title: NameError: name 'F' is not defined in Django ORM query
body: <p>I'm using the ipython (Python 3.7) console in PyCharm.  I'm trying got run a Django ORM query</p>
  
  <pre><code>Article.objects.filter((round(datetime.now(timezone.utc) - F("created_on")) / 300) * 300)
  </code></pre>
  
  <p>but I'm getting the following error</p>
  
  <pre><code>Traceback (most recent call last):
    File "/Users/davea/Documents/workspace/mainpage_project/venv/lib/python3.7/site-packages/IPython/core/interactiveshell.py", line 3267, in run_code
      exec(code_obj, self.user_global_ns, self.user_ns)
    File "&lt;ipython-input-12-53618885441a&gt;", line 1, in &lt;module&gt;
      Article.objects.filter((round(datetime.now(timezone.utc) - F("created_on")) / 300) * 300)
  NameError: name 'F' is not defined
  </code></pre>
  
  <p>I thought "F" was the way to reference a field in my model in an expression but I get the error above.  My Article model contains the field as below ...</p>
  
  <pre><code>class Article(models.Model):
      ...
      created_on = models.DateTimeField(default=datetime.now)
  </code></pre>