select_related_loops_sample.py

>>> Product.objects.filter(is_active=True).count()
200

category_names = set()

# Hits the database 201 times. 1 for fetching active products 
# and the rest 200 queries are for retrieving category for each product.
products = Product.objects.filter(is_active=True)
for product in products:
  category_names.add(product.category.name)
  

# using select_related

category_names = set()

# This is a single query and hits the database only once!!
products = Product.objects.filter(is_active=True).select_related('category')
for product in products:
  category_names.add(product.category.name)