Fix: Error displaying the error page: Unknown column 'a.client_id' in 'where clause': Call to undefined method JApplicationAdministrator::isClient() – Joomla

If you are trying to update your Joomla site using the Joomla! Update component in your backend to the latest Joomla version, you may receive an error during the update process or after the update attempts to complete. The failed Joomla update renders your Joomla website completely useless and you are stuck with a non-functioning website suddenly!

Error displaying the error page: Unknown column 'a.client_id' in 'where clause': Call to undefined method JApplicationAdministrator::isClient()

And or…

Call to undefined method JApplicationSite::isClient()

There is a known bug upgrading certain versions of Joomla 3.x to 3.6, 3.7 or 3.8 which cause these messaged. Luckily, there is a post-manual script developed which will recover your Joomla site and complete the update process succesfully. Follow the steps below.

1. After initiating a Joomla! Update in your site backend, you may receive a page with the following error message now:

Error displaying the error page: Unknown column 'a.client_id' in 'where clause': Call to undefined method JApplicationAdministrator::isClient()

2. Download the postupdate.php fix script and install this into your Joomla administrator folder path located below.

\Joomla Root Folder\administrator

3. Next, navigate to the post update script by typing in your Joomla site URL and pointing to the script. Example below.

myjoomlasite.com\administrator\postupdate.php

4. The first time you navigate to the script; you may see an error about failing to open a directory or no file found. Refresh the same link/page again.

5. After refreshing the link a 2^nd time, you should see a message similar to below. Update to 3.8.5 completed successfully.

6. Finally, remove the postupdate.php script from your administrator folder!

To learn more about the post-manual update scrip for Joomla, visit GitHub here.