title: RuntimeError: Model class doesn't declare an explicit app_label and isn't in an application in INSTALLED_APPS
body: <p>So, I upgraded my django version from 1.11 to 2.2 and ran <code>python manage.py runserver</code>.  I get the error:</p>
  <pre><code>Model class &lt;project_app_name&gt;.models.&lt;model_name&gt; doesn't declare an explicit app_label and isn't in an application in INSTALLED_APPS.
  </code></pre>
  <p>Obviously, all apps are registered including the &lt;project_app_name&gt;. So, the answers that try the solution of adding the app to the installed apps don't apply here.</p>
  <p>Any help would be appreciated</p>