Hi João,
I don't think you can do this with an aggregate. You can however achieve this scenario easily with an Advanced SQL:
SELECT {Customer}
.*, {Purchase}
.*
FROM {Customer} INNER JOIN
(
SELECT {Purchase}.[
customer_id],
MAX({Purchase}.[
date]) [
MaxDate]
FROM {Purchase}
GROUP BY {Purchase}.[
customer_id]
) MaxDates ON c.id = MaxDates.customer_id INNER JOIN
{Purchase}
ON MaxDates.customer_id = {Purchase}
.[customer_id]
AND MaxDates.MaxDate = {Purchase}
.[date]
This is an example of such a query.
Kind Regards,
João