old_contents: from django.db import models
  
  # Create your models here.
  class Category(models.Model):
      name = models.CharField(max_length=56)
  
      def __str__(self):
          return self.name
  
  class Tag(models.Model):
      name = models.CharField(max_length=56)
  
      def __str__(self):
          return self.name
  
  class Article(models.Model):
      title = models.CharField(max_length=128)
      date_time = models.DateTimeField(auto_now_add=True)
      content = models.TextField(blank=True, null=True)
  
      category = models.ForeignKey(Category, on_delete=models.CASCADE)
      tags = models.ManyToManyField(Tag, blank=True, null=True)
  
      def __str__(self):
          return self.title
  
      class Meta:
          ordering = ['-date_time']
  
new_contents: from django.db import models
  
  # Create your models here.
  class Category(models.Model):
      name = models.CharField(max_length=56)
  
      def __str__(self):
          return self.name
  
  class Tag(models.Model):
      name = models.CharField(max_length=56)
  
      def __str__(self):
          return self.name
  
  class Article(models.Model):
      title = models.CharField(max_length=128)
      date_time = models.DateTimeField(auto_now_add=True)
      content = models.TextField(blank=True)
  
      category = models.ForeignKey(Category, on_delete=models.CASCADE)
      tags = models.ManyToManyField(Tag, blank=True)
  
      def __str__(self):
          return self.title
  
      class Meta:
          ordering = ['-date_time']
  
subject: change some column to non-nullable