

[Tim Graham]

Thanks, the patch no longer merges cleanly so it needs to be rebased. It also needs documentation including a mention in the 1.7 release notes. If you could add that, I will try to review and commit it.

[Marc Tamlyn]

I'm pretty concerned about what the implementation for this looks like. It seems to me that every formset on every Django site would now have to have the extra "MIN" field. Whilst in theory this shouldn't break anyone's sites, it will almost certainly break everyone's tests. I'd prefer a less backwardsly problematic patch for a relatively minor feature.

That said, I think the feature is probably useful. Perhaps a patch could be devised which means the "MAX" field is not required either?

[Rogério Yokomizo <me@…>]

Patch rebased.

Could someone please review?

​https://github.com/django/django/pull/1444

About backward problematic, I don't know how can I do that. Any suggestions?

[Tim Graham]

I left comments on the pull request. I'm not sure there's actually any backwards compatibility concerns as the parameters aren't actually required in POST data as far as I can tell.

[Rogério Yokomizo]

Thanks timo!

I adjust the pull request based on your comments.

I rebased it. The old commit with comments can be seen at ​https://github.com/yokomizor/django/commit/35d40939f89990c01f4c31ac10a29e3ec809d26e.

Could someone please review?

​​https://github.com/django/django/pull/1444

About the backward problematic, maybe the only backwards compatibility concerns are in tests that test formset.as_X() return.

[Tim Graham <timograham@…>]

In df27803a55ffea71d468a6aab0d897caa2b7a797:

Fixed #9532 -- Added min_num and validate_min on formsets.

Thanks gsf for the suggestion.

[Tim Graham]

It would also be useful to add this to modelformset_factory, inlineformset_factory, etc. Ticket #17642 is open for that issue -- it has a patch and the branch from tback above also looks relevant.

[tchristiansen]

Why is min_num added to extra?
This makes no sense to me, and makes extra useless in combination with min_num.

extra=3 and min_num=3 resulting 6 forms, which is absolutely not what I would expect (3 forms).

[Rogério Yokomizo]

I think this is there because when you set min_num as 3 you want at least 3 extra forms.

Maybe we should remove it, or just clarify in the docs.

What about set extra equals to min_num when min_num is greater than extra?

[Tim Graham]

I agree a documentation improvement would be welcome.

@tchristiansen, is there a case you foresee where min_num=X and you wouldn't want at least X extra forms?

[Stephen Burrows]

@timo I would set min_num=3 if there should be at least three things. I would set extra=3 if I wanted three blank forms presented.

The implementation here means that there will *always* be six blank forms - existing objects are *not* taken into account.

To put it another way, if min_num is 3, and extra is 2, and there are already 2 existing objects, I would expect to have either 4 forms (2 extra on top of existing objects) or 5 forms (2 extra on top of the minimum.) With the code as it is, I will instead get seven forms - two for the existing objects, and an additional seven from extra.

Ran into this behavior while working on #17642. Possible this is only a modelformset issue.

[Tim Graham]

I agree existing objects should be taken into account so the number of blank forms should be (min_num + extra - existing objects). Let's call it a bug and fix it now.

[Stephen Burrows]

Opened a new ticket for this: #22628