old_contents: from django.contrib.auth import get_user_model
  from django.contrib.auth.models import User
  from django.db import models
  
  def get_null_user():
      return get_user_model().objects.get_or_create(username='deleted')
  
  
  class Post(models.Model):
      created = models.DateTimeField(auto_now_add=True)
      title = models.CharField(max_length=128)
      content = models.TextField(max_length=1024)
      author = models.ForeignKey(User, related_name='posts',
                                 on_delete=models.SET(get_null_user))
      slug = models.SlugField()
  
      def __str__(self):
          if self.title is not None:
              return self.title
          else:
              return "New Post"
  
new_contents: from django.contrib.auth import get_user_model
  from django.db import models
  
  from boards.models import Board
  
  
  def get_null_user():
      return get_user_model().objects.get_or_create(username='deleted')
  
  
  def get_null_board():
      return Board.objects.get_or_create(name='deleted')
  
  
  class Post(models.Model):
      created = models.DateTimeField(auto_now_add=True)
      title = models.CharField(max_length=128)
      content = models.TextField(max_length=1024)
      author = models.ForeignKey(get_user_model(), related_name='posts',
                                 on_delete=models.SET(get_null_user))
      board = models.ForeignKey(Board, related_name='posts', 
                                on_delete=models.SET(get_null_board))
      slug = models.SlugField()
      key = models.CharField(max_length=6)
      
      def __str__(self):
          return self.title
  
subject: add fields/methods to integrate board app.