[Solved]-Get foreign key objects in a single query

by

7👍

✅

As Ofri says, select_related only works on forwards relations, not reverse ones.

There’s no built-in way to automatically follow reverse relations in Django, but see my blog post for a technique to do it reasonably efficiently. The basic idea is to get all the related objects for every item at once, then associate them manually with their related item – so you can do it in 2 queries rather than n+1.

👤Daniel Roseman

1👍

Django ORM is a good thing but some some things is better to do manually.
You may import connection cursor and execute raw sql in single query.

from django.db import connection
cur=connection.cursor()
cur.execute(query)
rows = cur.fetchall()

your query should look like (for MySQL)

SELECT * FROM appname_modela INNER JOIN appname_modelb ON appname_modela.id=appname_modelb.modela_link_id
👤Shamanu4

1👍

The reason .select_related() didn’t work, is that .select_related() is used to follow foreign keys. Your ModelA doesn’t have a foreign key to ModelB. Its ModelB that has a foreign key to ModelA. (so a ModelA instance can have multiple ModelB instances related to it).

You could use this to do it in 2 queries, and a bit of python code:

list_b = ModelB.objects.all()
list_a = ModelA.objects.all()
for a in list_a:
    a.additional_data = [b for b in list_b if b.modela_link_id==a.id]
👤Ofri Raviv

0👍

from django.db.models import OuterRef, Subquery

newest = ModelB.objects.filter(modela_link=OuterRef('pk'))
ModelA.objects.annotate(newest=Subquery(newest))

https://docs.djangoproject.com/en/3.2/ref/models/expressions/#subquery-expressions

👤ykrop

Leave a comment