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
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/**
* In the calculation rendering, in default.php, we are keying by values not labels for join elements,
* so there's no point keying by label for joins. So I think we need to change this chunk so it just tests to see if
* there is a getJoinValueColumn method, and if so, use that. I'm leaving the original code intact though,
* while I wait to see if this causes Other Horrible Things To Happen.
*/