Annotating sum while still having access to foreign objects - django

Question

Let's say that I have a simple shopping cart system with these models:
from django.db import models
from django.utils import timezone
class Product(models.Model):
name = models.CharField(max_length=255)
# More fields...
#property
def featured_image(self):
try:
return self.productimage_set.all()[0]
except IndexError:
return None
def __str__(self):
return self.name
class ProductImage(models.Model):
product = models.ForeignKey(Product, on_delete=models.CASCADE)
image = models.FileField(upload_to='products/')
# More fields...
def __str__(self):
return self.product.name
class Order(models.Model):
created = models.DateTimeField(default=timezone.now)
# More fields...
def __str__(self):
return str(self.pk)
class OrderItem(models.Model):
order = models.ForeignKey(Order, on_delete=models.CASCADE)
product = models.ForeignKey(Product, on_delete=models.CASCADE)
quantity = models.PositiveIntegerField()
# More fields...
def __str__(self):
return 'Order %s: %s of %s' % (
self.order_id,
self.quantity,
self.product.name,
)
When a user places an order, an OrderItem object is created for each product they ordered, which lets me know (1) which product they purchased and (2) how many of that product they purchased.
Now, suppose I wanted to show the user which products they have ordered the most of. I'll use this data as an example:
Order 1:
Product A: 10
Order 2:
Product A: 10
Product B: 5
Order 3:
Product A: 10
Product B: 5
Product C: 5
This particular user should be shown the products in this order:
Product A: 30 total
Product B: 10 total
Product C: 5 total
This is the code that achieves this:
order_items_by_quantity = OrderItem.objects.values(
'product__name',
).annotate(
total_purchased=Sum('quantity'),
).order_by(
'-total_purchased',
)
for order_item in order_items_by_quantity:
print(order_item)
But with this approach, I can't use order_item.product.featured_image. Is there a way I can have both things? Namely, the quantities all summed up and ordered from most to least and the ability to use order_item.product.featured_image.

Answer 1

order_items_by_quantity = OrderItem.objects.values(
'product__name', 'product_id'
).annotate(
total_purchased=Sum('quantity'),
).order_by(
'-total_purchased',
)
for order_item in order_items_by_quantity:
pid = order_item['product_id']
image = ProductImage.objects.filter(product_id=pid).first()
print(order_item, image)

Answer 2

I don't think there's a way to get object property after you use values() as the queryset returns dicts with SELECT key-values and not model objects. You can do additional query to get products with featured images and map it to your order_items:
order_items_by_quantity = OrderItem.objects.values(
'product__name', 'product_pk'
).annotate(total_purchased=Sum('quantity')).order_by('-total_purchased')
product_image_map = {}
for product in Product.objects.prefetch_related('productimage_set'):
product_image_map[product.pk] = product.featured_image
for order_item in order_items_by_quantity:
order_item['featured_image'] = product_image_map[order_item['product_id']]
That way you ask database once for your products, not every time you iterate over order_items_by_quantity. Additionally you should do prefetch_related() on your images.