Query with Django ORM (joining models?) - django

Question

I have the following models:
class Thread(models.Model):
name = models.CharField(max_length=50)
category = models.ForeignKey(Category)
class Post(models.Model):
thread = models.ForeignKey(Thread)
datetime = models.DateTimeField(auto_now_add=True)
author = models.ForeignKey(User)
This is for my forum system.
I need to fetch all the Threads of category X and I need to have a few custom attributes like datetime and pk of the last post to the thread, for example:
threads = Thread.objects.filter(category=x)
last_post = Post.objects.filter(thread=threads[0]).order_by('-pk')[0]
Then I want to transfer everything to the template so that I get:
{{ t.pk }} as thread_pk
{{ t.last_post_pk }} as last_post_pk
And so on.

Answer 1

To get last post for every thread you should use aggregation:
Post.objects.filter(thread__in=threads).values('thread').annotate(last_id=Max('id')).order_by()
This will get you last_id from every thread in threads.

Answer 2

Since Django doesn't do joining like you would expect with SQL joins (see my comment for further explanation), I would solve it in the view (or perhaps a method added to the Thread class). Consider the following example view:
def index(request):
res = []
for t in Thread.objects.filter(category__name='x'):
res.append((t, Post.objects.filter(thread=t).order_by('-datetime')[0]))
# Optionally sort by datetime here:
# res.sort(key=lambda x: x[1].datetime, reverse=True)
return render_to_response('index.html', {'result': res})
You could then use it in the template as follows:
{% for r in res %}
Thread {{ r.0.name }}, last post by {{ r.1.author.name }} on {{ r.1.datetime }}.
{% endfor %}
# This would give something like this:
# Thread mythread, last post by john on Oct. 21, 2011, 1:52 a.m..
By creating a tuple of Threads and Posts, you don't really join them, but do make them easily available in the template.