title: Django AutoField with primary_key vs default pk
body: <p>I am curious about Django database model pk.</p>
  
  <p>Is there any difference this </p>
  
  <pre><code>class Category(models.Model):
      category_id = models.AutoField(primary_key=True)
      category_name = models.CharField(max_length=50)
  </code></pre>
  
  <p>between this?</p>
  
  <pre><code>class Category(models.Model):
     category_name = models.CharField(max_length=50)
  </code></pre>
  
  <p>Are the same things <code>AutoField with primary_key</code> and <code>default pk</code>?</p>