This is equivalent to work done for Leave Request, at:
185285c4
(merge_request 493)

Follow a simple / non-simple approach, like for Task and its lines.

In default case, user would need only one Presence Request Period, which can be edited directly on PresenceRequest_view. So, in this case, hide the Listbox containing the Presence Request Periods and have default_presence_request_period being created / edited on background, which will acquire quantity, resource, start_date and stop_date from what is set at the view of the Parent.
In case one or more Presence Request Period are added manually, user would get listbox with sub-objects, and editing should be done directly on them, i.e. fields like my_default_presence_request_period_start_date will be disabled fromPresenceRequest_view.
Additionally, now Presence Request has it's own:
start_date: equal to the minimum start_date of all Presence Request Periods
stop_date: equal to the maximum stop_date of all Presence Request Periods
These values are set by interaction on Presence Request Period, triggered on setStartDate, _setStopDate. This helps so that presence_request_module listbox can have sortable dates.
Commit contains also test that should make it easier to understand.