old_contents: from django.db import models
  
  class HistoryFieldsMixin(models.Model):
      created_at = models.DateTimeField(auto_now_add=True)
      modified_at = models.DateTimeField(auto_now=True)
      deleted_on = models.DateTimeField(blank=True, null=True)
      changed_by = models.ForeignKey('auth.User', null=True, blank=True)
  
      # Used by django-simple-history
      @property
      def _history_user(self):
          return self.changed_by
  
      @_history_user.setter
      def _history_user(self, value):
          self.changed_by = value
      # END: Used by django-simple-history
  
      class Meta:
          abstract = True
new_contents: from django.db import models
  
  class HistoryFieldsMixin(models.Model):
      '''
      Adds common history fields to a model. changed_by field you need to populate yourself.
      Usage: 
      from ehealth_tools.django_tools.mixins import HistoryFieldsMixin
      class Area(HistoryFieldsMixin, models.Model):
          pass
      '''
      created_at = models.DateTimeField(auto_now_add=True)
      modified_at = models.DateTimeField(auto_now=True)
      deleted_on = models.DateTimeField(blank=True, null=True)
      changed_by = models.ForeignKey('auth.User', null=True, blank=True)
  
      # Used by django-simple-history
      @property
      def _history_user(self):
          return self.changed_by
  
      @_history_user.setter
      def _history_user(self, value):
          self.changed_by = value
      # END: Used by django-simple-history
  
      class Meta:
          abstract = True
  
subject: Add docu to mixin