old_contents: from django.contrib.auth.models import User
  from django.db import models
  from django.db.models import fields
  
  
  class Receipt(models.Model):
      title = fields.CharField(max_length=255)
      price = fields.DecimalField(max_digits=10, decimal_places=2)
      user = models.ForeignKey(User)
  
      def __str__(self):
          return "{}_{}".format(self.title, self.price)
  
new_contents: from django.contrib.auth.models import User
  from django.db import models
  from django.db.models import fields
  from django.utils import timezone
  
  
  class Receipt(models.Model):
      title = fields.CharField(max_length=255)
      price = fields.DecimalField(max_digits=10, decimal_places=2)
      created = fields.DateTimeField(auto_now=True)
      modified = fields.DateTimeField(default=timezone.now())
      user = models.ForeignKey(User)
  
      def __str__(self):
          return "{}_{}".format(self.title, self.price)
  
subject: Add created and modified fields to Receipt