Re: Admin validation activated just once after final call to save_model()

I had a similar problem that I've worked through. I recommend solutions
at three levels:

1. The UI - Make it more intuitive for the race admins to rank the results. Rather than give them an integer input field, use something like django-admin-sortable2[b] to allow the race admin to drag-n-drop to reorder the results.
This ensures that the user can't accidentally try to set two results as
first place, for example.

2. The change list page ModelFormSet validation - The Django admin change list page uses a ModelFormSet for list_editable. You can override the formset[a] and implement the `clean` method[c] to validate that the results are in strict order from 1 to N. This ensures that *if* the user violates the ordering rules, the results won't get saved and the page will display a nice error.

3. The database via transaction - Django's database transaction support allows automatic rollback if an exception is raised via `atomic`[d]. Override the change list page's ModelFormSet save method and wrap the individual form.save calls in a `with transaction.atomic()`. After the saves are done, check to make sure the results are in a strict order from 1 to N and raise an exception otherwise to rollback the changes.

You can choose to use any or all of the methods above together.

[a] https://docs.djangoproject.com/en/4.1/ref/contrib/admin/#django.contrib.admin.ModelAdmin.get_changelist_formset
[b] https://django-admin-sortable2.readthedocs.io/en/latest/
[c] https://docs.djangoproject.com/en/4.1/topics/forms/formsets/#custom-formset-validation
[d] https://docs.djangoproject.com/en/4.1/topics/db/transactions/#controlling-transactions-explicitly

On Sun, Jan 15, 2023 at 07:51:40AM -0800, David Wallace wrote:
> I am attempting to modify a page on my Django admin so that a trusted race
> administrator can manually rearrange a leader board of race results. The
> results go from 1 to N competitors in a race, and are mutually exclusive.
> Draw results are not allowed, so race administrators apply often complex
> and arcane tie breaking rules to avoid draws, which is the reason why they
> require the facility to manually re-arrange results.
>
> I have a model and validators here:
> ```
> def number_of_ranks(value):
> number_of_results = Rank.objects.all().count()
> if value > number_of_results:
> raise ValidationError(
> _('%(value)s is bigger than number of results which is
> %(number_of_results)s'),
> params={'value': value, 'number_of_results': number_of_results},
> )
>
>
> class Rank(models.Model):
>
> result = models.PositiveIntegerField(validators=[
> MinValueValidator(1),
> number_of_ranks]) # 1st, 2nd, 3rd = 1,2,3 etc - may be
> assigned by administrator
> team = models.ForeignKey( # team that achieved this
> result
> 'team',
> related_name='team_in_rank',
> on_delete=models.PROTECT
> )
> ```
> and an admin page for Rank here:
> ```
> @admin.register(Rank)
>
> class RankAdmin(admin.ModelAdmin):
> list_display = ('result', 'team',)
> sortable_by = ('')
> ordering = ('result',)
> list_editable = ('result',)
> list_display_links = None
>
> def has_add_permission(self, request):
> return False
>
> def has_delete_permission(self, request):
> return False
>
> def save_model(self, request, obj, form, change):
> print('-------------save_model called------------')
> print(request)
> print('================================')
> print('All objects and results BEFORE change')
> print([(rank, rank.result) for rank in Rank.objects.all()])
> print('================================')
> print('Changed object and changed result')
> print(obj, obj.result)
> print('================================')
> super().save_model(request, obj, form, change)
> print('All objects and results AFTER change')
> print([(rank, rank.result) for rank in Rank.objects.all()])
> ```
> This achieves some of what I want. That is
>
> 1. The admin can neither remove or add teams in the race, nor change the
> names of any of the teams, only results.
> 2. Any result must be in the range 1 to N competitors.
> 3. The display of the teams is arranged in order of result. 1 on the top
> row and N on the Nth row.
>
> The over-ride of save_model() achieves nothing functional but is merely an
> effort to discover something using print() calls about the validation and
> save sequence. It seems to indicate to me that there is a separate and
> individual call made to save_model() for each and every record that is
> changed.
>
> So that if I have four teams initially
> 1 Red, 2 Blue, 3 Yellow, 4 Green
> and the admin wants to change this to
> 1 Blue, 2 Red, 3 Yellow, 4 Green
> then that triggers two separate calls to save_model(): eg
> 1 Red, 2 Blue, 3 Yellow, 4 Green -> 1 Red, 1 Blue, 3 Yellow, 4 Green
> and
> 1 Red, 1 Blue, 3 Yellow, 4 Green -> 1 Blue, 2 Red, 3 Yellow, 4 Green
> (the actual stages can vary - the reds could change first)
>
> What I want to be able to do is perform a final validation only after the
> final record is changed so that I can check that every record has a unique
> result in the range 1..N - i.e. there are no duplicated results. But in the
> intermediate call(s) to save_model() there will inevitably be at least one
> duplicated result - just one if the admin exchanges two results, but
> potentially more if he attempts a more complicated reshuffle.
>
> So I want to hook into / over-ride a procedure which is called just once
> when triggered by pressing the Save button, after all the separate
> save_model() calls are complete. I haven't been able to discover from the
> docs how to do this, or even if it can be done. Possibly my approach to
> this is wrong, either entirely or in part?
>
> Any advice would be helpful.
> Thanks
>
>
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