How QuerySets are evaluated in Django? - django

Question

I have following models:
class Product(models.Model):
"""
Basic product
"""
name = models.CharField(max_length=100, db_column='name', unique=True)
url = models.SlugField(max_length=100, db_column="url", unique=True, db_index=True)
description = HTMLField(db_column='description')
category = models.ForeignKey(Category, db_column='category', related_name='products')
class FirstObject(Product):
pass
class FirstProduct(models.Model):
product = models.ForeignKey(FirstObject, db_column='product')
color = models.ForeignKey(Color, db_index=True, db_column='color')
class SecondObject(Product):
pass
class SecondProduct(models.Model):
product = models.ForeignKey(SecondObject, db_column='product')
diameter = models.PositiveSmallIntegerField(db_column='diameter')
In other words I have two different types of products (with different parameters).
I want for particular category (in category can be only one type of product and I know what) select all products with appropriate parameters.
How can this be accomplished efficiently?
If I write Category.objects.get(id=id).products.all() and then use related manager to fetch parameters of particular product, does it mean that database is hitted for every product?
Second approach is to fetch all products in one query and then fetch all parameters.
Then group them in list/dictionary.
What approach is the best? Or maybe there is another approach?
Thank you.

Answer 1

Your schema really does not lend itself well to querying. You will very quickly hit the worst case query behaviour (2 queries for every type of modification to a product. I suggest you have a look at the schema for django-shop-simplevariations and see how they are able to achieve fast lookups (Hint, the schema is structured for prefetch_related to be effective).