fix.py

def get_reserved_tickets(self, performance=None):
    Offer = apps.get_model('events', 'Offer')
    Order = apps.get_model('events', 'Order')
    offers = Offer.objects.published().filter(
        state=1,
        ticket_price=self,
    )
    if performance:
        offers = offers.filter(performance=performance)
    pending_tickets = offers.filter(basket__state=0).distinct() \
        .aggregate(count=models.Sum('quantity'))['count']
    pending_tickets = pending_tickets if pending_tickets else 0

    booked_tickets = offers = offers.filter(basket__state=1) \
        .values_list('basket_id', flat=True)

    booked_tickets = Order.objects \
        .filter(state=1, basket_id__in=list(booked_tickets)) \
        .aggregate(count=models.Sum('tickets_count'))['count']
    booked_tickets = booked_tickets if booked_tickets else 0

    return pending_tickets + booked_tickets