Identify the changed fields in django post_save signal

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If you want to compare state before and after save action, you can use pre_save signal which provide you instance as it should become after database update and in pre_save you can read current state of instance in database and perform some actions based on difference. 

from django.db.models.signals import pre_save
from django.dispatch import receiver


@receiver(pre_save, sender=MyModel)
def on_change(sender, instance: MyModel, **kwargs):
    if instance.id is None: # new object will be created
        pass # write your code here
    else:
        previous = MyModel.objects.get(id=instance.id)
        if previous.field_a != instance.field_a: # field will be updated
            pass  # write your code here

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