"Cannot group by undefined identification or result variable." when applying filter

[hvanoch]

When we use a SQLfunction inside the Group By statement in the datagrid we get the error Cannot group by undefined identification or result variablewhen applying a filter on a toManyrelation.
This is caused by

platform/src/Oro/Bundle/FilterBundle/Filter/AbstractFilter.php

Line 121 in d93ef23

$subQb->groupBy(sprintf('%s, %s', $groupBy, $fieldExpr));

when using a doctrine function in a group by it should also be put inside the select (as AS HIDDEN)

[anyt]

Internal ticket id BAP-20456.