Create json object from tree structure in Postgresql - json

Question

I have a nested tree data structure of groups and layers that need to be generated from a couple of joined database tables and then be stored as JSON in a PostgreSQL (version 12.8) database. Groups can contain layer objects, but also (sub)groups. The desired JSON output (generated via SQL and/or function) would look like shown below.
[{
"title": "Folder 1",
"type": "group",
"folded": false,
"layers": [{
"title": "Layer 1",
"type": "WMS",
"visible": true
},
{
"title": "Folder 2",
"type": "group",
"folded": true,
"layers": [{
"title": "Layer 2",
"type": "WMS",
"visible": true
},
{
"title": "Layer 3",
"type": "WMS",
"visible": true
},
{
"title": "Folder 3",
"type": "group",
"folded": true,
"layers": [{
"title": "Layer 4",
"type": "WMS",
"visible": false
},
{
"title": "Layer 5",
"type": "WMS",
"visible": true
}
]
}
]
}
]
}]
These are the database tables and sample data:
CREATE TABLE IF NOT EXISTS jtest.folders
(
id serial,
item_id integer,
title text,
folded boolean NOT NULL
);
CREATE TABLE IF NOT EXISTS jtest.layers
(
id serial,
item_id integer,
title text,
type text,
visible boolean
);
CREATE TABLE IF NOT EXISTS jtest."connect"
(
id serial,
item_id integer,
child_id integer,
layer boolean DEFAULT true
);
--data
INSERT INTO jtest.folders(id, item_id, title, folded) VALUES
(1,1,'Folder 1',false),(2,2,'Folder 2', true),(3,3,'Folder 3', true);
INSERT INTO jtest.layers(item_id,id,title,type,visible) VALUES
(1,4,'Layer 1','WMS',true),(2,5,'Layer 2','WMS',true),(3,6,'Layer 3','WMS',true),(4,7,'Layer 4','WMS',true),(5,8,'Layer 5','WMS',true);
INSERT INTO jtest.connect (item_id, child_id, layer) VALUES
(1,4,true),(1,2,false),(2,5,true),(2,6,true),(2,3,false),(3,7,true),(3,8,true);
I partially succeeded to generate a JSON output for a simple folder-layer tree using queries like the one below, but could not figure out how to correctly handle folders nested into a list of layers (aka sub folders).
SELECT jsonb_agg(sub)
from (
SELECT f.id, f.folded,f.item_id,f.title, to_jsonb(array_agg(l.*)) as layers
FROM jtest.folders f
JOIN jtest.connect c ON f.item_id = c.item_id
JOIN jtest.layers l ON l.item_id = c.child_id
where c.layer is true
GROUP BY f.id,f.folded,f.item_id,f.title
ORDER BY f.item_id ) sub;
Any ideas or examples on how to solve this ?