Django模型查询是否被缓存？

Question

Let's say I have a model_a that has foreign key to model_b and I am accessing model_b 's properties like so:

model_a = ModelA.objects.get(id=id)

x = model_a.model_b.x
y = model_a.model_b.y
z = model_a.model_b.z

Is Django going to the DB and joining the tables to get x , y , and z fields on each line? Or does Django store the result of the first join and then no more joins are needed?

Is this in any way different, performance-wise?

model_b = model_a.model_b  # only one join
x = model_b.x
y = model_b.y
z = model_b.z

Just curious

Answer 1

You can see the SQL queries that Django with connection.queries .

from django.db import connection
model_a = ModelA.objects.get(id=1)
x = model_a.model_b.x
y = model_a.model_b.y
z = model_a.model_b.z
print(connection.queries)

This should show you two queries, one to fetch model_a and another to fetch model_b . Django caches model_a.model_b , so y = model_a.model_b.y model_a.model_b.z do not cause further lookups for model_b . See the docs on understanding cached attributes for more info.

You may also find the debug toolbar useful for seeing which SQL queries are running.

The most important tools to reduce sql queries are to use select_related and prefetch_related . For your example, you can use select_related . Since you are fetching a single ModelA instance, it will reduce the number of queries from two to one, so it probably won't make a noticeable difference to performance.

model_a = ModelA.objects.select_related('model_b').get(id=1)
x = model_a.model_b.x