Get Foreign Key Objects In A Single Query

I have 2 models in my Django code: class ModelA(models.Model): name = models.CharField(max_length=255) description = models.CharField(max_length=255) created_by = model

Solution 1:

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.

Solution 2:

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.dbimport connection
cur=connection.cursor()
cur.execute(query)
rows = cur.fetchall()

your query should look like (for MySQL)

SELECT*FROM appname_modela INNERJOIN appname_modelb ON appname_modela.id=appname_modelb.modela_link_id

Solution 3:

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]

Solution 4:

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