old_contents: from django.db import models
  
  from django.contrib.auth.models import User
  
  # class with name URL does not work with Flask in previous project
  # (URL - Uniform Resource Locator)
  class Locator(models.Model):
      url = models.CharField(max_length=500)
      title = models.CharField(max_length=500)
      groupname = models.CharField(max_length=25)
      date = models.DateTimeField(auto_now_add=True)
      username = models.ForeignKey(User, on_delete=models.CASCADE)
  
      def __str__(self):
          return self.title
  
new_contents: from django.db import models
  
  from django.contrib.auth.models import User
  
  # class with name URL does not work with Flask in previous project
  # (URL - Uniform Resource Locator)
  class Locator(models.Model):
      url = models.CharField(max_length=500)
      title = models.CharField(max_length=500)
      groupname = models.CharField(max_length=25, blank=True)
      date = models.DateTimeField(auto_now_add=True)
      username = models.ForeignKey(User, on_delete=models.CASCADE)
  
      def __str__(self):
          return self.title
  
subject: add 'blank=True' to 'groupname', because this field is not required