Converting select statement directly into json array - postgresql

Question

From here, and here I have figured out that if I want to aggregate a set of related rows into an array of objects I have to use this syntax:
(select to_json(C) from ( /* subquery */ ) C)
So, if I have three tables: user, creature and their junction table user_creature:
And I want to retrieve each user, and each creature that belongs to this user, I would have to do something like this:
select to_json(T)
from (
select "user".id as user_id,
(select to_json(C) -- !!! There it is
from (
select name, height
from creature
inner join "user_creature" uc on creature.id = "uc".creature_id
inner join "user" u on "uc".user_id = u.id
where u.id = user_id
) C) as "creatures" -- !!! There it is
from "user"
) T;
This query successfully retrieves a list of users and their related creatures:
Is there a way to drop select and from keywords from the query, so that I can write my query like this:
select to_json(T)
from (
select "user".id as user_id,
to_json( -- !!! Calling to_json directly on select statement
select name, height
from creature
inner join "user_creature" uc on creature.id = "uc".creature_id
inner join "user" u on "uc".user_id = u.id
where u.id = user_id
) as "creatures"
from "user"
) T;

Answer 1

It is possible to use a subquery as the argument to to_json, but not practical:
You need to wrap the subquery in a grouping parenthesis: to_json( (SELECT … FROM …) )
The subquery must return exactly one row (but that's normal)
The subquery must return exactly one column. This is a bit harder - you can return a record, but if you build it dynamically (e.g. from a selection of columns, you can hardly control the field names)
(See a demo here).
Instead, use json_build_object if you want to write a single SELECT query only:
SELECT json_build_object(
'user_id', u.id,
'creatures', (
SELECT json_build_object(
'name', c.name,
'height', c.height
)
FROM creature c
INNER JOIN "user_creature" uc ON c.id = uc.creature_id
WHERE uc.user_id = u.id
)
)
FROM "user" u;
And, if you want to be able to retrieve multiple rows use SELECT json_agg(json_build_object(…)) FROM … or ARRAY(SELECT json_build_object(…) FROM …):
SELECT json_build_object(
'user_id', u.id,
'creatures', (
SELECT json_agg(json_build_object(
'name', c.name,
'height', c.height
))
FROM creature c
INNER JOIN "user_creature" uc ON c.id = uc.creature_id
WHERE uc.user_id = u.id
)
)
FROM "user" u;