title: Unsupported lookup 'lower' for CharField or join on the field not permitted
body: <p>I'm attempting to perform a slightly more advanced lookup on a queryset, as outlined here: <a href="https://docs.djangoproject.com/en/3.0/topics/db/search/" rel="nofollow noreferrer">https://docs.djangoproject.com/en/3.0/topics/db/search/</a></p>
  
  <p>My queryset is as follows:</p>
  
  <pre><code>            queryset = queryset.filter(
                  Q(user__username__unaccent__lower__trigram_similar=search) |
                  Q(user__first_name__unaccent__lower__trigram_similar=search) |
                  Q(user__last_name__unaccent__lower__trigram_similar=search)
              )
  </code></pre>
  
  <p>I've included the <code>django.contrib.postgres</code> app in INSTALLED_APPS in my <code>settings.py</code>.</p>
  
  <p>However, I receive the following error:</p>
  
  <p><code>django.core.exceptions.FieldError: Unsupported lookup 'lower' for CharField or join on the field not permitted.</code></p>
  
  <p>Does anyone know the correct procedure for search a search?</p>
  
  <p>I am using Django >3 and postgres engine version 11.</p>