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Use case
When the 'Number field' custom field is changed in a sub-task, the corresponding field in the parent, representing the sum of those field values, gets updated with the updated result.
Field Changed Event
Create a new rule and name it appropriately.
Providing a description will help you to identify what the rule does but this step is optional.
Add a trigger and choose the Field Changed Event
Choose Number Field in Fields* and Any value in Changed to*
No further configuration needed. The automation rule is triggered every time an issue is being transitioned.
Issue Selector
Add Selector → Issue Selector
Update Field
Add Action → Update Field
Field*: Number Field
Update to*: Value returned by parsed expression
Parser Expression*
Set it to Advanced and use the following expression:
sum(fieldValue({issue.cfNNNNN}, subtasks()))
NNNNN is the field code for the field to be summed up.
Activate the rule by clicking on the Activate rule button →
Screencast
This is how the configuration above should look on your screen