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

In default case, user would need only one Leave Request Period, which
can be edited directly on LeaveRequest_view. So, in this case, hide the Listbox
containing the Leave Request Periods and have default_leave_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 Leave 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_leave_request_period_start_date will be disabled from LeaveRequest_view.

Additionally, now Leave Request has it's own:
- start_date: equal to the minimum start_date of all Leave Request Periods
- stop_date: equal to the maximum stop_date of all Leave Request Periods

These values are set by interaction on Leave Request Period, triggered on
_setStartDate, _setStopDate. This helps so that leave_request_module listbox
can have sortable dates.

Commit contains also <a href="https://lab.nexedi.com/georgios.dagkakis/erp5/blob/leave_request/product/ERP5/tests/testCalendar.py#L1626">test</a> that should make it easier to understand.

Test results <a href="https://nexedi.erp5.net/test_result_module/20171113-20E90B8F">here</a>

@seb we have talked on this and @cedric.leninivin I know you dealt with Leave Requests. Who else should be notified?





/reviewed-on nexedi/erp5!493