'Return a query for hrm_human_resource filtering
for entries which are linked to a current shift'

@staticmethod
def fire_staff_on_duty(station_id=None):

    db = current.db
    staff = db.hrm_human_resource
    roster = db.fire_shift_staff
    query = ((staff.id == roster.human_resource_id) & (roster.deleted != True))
    if (station_id is not None):
        query &= (roster.station_id == station_id)
    return query
