row_to_json on a subset of columns.

I'm using a JSON column to store some aggregate data, like so:

UPDATE courses_table
SET aggregates = agg.aggregates
FROM (
SELECT course_id, row_to_json(sub) AS aggregates
FROM (
SELECT course_id, avg(rating) AS rating, sum(reviews_count) AS
reviews_count,
sum(user_started_count) AS user_started_count,
sum(all_user_started_count) AS all_user_started_count,
sum(user_completed_count) AS user_completed_count,
sum(all_user_completed_count) AS all_user_completed_count
FROM course_details_table
GROUP BY course_id
) sub
) agg
WHERE courses_table.id = agg.course_id;

This works, but also stores the course_id in the JSON document. Is
there a relatively clean way to remove it? The suggestions I got in
#postgresql on freenode were to remove the course_id from the
innermost select, but that would break the outer queries, or to use
row() to select only a few of the columns, which loses their column
names. I'm on PG 9.3.3.

Thanks!
Chris