TIL: Django relations are not cached in model instance
Evaluating relation multiple times using .all() performs n queries
Input:
order = Order.objects.first()
with query_debugger("without prefetch"):
list(order.lines.all())
list(order.lines.all())
list(order.lines.all())
Output:
without prefetch: 3 queries
Evaluating a related queryset stored in a variable only performs one query
Input:
order = Order.objects.first()
with query_debugger("storing related queryset in a variable"):
lines = order.lines.all()
list(lines)
list(lines)
list(lines)
Output:
storing related queryset in a variable: 1 queries
Evaluating a prefetched related queryset multiple times using .all() doesn’t perform any extra query
Input:
order = Order.objects.prefetch_related("lines").first()
with query_debugger("with prefetching"):
list(order.lines.all())
list(order.lines.all())
list(order.lines.all())
Output:
with prefetching: 0 queries
Models and tools used for this test
Models
from django.db import models
class Order(models.Model):
pass
class Line(models.Model):
order = models.ForeignKey("order.Order", on_delete=models.CASCADE, related_name="lines")
and a context decorator that measures how many queries are done
import time
from contextlib import contextmanager
from django.db import connection
@contextmanager
def query_debugger(name: str):
start_queries = len(connection.queries)
yield
end_queries = len(connection.queries)
query_count = end_queries - start_queries
print(f"{name}: {query_count} queries")