Nested array of objects in postgres

select posts.id,
       title,
       (case when count(c) = 0 then cast('[]' as json) else array_to_json(array_agg(c.obj)) end) AS comments
       from posts
left join (select post_id, json_build_object('id', id, 'message', message) as obj from comments) c on posts.id = c.post_id
group by posts.id
order by title