How to make Django comments use select_related() on "user" field? - django

Question

I'm using django comments frameworks. All the comments are posted by authenticated users. Near the comment, I'm showing some user profile info using {{ comment.user.get_profile }}
{# custom comment list templates #}
<dl id="comments">
{% for comment in comment_list %}
<dt id="c{{ comment.id }}">
{{ comment.submit_date }} - {{ comment.user.get_profile.display_name }}
</dt>
<dd>
<p>{{ comment.comment }}</p>
</dd>
{% endfor %}
</dl>
Problem is that django's comment queries does not use select_related() and for 100 comments I get 101 hit on the database.
Is there a way to make django comments framework to select user profile for each comment in one go?

Answer 1

I tested rendering 100 comments for an object with the default {% get_comment_list %} tag and django did 200 comment related queries to list the comments + user + profile because...
Comment.__unicode__ actually calls Comment.user if a user_id exists. +1 query
get_profile +1 query
Ouch!
I went from 203 queries in ~25ms to 3 in ~2ms.
Populate comment_list yourself
I would highly suggest building the comment_list QuerySet yourself using the appropriate select_related() calls. If it's used often, create a utility function called from your other views.
def get_comments_with_user_and_profile(obj):
content_type =ContentType.objects.get_for_model(obj)
return (Comment.objects
.filter(content_type=content_type, object_pk=obj.id)
.select_related('user__profile'))
If you want the entire framework to behave this way... You'll have to monkey patch.
It's not something I would do lightly. There are other ways around this specific problem but you did ask "in one go".
Put this somewhere in your INSTALLED_APPS models.py files. I actually have a monkey_patch app for modifying django.contrib.auth.User.username lengths and such (which is a last resort unlike here).
from django.contrib.comments.models import Comment
from django.contrib.comments.managers import CommentManager
class CommentManager(CommentManager):
def get_query_set(self):
return (super(CommentManager, self)
.get_query_set()
.select_related('user__profile'))
Comment.add_to_class('objects', CommentManager())
Gotchas with profiles and select_related()
Note that your UserProfile class needs a OneToOneField to User with a related_name equal to what you pass to select_related(). In my example it's profile and you need django 1.2+. I recall stumbling on that before.
class UserProfile(models.Model):
user = models.OneToOneField(User, related_name='profile')
# example to use User.objects.select_related('profile')

Answer 2

Assuming that you have a setup like so:
class UserProfile(models.Model):
user = models.ForeignKey(User, related_name='profile')
...
You can use the following select related: Comments.objects.select_related('user__pk','user__profile__pk') and that should do what you want.
You'll have to extend the comments framework. This is fairly straightforward. Basically, create your own comments app. You can look at django-threadedcomments for inspiration (and, actually, in some ways it's already a better implementation to use anyway).
Here's code you can insert into the django-threaded comments app to make sure it always uses the select related (in models.py):
class RelatedCommentManager(CommentManager):
def filter(self, *args, **kwargs):
return super(RelatedCommentManager, self).select_related('user__pk','user__profile__pk').filter(*args, **kwargs)
def exclude(self, *args, **kwargs):
return super(RelatedCommentManager, self).select_related('user__pk','user__profile__pk').exclude(*args, **kwargs)
def all(self)
return super(RelatedCommentManager, self).select_related('user__pk','user__profile__pk').all()
and replace
objects = CommentManager()
with
objects = RelatedCommentManager()
Follow the instructions for integrating threadedcomments into your app.
Then, in the template, I think you'll have to reference .profile instead of .get_profile.
It may be that Django automatically factors this in, so get_profile will not generate another db hit so long as .profile is available.

Answer 3

You can't use select_related() in this example, because User is foreign key of profile, not vice versa.
To avoid using cache (which is probably the best option) you could create proxy model for Comment with foreign key to your profile model. then you could write:
{{ comment.submit_date }} - {{ comment.user.profile.display_name }}