Get SUM of Child Object on Form Load

[marc]

AwareIM 3.0 (1028)
I have a BO based on an table which has an relation over an third table to an child table.
I would like to show the SUM in the BO Form from an child table field.

BO Field: amounts (here should be the SUM)
Child Table Field amounts AND payment_status (Value for testing = Paid)

I am using the following Rule (Actions):
FIND member_package_payment WHERE member_package_payment_editor.member_contract_releation=member_package_payment.member_contract_relation

member_package_payment_editor.amounts=SUM member_package_payment.amounts WHERE (member_package_payment.payment_status='Paid')

The Value in the Form Field is always empty.
The Form Field is also empty if the Rule is (For testing only):
member_package_payment_editor.amounts='1'

How can i show an value in an Form on Load using a Rule in the BO?

[marc]

I have done further tests, the rule is working as it should but only when clicking SAVE the update is performed and the value is visible.
How can i get the updated values shown in the form on opening the from without to click SAVE first?

[aware_support]

Try checking the flag "used during initialization" from the Advanced properties of the rule.