Depending on your "relation" fetch it from the database.
http://fabrikar.com/forums/index.php?wiki/common-php-tasks/#select
http://fabrikar.com/forums/index.php?wiki/common-php-tasks/#select
For running J!5.1 you must https://fabrikar.com/forums/index.php?wiki/update-from-github/ or include the new file manually https://fabrikar.com/forums/index.php?threads/joomla-5-1-and-fabrik-cannot-find-files-error.54473/post-285151 See also Announcements
See Announcement
We are still working on the Joomla update.