What is most efficient way to get ranking in QuerySet?

Question

I'm trying to do ranking of a QuerySet efficiently (keeping it a QuerySet so I can keep the filter and order_by functions), but cannot seem to find any other way then to iterate through the QuerySet and tack on a rank. I dont want to add rank to my model if I don't have to.

I know how I can get the values I need through SQL query, but can't seem to translate that into Django:

SET @rank = 0, @prev_val = NULL;
SELECT rank, name, school, points FROM
    (SELECT @rank := IF(@prev_val = points, @rank, @rank+1) AS rank, @prev_val := points, points, CONCAT(users.first_name, ' ', users.last_name) as name, school.name as school
    FROM accounts_userprofile
        JOIN schools_school school ON school_id = school.id
        JOIN auth_user users ON user_id = users.id
    ORDER BY points DESC) as profile
ORDER BY rank DESC

I found that if I did iterate through the QuerySet and tacked on 'rank' manually and then further filtered the results, my 'rank' would disappear - unless is turned it into a list (which made filtering and sorting a bit of pain). Is there any other way you can think of to add rank to my QuerySet? Is there any way I could do the above query and get a QuerySet with filter and order_by functions still intact? I'm currently using the jQuery DataTables with Django to generate a leaderboard with pagination (which is why I need to preserver filtering and order_by).

Thanks in advance! Sorry if I did not post my question correctly - any help would be much appreciated.

Answer 1

我自己没有使用它，但我很确定你可以用extra（）方法做到这一点。

Answer 2

Looking at your raw SQL I don't see anything special in your logic. You are simply enumerating all SQL rows on the join ordered by points with a counter from 1, while collapsing the same point values to a same rank.

My suggestion would be to write a custom manager that uses raw() or extra() method. In your manager you would use python's enumerate on all model instances as a rank previously ordered by points . Of course you would have to keep current max value of points and override what enumerate returns to you if they have the same amount of points. Look here for an example of something similar.

Then you could do something like:

YourQuerySet.objects.with_rankings().all()