Question

I’m trying to use .extra() where the query return more than 1 result, like :

'SELECT "books_books"."*" FROM "books_books" WHERE "books_books"."owner_id" = %s' % request.user.id

I got an error : only a single result allowed for a SELECT that is part of an expression

Try it on dev-server using sqlite3. Anybody knows how to fix this? Or my query is wrong?

EDIT:

I’m using django-simple-ratings, my model like this :

class Thread(models.Model):
    #
    #
    ratings = Ratings()

I want to display each Thread‘s ratings and whether a user already rated it or not. For 2 items, it will hit 6 times, 1 for the actual Thread and 2 for accessing the ratings. The query:

    threads = Thread.ratings.order_by_rating().filter(section = section)\
                .select_related('creator')\
                .prefetch_related('replies')

    threads = threads.extra(select = dict(myratings = "SELECT SUM('section_threadrating'.'score') AS 'agg' FROM 'section_threadrating' WHERE 'section_threadrating'.'content_object_id' = 'section_thread'.'id' ",)

Then i can print each Thread‘s ratings without hitting the db more. For the 2nd query, i add :

   #continue from extra 
   blahblah.extra(select = dict(myratings = '#####code above####',
                                voter_id = "SELECT 'section_threadrating'.'user_id' FROM 'section_threadrating' WHERE ('section_threadrating'.'content_object_id' = 'section_thread'.'id' AND 'section_threadrating'.'user_id' = '3') "))

Hard-coded the user_id. Then when i use it on template like this :

{% ifequal threads.voter_id user.id %}
#the rest of the code

I got an error : only a single result allowed for a SELECT that is part of an expression

Let me know if it’s not clear enough.

Answer 1

The problem is in the query. Generally, when you are writing subqueries, they must return only 1 result. So a subquery like the one voter_id:

select ..., (select sectio_threadrating.user_id from ...) as voter_id from ....

is invalid, because it can return more than one result. If you are sure it will always return one result, you can use the max() or min() aggregation function:

blahblah.extra(select = dict(myratings = '#####code above####',
                             voter_id = "SELECT max('section_threadrating'.'user_id') FROM 'section_threadrating' WHERE ('section_threadrating'.'content_object_id' = 'section_thread'.'id' AND 'section_threadrating'.'user_id' = '3') "))

This will make the subquery always return 1 result.

Removing that hard-code, what user_id are you expecting to retrieve here? Maybe you just can’t reduce to 1 user using only SQL.