code: def _check_field_changed(instance, old_instance, field_name, update_fields=None):
      
      if update_fields is not None and field_name not in update_fields:
          return False
  
      return getattr(instance, field_name) != getattr(old_instance, field_name)
docs: Examines update_fields and an attribute of an instance to determine if
      that attribute has changed prior to the instance being saved.
  
      Parameters
      ----------
      field_name : str
      instance : object
      old_instance : object
      update_fields : list of str, optional