title: Filtering starting from a Foreign key users that are not staff combined with other filters
body: <p>I have the following model:</p>
  
  <pre><code>class Post(models.Model):
      user = models.ForeignKey(settings.AUTH_USER_MODEL, related_name='activity',
   sent_at = models.DateTimeField(blank=True, null=True) on_delete=models.CASCADE)
  </code></pre>
  
  <p>I need a query that exclude all users that are not staff and sent is null.</p>
  
  <p>So I tried to use exclude:</p>
  
  <pre><code>Post.object.all().exclude(user.is_staff=True)
  </code></pre>
  
  <p>and I get the following error:</p>
  
  <pre><code>keyword can't be an expression
  </code></pre>