select_related_joins.py

product = Product.objects.get(id=10) # Hits the database.

category = product.category # Hits the database again.
user = category.created_by # Hits the database again!


# using select_related
product = Product.objects.select_related('category__created_by').get(id=10) # Hits the database

category = product.category # Doesn't hit the database
user = category.created_by # Doesn't hit the database