[html-formfu] Question on dynamic select

Hi Folks,
I'm trying to include a query-driven select field in a Repeatable section
(subset shown below).

Where does the "select_from_db" subroutine live and how does the query
result need to get "munged" in order to work?

Thanks,
-Joe

- type: Repeatable
nested_name: cust_details
increment_field_names: 1
db:

elements:
- type: Hidden
name: pkey

- type: Select
options: __dynamic(select_from_db)__