title: TypeError: __init__() missing 1 required positional argument: 'on_delete'
body: <p>I'm trying to run my code and I'm getting this error:</p>
  
  <pre><code>  File "C:\Users\JOSHUA\Documents\ypgforum\myproject\boards\models.py", line 13, in &lt;module&gt;
      class Topic(models.Model):
    File "C:\Users\JOSHUA\Documents\ypgforum\myproject\boards\models.py", line 16, in Topic
      board = models.ForeignKey(Board, related_name='topics')
  TypeError: __init__() missing 1 required positional argument: 'on_delete'
  </code></pre>
  
  <p>This is the models.py it pointed to:</p>
  
  <pre><code>class Topic(models.Model):
      subject = models.CharField(max_length=255)
      last_updated = models.DateTimeField(auto_now_add=True)
      board = models.ForeignKey(Board, related_name='topics')
      starter = models.ForeignKey(User, related_name='topics')
      last_updated = models.DateTimeField(auto_now_add=True)
  </code></pre>