ManyToMany Field data validation

Using Django Getting Started
1

I’m having some difficulties to add some data validation to a manyTomany field I have in my model. The code I have until now is the following:

#models.py
class Cargo(models.Model):
    name = models.CharField(max_length=100)

    def __str__(self) -> str:
        return self.name

class Committee(models.Model):
    MAX_CARGOS = 3

    name = models.CharField(max_length=100)
    image = models.ImageField(upload_to='committee/', blank=True)
    cargos = models.ManyToManyField(Cargo, blank=True, related_name='cargos')

    def __str__(self) -> str:
        return self.name

#admin.py
class CommitteeCargoInline(admin.TabularInline):
    model = Committee.cargos.through
    max_num = 5

class CommitteeAdmin(admin.ModelAdmin):
    fieldsets = [
        (None,               {'fields': ['name']}),
    ]
    inlines = [CommitteeCargoInline]

admin.site.register(Committee, CommitteeAdmin)
admin.site.register(Cargo)

The aim here is to ensure that when admin user clicks on one of the “Save” buttons in admin panel Committee page, it is checked that a Committee has no more than MAX_CARGOS.

At the moment I’m kinda lost because I couldn’t find anything that says what is the correct and standard way to achieve this, I’d like feedback on this point.

I already tried multiple things:

  • Started by trying to add a field validator to my cargos attribute but then I found at Django Docs that field validators are not supported for ManyToMany fields (“ManyToManyField does not support validators.”)

  • Then I tried implement Committe save() method but it also didn’t work:

 def save(self, force_insert, force_update, using, update_fields):
      if self.cargos.count() > self.MAX_CARGOS:
          raise ValidationError(f"A Committee can have at most {self.MAX_CARGOS} cargos.")
      super().save(force_insert, force_update, using, update_fields)

Anyway this is not what I wanted since this implementation will only take into account the old model values and not the updated ones.

  • Then I tried to implement it through signals, it seems many people use this solution. Here, I tried to listen m2m_changed, but no signal of this type seems to be triggered when data is modified through admin panel. I was able to get a signal pre_save, but with this signal I don’t know how to get the new values:
def validate_committee_cargos(sender, instance, **kwargs):
    logging.info("We got a signal")
    logging.info(sender)
    logging.info(sender.objects.get(pk=instance.pk))

    updated = sender.objects.get(pk=instance.pk).cargos.all()

    for item in updated:
        logging.info(item)

    logging.info(kwargs)

    my_m2m_field = instance.cargos.all()

    for item in my_m2m_field:
        logging.info(item)
   
pre_save.connect(validate_committee_cargos, sender=Committee, weak=False)
  • Also saw some people implementing these validations only at forms level. IMO this is good to make some pre checks and to save some round-trips, but at backend level this validation should be done too, since it is part of business / model logic.

Are signals the standard way to do this? If yes, how can I achieve this data validation? Should I use managers, since Django Docs say “Signals can make your code harder to maintain. Consider implementing a helper method on a custom manager, to both update your models and perform additional logic, or else overriding model methods before using model signals.”?

Using Django 4.1. Thanks in advance

2

Right idea, but wrong location.

As you may have identified, the saving of the inlines is the second step of a two-part process.

The admin first saves the base model, then saves the related objects.

The method you’re looking for here is save_related.

(And yes, I always recommend against using signals in any situation other than those in which they are truly necessary.)

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