Django Query or SQL

Question

I have these models:

class Person(models.Model):
    user = models.OneToOneField(User)

class Post(models.Model):
    author = models.ForeignKey(Person, null=False)
    text = models.TextField(null=False, blank=False)

and want a Queryset at least with the below fields,

author.user.username
text

I have read select_related() queries but when I try to use that with this view can't get username field

posts = Post.objects.select_related('person__user')[:10]

can I use Django query or have to use SQL raw ?

Thanks for any help

what is the output you get when you print posts[0].author.user.username? — lmc
– lmc, Commented Aug 24, 2014 at 5:44
It should be select_related('author__user') if you want to prefetch the author information, but that alone shouldn't prevent you from doing e.g. posts[0].author.user.username as select_related is an optimization only. — Andrew Gorcester
– Andrew Gorcester, Commented Aug 24, 2014 at 5:49
oh, sorry! i got the right username! but my problem is accessing username after serializing "posts" can u help me with that ? does posts[0].author.user.username hit database again ? — Bheid
– Bheid, Commented Aug 24, 2014 at 5:52
thank u @Imc ! please answer "does posts[0].author.user.username hit database again ?" — Bheid
– Bheid, Commented Aug 24, 2014 at 6:05

Bheid · Accepted Answer · 2014-08-24 06:52:19Z

2

You can serialize like this:

import json
from django.core.serializers.json import DjangoJSONEncoder
json_data = json.dumps(list(Post.objects.values('author__user__username', 'text')[:10]), cls=DjangoJSONEncoder)

edited Aug 24, 2014 at 6:52

Bheid

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answered Aug 24, 2014 at 6:41

ruddra

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Comments

Josh Smeaton · Accepted Answer · 2014-08-24 06:48:41Z

0

select_related should be called with the field names, not the type.

posts = Post.objects.select_related('author__user')[:10]  
for post in posts:
    print(post.person.user.username)
    print(post.text)

All the select_related does is ensure that the foreign fields can be accessed without extra queries (select_related constructs joins to the relevant tables).

answered Aug 24, 2014 at 6:48

Josh Smeaton

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