title: How can I check out only specified fields when I query the models?
body: <p>How can I query the model by constraint some fields, such as I don't want to query out the id, and password.</p>
  
  <pre><code>Class AppUser(models.Model):
      username = models.CharField(max_length=16)
      password = models.CharField(max_length=64)
      email = models.CharField(max_length=16)
  </code></pre>
  
  <p>When I query it now the situation is:</p>
  
  <pre><code>app_users = models.AppUser.objects.all()  # all the fields will be find out. 
  </code></pre>
  
  <p>So, how can I check out only specified fields when I query it, I don't want to shows the id and password?</p>