old_contents: from django.db import models
  
  class Tag(models.Model):
      name = models.CharField(max_length=255)
  
  class Author(models.Model):
      name = models.CharField(max_length=255)
  
  class Article(models.Model):
      title = models.CharField(max_length=255)
      content = models.TextField()
      is_published = models.BooleanField()
      created_at = models.DateTimeField()
      tags = models.ManyToManyField(Tag, blank=True)
      author = models.ForeignKey(Author, related_name='articles')
  
new_contents: from django.db import models
  
  class Tag(models.Model):
      name = models.CharField(max_length=255)
  
      def serialize(self):
          return {
              'id': self.id,
              'name': self.name
          }
  
  class Author(models.Model):
      name = models.CharField(max_length=255)
  
  class Article(models.Model):
      title = models.CharField(max_length=255)
      content = models.TextField()
      is_published = models.BooleanField()
      created_at = models.DateTimeField()
      tags = models.ManyToManyField(Tag, blank=True)
      author = models.ForeignKey(Author, related_name='articles')
  
subject: Add test for Model#serialize