Question

I have the following (simplified) models:

class Post(models.Model):
    title=models.CharField(max_length=500)

class Recommendation(models.Model):
    post=models.ForeignKey(Post)
    submit_time=models.DateTimeField(auto_now_add=True)

And I want to get the list of distinct Posts ordered by Recommendation's submit time.
The first way I tried was the straighforward:

Post.objects.order_by('recommendation__submit_time').distinct()

But surprisingly this gave a QuerySet with duplicate Post objects. Turns out the rows are actually different because Django adds extra columns for the ordering, but does not return them in the results.

Looking around I found a couple answers on SO, including to use aggregation instead of ordering:

Post.objects.all().annotate(Max('recommendation__submit_time')).order_by('recommendation__submit_time__max')

Or to de-normalize the model and add a last_recommended_time field to Post.

Most of the questions/answers already in SO are a couple years old, so I was wondering if there’s a more idiomatic and straightforward way to do this than those suggested hacks.

EDIT:
Just thought I made it clear:
The solutions listed above do work and I’m using them (albeit not in production). I’m just interested in better solutions to this issue.

Answer 1

Have you thought about using raw sql

Post.objects.raw("""
    SELECT DISTINCT post FROM
        (SELECT appname_post.post_id AS post, appname_recommendation.submit_time
        FROM appname_post
        INNER JOIN appname_recommendation 
        ON appname_post.post_id = appname_recommendation.post_id
        ORDER_BY appname_recommendation.submit_time)
    """)