models.py

class Item(TimeStampedModel):
    LEADTIME_PERIOD = (
        ('D', 'DAYS'),
        ('W', 'WEEKS'),
        ('M', 'MONTHS'),
    )

    sku = models.CharField(max_length=50, blank=False, null=False)
    item_name = models.CharField(max_length=50, blank=False, null=False)
    item_description = models.TextField()
    item_search = models.CharField(max_length=50, blank=False, null=False)
    item_category = models.ForeignKey('ItemCategory')
    item_group = models.ForeignKey('ItemGroup')
    item_lead_time = models.PositiveIntegerField()
    item_lead_time_period = models.CharField(max_length=1, choices=LEADTIME_PERIOD,
                                             default='D', null=True, blank=True)
    reorder_point = models.PositiveIntegerField()
    unit_price = models.DecimalField(max_digits=5, decimal_places=2, blank=True)
    unit_cost = models.DecimalField(max_digits=5, decimal_places=2, blank=True)
    manufacturer = models.ForeignKey(Manufacturer, on_delete=models.CASCADE)
    vendor = models.ForeignKey(Vendor, on_delete=models.CASCADE)
    total_qty_on_hand = models.PositiveSmallIntegerField(default=0, null=True, blank=True)
    total_qty_on_order = models.PositiveSmallIntegerField(default=0, null=True, blank=True)
    total_qty_in_transit = models.PositiveIntegerField(default=0, null=True, blank=True)
    is_consumable = models.BooleanField(default=False)
    
    def update_totals(self):
        total_qty_on_hand = 0
        total_qty_on_order = 0
        total_qty_in_transit = 0

        for item in self.itemlocation_set.all():
            total_qty_on_hand += item.qty_on_hand
            total_qty_on_order += item.qty_on_order
            total_qty_in_transit += item.qty_in_transit
            self.total_qty_on_hand = total_qty_on_hand
            self.total_qty_on_order = total_qty_on_order
            self.total_qty_in_transit = total_qty_in_transit
            self.save()

    def __unicode__(self):
        return unicode(str(self.item_name + '-' + self.sku))

    class Meta:
        ordering = ('item_name',)


class ItemLocation(models.Model):
    item = models.ForeignKey(Item, on_delete=models.CASCADE)
    location = models.ForeignKey(Location, on_delete=models.CASCADE)
    qty_on_hand = models.PositiveIntegerField()
    qty_on_order = models.PositiveIntegerField()
    qty_in_transit = models.PositiveIntegerField()

    def __unicode__(self):
        return "%s - %s - %s" % (self.item, self.location, self.qty_on_hand)

    class Meta:
        verbose_name = 'Items By Location'
        ordering = ('-id',)