Recursive parent child JSON Postgresql - json

Question

I have the following recursive PostgreSQL table.
CREATE TABLE public."data" (
id int4 NULL,
parentid int4 NULL,
menu varchar NULL
);
I am trying to create a JSON object from the table.
[
{
"menu": "home",
"children": [
{
"menu": "home 1",
"children": []
},
{
"menu": "home 2",
"children": [
{
"menu": "home 2.1",
"children": []
}
]
}
]
},
{
"menu": "config",
"children": []
}
]
How could I create a hierarchal object like this?

Answer 1

First you should use the jsonb format instead of the json format in postgres, see the documentation here :
In general, most applications should prefer to store JSON data as
jsonb, unless there are quite specialized needs, such as legacy
assumptions about ordering of object keys..
Then, assuming your table is a set of (id, menu) tuples and parentid is the id of the parent of this tuple, you can try :
CREATE VIEW parent_children (parent, children, root, cond) AS
( SELECT jsonb_build_object('menu', p.menu, 'children', '[]' :: jsonb) :: text AS parent
, jsonb_agg(jsonb_build_object('menu', c.menu, 'children', '[]' :: jsonb)) :: text AS children
, array[c.parentid] AS root
, array[c.parentid] AS cond
FROM public.data AS c
LEFT JOIN public.data AS p
ON p.id = c.parentid
GROUP BY c.parentid
) ;
WITH RECURSIVE list(parent, children, root, cond) AS
( SELECT parent, children, root, cond
FROM parent_children
WHERE root = array[NULL] -- start with the root parents
UNION
SELECT p.parent
, replace(p.children, c.parent, replace(c.parent, '[]', c.children))
, p.root
, p.cond || c.cond
FROM list AS p
INNER JOIN parent_children AS c
ON position(c.parent IN p.children) > 0
AND NOT p.cond #> c.root -- condition to avoid circular path
)
SELECT children :: jsonb
FROM list AS l
ORDER BY array_length(cond, 1) DESC
LIMIT 1 ;