sucky.py

class InventoryEvent(AbstractEvent):

    booking_stock_item = models.ForeignKey('office.BookingStockItem', null=True, blank=True)
    booking_consumable = models.ForeignKey('office.BookingConsumable', null=True, blank=True)
    purchase = models.ForeignKey('inventory.Purchase', null=True, blank=True)
    cross_hire = models.ForeignKey('inventory.CrossHire', null=True, blank=True)
    booking_cross_hire = models.ForeignKey('office.BookingCrossHire', null=True, blank=True)
    repair = models.ForeignKey('inventory.Repair', null=True, blank=True)
    missing = models.ForeignKey('inventory.Missing', null=True, blank=True)