title: Python:django: TypeError: __init__() missing 1 required positional argument: 'on_delete'
body: <pre><code>from django.db import models
  
  class Topic(models.Model):
      text = models.CharField(max_length=200)
      date_added = models.DateTimeField(auto_now_add=True)
  
  
      def _str_(self):
          return self.text
  class Entry(models.Model):
      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 self.text[:50]+"..."
  </code></pre>
  
  <p><strong>This is my code</strong>,When i run:</p>
  
  <pre><code>python3 manage.py makemigrations learning_logs
  </code></pre>
  
  <p>The answer is :</p>
  
  <pre><code> TypeError: __init__() missing 1 required positional argument: 'on_delete'
  </code></pre>
  
  <p>why this warning ? I read the documentation too, but no help.</p>