Allow certain workflow transitions in "issue A" only if all their sub-tasks have a value on custom fields "user" and "date".
Add theĀ Logical validator to the desired workflow transition.
Add the following expression:
count(filterByPredicate(subtasks(), ^%{issue.cfnnnnn} != null and ^{issue.dueDate} != null)) = count(subtasks())
Replace ^%{issue.cfnnnnn} with the actual field code of the custom field.
Logical validator
Logical condition
count()
filterByPredicate()
subtasks()