Selecting a JSON object of arrays from a PostgreSQL table - sql

Question

I have prepared a simple SQL Fiddle demonstrating my problem -
In a two-player game I store user chats in a table:
CREATE TABLE chat(
gid integer, /* game id */
uid integer, /* user id */
created timestamptz,
msg text
);
Here I fill the table with a simple test data:
INSERT INTO chat(gid, uid, created, msg) VALUES
(10, 1, NOW() + interval '1 min', 'msg 1'),
(10, 2, NOW() + interval '2 min', 'msg 2'),
(10, 1, NOW() + interval '3 min', 'msg 3'),
(10, 2, NOW() + interval '4 min', 'msg 4'),
(10, 1, NOW() + interval '5 min', 'msg 5'),
(10, 2, NOW() + interval '6 min', 'msg 6'),
(20, 3, NOW() + interval '7 min', 'msg 7'),
(20, 4, NOW() + interval '8 min', 'msg 8'),
(20, 4, NOW() + interval '9 min', 'msg 9');
And I can fetch the data by running the SELECT query:
SELECT ARRAY_TO_JSON(
COALESCE(ARRAY_AGG(ROW_TO_JSON(x)),
array[]::json[])) FROM (
SELECT
gid,
uid,
EXTRACT(EPOCH FROM created)::int AS created,
msg
FROM chat) x;
which returns me a JSON-array:
[{"gid":10,"uid":1,"created":1514813043,"msg":"msg 1"},
{"gid":10,"uid":2,"created":1514813103,"msg":"msg 2"},
{"gid":10,"uid":1,"created":1514813163,"msg":"msg 3"},
{"gid":10,"uid":2,"created":1514813223,"msg":"msg 4"},
{"gid":10,"uid":1,"created":1514813283,"msg":"msg 5"},
{"gid":10,"uid":2,"created":1514813343,"msg":"msg 6"},
{"gid":20,"uid":3,"created":1514813403,"msg":"msg 7"},
{"gid":20,"uid":4,"created":1514813463,"msg":"msg 8"},
{"gid":20,"uid":4,"created":1514813523,"msg":"msg 9"}]
This is close to what I need, however I would like to use "gid" as JSON object properties and the rest data as values in that object:
{"10": [{"uid":1,"created":1514813043,"msg":"msg 1"},
{"uid":2,"created":1514813103,"msg":"msg 2"},
{"uid":1,"created":1514813163,"msg":"msg 3"},
{"uid":2,"created":1514813223,"msg":"msg 4"},
{"uid":1,"created":1514813283,"msg":"msg 5"},
{"uid":2,"created":1514813343,"msg":"msg 6"}],
"20": [{"uid":3,"created":1514813403,"msg":"msg 7"},
{"uid":4,"created":1514813463,"msg":"msg 8"},
{"uid":4,"created":1514813523,"msg":"msg 9"}]}
Is that please doable by using the PostgreSQL JSON functions?

Answer 1

I think you're looking for json_object_agg for that last step. Here is how I'd do it:
SELECT json_object_agg(
gid::text, array_to_json(ar)
)
FROM (
SELECT gid,
array_agg(
json_build_object(
'uid', uid,
'created', EXTRACT(EPOCH FROM created)::int,
'msg', msg)
) AS ar
FROM chat
GROUP BY gid
) x
;
I left off the coalesce because I don't think an empty array is possible. But it should be easy to put it back if your real query is something more complicated that could require it.