Updating a model when an object it references through a ManyToMany relation is deleted

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I have a model, for instance

class ModelA(models.Model):
  name = models.CharField()

class ModelB(models.Model):
  last_modified_time = models.DateTimeField()
  as = models.ManyToManyField('ModelA')

when I delete an instance of ModelA I would like to also update the last_modified_time field on all ModelB instances that referred to ModelA. I can use a predelete signal for this, i.e.

def pre_delete_handler(sender=None, instance=None, *args, **kwargs):
   # update all using instance.modelbs...

models.signals.pre_delete.connect(pre_delete_handler, sender=ModelA)

but then every time a new model references it via a ManyToMany this bit of code needs to be updated which isn't the best from a maintenance point of view. I am looking for a way to enumerate all models that ModelA is referenced by so I can have a single update that covers all cases but can't figure out the best way to do this. What is the proper way to enumerate all objects that ModelA is related to?

django-queryset django-models django
1 Answers

If all the models that refer to A as many-to-many have the field last_modified_time, then something like this can work:

from django.db.models.fields.reverse_related import ManyToManyRel
from django.utils import timezone


def pre_delete_handler(sender=None, instance=None, *args, **kwargs):
    if not instance:
        return

    # Get all reverse m2m fields
    reverse_m2m_fields = [m for m in instance._meta.model._meta.get_fields() if isinstance(m, ManyToManyRel)]
    # or [m for m in ModelA._meta.get_fields() if isinstance(m, ManyToManyRel)]

    for f in reverse_m2m_fields:
        # Find the name of the reverse m2m field from this model
        field_name = f.related_name or f.related_query_name or f'{f.name}_set'
        getattr(instance, field_name).update(last_modified_time=timezone.now())


models.signals.pre_delete.connect(pre_delete_handler, sender=ModelA)
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