from django.db.models import Q
from django.utils import timezone

def filter_for_when(queryset, when=None):
    """
    Return a filtered queryset which is valid for the given date

    Logic:

    Look for entries that have a starting date before when
    and either
    - No ending date
    - Ending date after "when"

    Returns a queryset, you'll neet to apply .first() or similar on it

    """

    if not when:
        when = timezone.now()

    return queryset.filter(starting_date__lte=when).filter(Q(ending_date__gte=when) |
                                                           Q(ending_date__isnull=True))