Django ORM: Joining a table to itself and aggregating - django

Question

I have a table in my Django app, UserMonthScores, where every user has a "score" for every month. So, it looks like
userid | month | year | score
-------+-------+------+------
sil | 9 | 2014 | 20
sil | 8 | 2014 | 20
sil | 7 | 2014 | 20
other | 9 | 2014 | 100
other | 8 | 2014 | 1
I'd like to work out which position a specific user was in, for each month, in the ranking table. So in the above, if I ask for monthly ranking positions for user "sil", per month, I should get a response which looks like
month | year | rank
------+------+-----
9 2014 2 # in second position behind user "other" who scored 100
8 2014 1 # in first position ahead user "other" who scored 1
7 2014 1 # in first position because no-one else scored anything!
The way I'd do this in SQL is to join the table to itself on month/year, and select rows where the second table was for the specific user and the first table had a larger score than the second table, group by month/year, and select the count of rows per month/year. That is:
select u1.month,u1.year,count(*) from UserMonthScores u1
inner join UserMonthScores u2
on u1.month=u2.month and u1.year=u2.year
and u2.userid = 'sil' and u1.score >= u2.score
group by u1.year, u1.month;
That works excellently. However, I do not understand how to do this query using the Django ORM. There are other questions about joining a table to itself, but they don't seem to cover this use case.