Raw SQL -> Django ORM Code: Query that returns all blog posts with most recent approved comment for a specific user - django

Question

I'm working on a simple blog system. Here is my models.py file:
from django.contrib.auth.models import User
from django.db import models
class Comment(models.Model):
user = models.ForeignKey(User)
post = models.ForeignKey('Post')
content = models.TextField()
approved = models.NullBooleanField()
class Meta:
ordering = ('-id',)
def __unicode__(self):
return u'Comment by %s' % self.user
class Post(models.Model):
user = models.ForeignKey(User)
title = models.CharField(max_length=200)
slug = models.CharField(max_length=50)
content = models.TextField()
class Meta:
ordering = ('title',)
def __unicode__(self):
return self.title
Here is some test data in a fixture which I've named testdata.json (the "some_author" user is a superuser and the password is "Stack Overflow"):
[
{
"pk": 1,
"model": "auth.user",
"fields": {
"username": "some_author",
"first_name": "Some",
"last_name": "Author",
"is_active": true,
"is_superuser": true,
"is_staff": true,
"last_login": "2014-07-02T20:18:49Z",
"groups": [],
"user_permissions": [],
"password": "pbkdf2_sha256$12000$PTl1hfgcIGZy$/0w1jNMBuKi9zk11JXhoS5WrbMBUgMDkZAhEvNEelbs=",
"email": "some_author#example.com",
"date_joined": "2014-07-02T20:18:29Z"
}
},
{
"pk": 2,
"model": "auth.user",
"fields": {
"username": "some_reader",
"first_name": "Some",
"last_name": "Reader",
"is_active": true,
"is_superuser": false,
"is_staff": false,
"last_login": "2014-07-02T20:21:10Z",
"groups": [],
"user_permissions": [],
"password": "pbkdf2_sha256$12000$CtTGfFeOaRhd$oVR6zFSpK2qg1AZ4fgdBG/wt6Sr56dHsEIxFO99mHC8=",
"email": "some_reader#example.com",
"date_joined": "2014-07-02T20:21:10Z"
}
},
{
"pk": 3,
"model": "auth.user",
"fields": {
"username": "another_reader",
"first_name": "Another",
"last_name": "Reader",
"is_active": true,
"is_superuser": false,
"is_staff": false,
"last_login": "2014-07-02T20:21:34Z",
"groups": [],
"user_permissions": [],
"password": "pbkdf2_sha256$12000$ZPnmV7fVeie3$08H2vv3A8Py4E92+uVAIiEaeg8CAL5deTyNAZj1YJMs=",
"email": "another_reader#example.com",
"date_joined": "2014-07-02T20:21:34Z"
}
},
{
"pk": 1,
"model": "blog.comment",
"fields": {
"content": "Comment 1 of 1 on post 1: approved",
"post": 1,
"user": 2,
"approved": true
}
},
{
"pk": 2,
"model": "blog.comment",
"fields": {
"content": "Comment 1 of 1 on post 2: not approved",
"post": 2,
"user": 2,
"approved": false
}
},
{
"pk": 3,
"model": "blog.comment",
"fields": {
"content": "Comment 1 of 2 on post 3: approved",
"post": 3,
"user": 2,
"approved": true
}
},
{
"pk": 4,
"model": "blog.comment",
"fields": {
"content": "Comment 2 of 2 on post 3: not approved",
"post": 3,
"user": 2,
"approved": false
}
},
{
"pk": 5,
"model": "blog.comment",
"fields": {
"content": "Comment 1 of 2 on post 4: not approved",
"post": 4,
"user": 2,
"approved": false
}
},
{
"pk": 6,
"model": "blog.comment",
"fields": {
"content": "Comment 2 of 2 on post 4: approved",
"post": 4,
"user": 2,
"approved": true
}
},
{
"pk": 7,
"model": "blog.comment",
"fields": {
"content": "Comment 1 of 2 on post 5: approved",
"post": 5,
"user": 2,
"approved": true
}
},
{
"pk": 8,
"model": "blog.comment",
"fields": {
"content": "Comment 2 of 2 on post 5: approved",
"post": 5,
"user": 2,
"approved": true
}
},
{
"pk": 9,
"model": "blog.comment",
"fields": {
"content": "Comment 1 of 2 on post 6: not approved",
"post": 6,
"user": 2,
"approved": false
}
},
{
"pk": 10,
"model": "blog.comment",
"fields": {
"content": "Comment 2 of 2 on post 6: not approved",
"post": 6,
"user": 2,
"approved": false
}
},
{
"pk": 11,
"model": "blog.comment",
"fields": {
"content": "Comment 1 of 1 on post 7: approved",
"post": 7,
"user": 3,
"approved": true
}
},
{
"pk": 1,
"model": "blog.post",
"fields": {
"content": "First post",
"slug": "post-1",
"user": 1,
"title": "Post 1"
}
},
{
"pk": 2,
"model": "blog.post",
"fields": {
"content": "Second post",
"slug": "post-2",
"user": 1,
"title": "Post 2"
}
},
{
"pk": 3,
"model": "blog.post",
"fields": {
"content": "Third post",
"slug": "post-3",
"user": 1,
"title": "Post 3"
}
},
{
"pk": 4,
"model": "blog.post",
"fields": {
"content": "Fourth post",
"slug": "post-4",
"user": 1,
"title": "Post 4"
}
},
{
"pk": 5,
"model": "blog.post",
"fields": {
"content": "Fifth post",
"slug": "post-5",
"user": 1,
"title": "Post 5"
}
},
{
"pk": 6,
"model": "blog.post",
"fields": {
"content": "Sixth post",
"slug": "post-6",
"user": 1,
"title": "Post 6"
}
},
{
"pk": 7,
"model": "blog.post",
"fields": {
"content": "Seventh post",
"slug": "post-7",
"user": 1,
"title": "Post 7"
}
},
{
"pk": 8,
"model": "blog.post",
"fields": {
"content": "Eighth post",
"slug": "post-8",
"user": 1,
"title": "Post 8"
}
}
]
I'm trying to query the database for all of the blog posts along with each blog post's most recent comment that meets both of these two conditions:
The comment was made by "Some Reader" (user_id = 2)
The comment has been approved
I want the query to return all of the blog posts, even if they don't have a comment that meets the above two conditions. For the blog posts that don't have a comment that meets the above two conditions, the returned comment column should just be NULL. I have this working with raw SQL:
for p in Post.objects.raw(
'''
SELECT blog_post.id,
blog_post.title,
blog_comment.content
FROM blog_post
LEFT OUTER JOIN (SELECT post_id,
MAX(id) AS latest
FROM blog_comment
WHERE user_id = 2
AND approved = 1
GROUP BY post_id) AS x
ON x.post_id = blog_post.id
LEFT OUTER JOIN blog_comment
ON blog_comment.post_id = x.post_id
AND blog_comment.id = x.latest
ORDER BY blog_post.id;
'''
):
print '%s: %s' % (
p.title,
p.content,
)
The above code outputs this (which is what I want):
Post 1: Comment 1 of 1 on post 1: approved
Post 2: None
Post 3: Comment 1 of 2 on post 3: approved
Post 4: Comment 2 of 2 on post 4: approved
Post 5: Comment 2 of 2 on post 5: approved
Post 6: None
Post 7: None
Post 8: None
My question is this: is it possible to (efficiently) do this same thing, but without resorting to raw SQL? I like to avoid raw queries whenever possible.

Answer 1

You can't do it without raw sql inside django orm paradigm.
But you can do it with two queries to db:
from django.db.models import Max
posts = Post.objects.annotate(Max('comment_set__id'))
comments_cache = Comment.objects.filter(id__in= posts.values('id', flat=True))
comments_dict = dict([(item.id, item) for item in comments_cache])
for item in posts:
print post, comments_dict[item.id]
I often make complex queries and still can't find better way than get all data I need with few queries inside cache-object and than group it as I need.
Please don't use code like:
#get_comment: return self.comment_set.filter(user=user, approved=True).latest('id')
for post in Post.objects.all():
print post.get_comment(request.user)
It will produce len(posts) sql-queries to database. It's bad practice.

Answer 2

You want all posts, but comments should be the one that matches specified criteria otherwise None. For that you can add a method/property inPost` model to get recent comment the way you wanted.
Don't think there is good way of doing this with ORM.
Add following method in your model,
class Post(models.Model):
...
#your code
def get_comment(self, user):
comment = None
try:
comment = self.comment_set.filter(user=user, approved=True).latest('id')
except Comment.DoesNotExist:
comment = None #no comment matching criteria
return comment
From you view or other code, you can do
for post in Post.objects.all():
print post.get_comment(request.user)