title: on_delete error for Django
body: <p>I'm currently doing some projects from a book called Python Crash Course by Eric Matthes. Currently, I'm doing lab 19.1 where I have to use previous code to make a Django project while changing some of his own code but ran into a problem.
  Every time I want to run this command</p>
  
  <pre><code>&gt;&gt;&gt;python manage.py makemigration blogs
  </code></pre>
  
  <p>in return, I get this code</p>
  
  <blockquote>
    <p>TypeError: <strong>init</strong>() missing 1 required positional argument:
    'on_delete'</p>
  </blockquote>
  
  <p>His original models.py code:</p>
  
  <pre><code>from django.db import models
  
  class Topic(models.Model):
      """A topic the user is learning about."""
      text = models.CharField(max_length=200)
      date_added = models.DateTimeField(auto_now_add=True) 
  
      def __str__(self):
          """Return a string representation of the model."""
          return self.text
  
  class Entry(models.Model):
      """Something specific learned about a topic."""
      topic = models.ForeignKey(Topic)
      text = models.TextField()
      date_added = models.DateTimeField(auto_now_add=True)
  
      class Meta:
          verbose_name_plural = 'entries'
  
      def __str__(self):
          """Return a string representation of the model."""
          return self.text[:50] + "..."
  </code></pre>
  
  <p>and my current code:</p>
  
  <pre><code>from django.db import models
  
  # Create your models here.
  
  class BlogPost(models.Model):
      title = models.CharField(max_length=200)
      text = models.CharField(max_length=200)
      date_added = models.DateTimeField(auto_now_add=True)
  
      def __str__(self):
          """Return a string representation of the model."""
          return self.text
  
  class Post(models.Model):
      """SThe post"""
      topic = models.ForeignKey(BlogPost)
      text = models.TextField()
      date_added = models.DateTimeField(auto_now_add=True)
  
      class Meta:
          verbose_name_plural = 'posts'
  
      def __str__(self):
          if len(self.text) &gt;= 50:
              """Return a string represerntation of the model"""
              return self.text[:50] + "..."
          else:
              return self.text
  </code></pre>
  
  <p>I honestly do not know why I'm getting this error code I've check to see if I messed something up but I cannot find anything. Is there anyone that may know?</p>