title: Django model.foreignKey and return self.text errors
body: <p>So I'm working on the model.py in Django and i'm getting 2 pylint errors.
  I don't understand why? is this an issue with pylint or something i'm doing wrong in my code.</p>
  
  <pre><code>E1120:No value for argument 'on_delete' in constructor call    
  E1136:Value 'self.text' is unsubscriptable
  </code></pre>
  
  <ul>
  <li><p>The first is on line 19, in Entry
     topic = models.ForeignKey(Topic)</p></li>
  <li><p>The second is on line 24 self.text[:50]</p></li>
  </ul>
  
  <p>If I remove the entry class the code works</p>
  
  <pre><code>from django.db import models
  
  # Create your models here.
  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>