Postgres LTREE display by grouping parents and children - sql

Question

I have a sample Ltree Structure than I want to return it as JSON structure.
I've tried searching on stack overflow but the results are giving wrong responses.
create table node
(
id integer not null,
name varchar(255),
path ltree not null
);
and I have these data
INSERT INTO node (id,name,path) VALUES (1,'Residential','1');
INSERT INTO node (id,name,path) VALUES (2,'Commercial','2');
INSERT INTO node (id,name,path) VALUES (3,'Industrial','3');
INSERT INTO node (id,name,path) VALUES (4,'Res type 1','1.4');
INSERT INTO node (id,name,path) VALUES (5,'Comm type 1','2.5');
INSERT INTO node (id,name,path) VALUES (6,'Industrial 1','3.6');
INSERT INTO node (id,name,path) VALUES (7,'Residential 2','1.4.7');
INSERT INTO node (id,name,path) VALUES (8,'Commercial 2','2.5.8');
INSERT INTO node (id,name,path) VALUES (9,'Industrial 2','3.6.9');
and this is what I want to collect with my query
[
{
"name": "Residentioal",
"children": [
{
"name": "Res type 1",
"children": [
{
"name": "Residential 2",
"children": []
}
]
}
]
},
{
"name": "Commercial",
"children": [
{
"name": "Comm type 1",
"children": [
{
"name": "Commercial 2",
"children": []
}
]
}
]
},
{
"name": "Industrial",
"children": [
{
"name": "Industrial 1",
"children": [
{
"name": "Industrial 2",
"children": []
}
]
}
]
}
]
I tried recursive with .. but it keeps looping through without returning proper value.

Answer 1

You need two parts, the recursion one and additionally a function. I explained this already here, here and here, so please have a look there for further explanations.
demo:db<>fiddle
Recursion
WITH RECURSIVE cte AS (
SELECT
id,
name,
path,
json_build_object('name', name, 'children', ARRAY[]::text[]) AS jsonobject,
ARRAY[]::text[] || (row_number() OVER () - 1)::text as jsonpath,
0 as depth
FROM node
WHERE path = subpath(path, 0, 1) --parents
UNION ALL
SELECT
n.id,
n.name,
n.path,
json_build_object('name', n.name, 'children', ARRAY[]::text[]),
jsonpath || '{children}' || (row_number() OVER (PARTITION BY subpath(n.path, depth, 1)::text ORDER BY subpath(n.path, depth + 1, 1)::text::int) - 1)::text,
c.depth + 1
FROM
node n
JOIN cte c
ON c.id = subpath(n.path, depth, 1)::text::int
AND nlevel(n.path) = depth + 2 AND subpath(n.path, depth + 1, 1)::text::int = n.id
)
SELECT * FROM cte
The function
CREATE OR REPLACE FUNCTION nested_json() RETURNS jsonb AS $$
DECLARE
_json_output jsonb;
_temprow record;
BEGIN
_json_output := '[]'::jsonb;
FOR _temprow IN
-- <Add the CTE from above here>
LOOP
SELECT
jsonb_insert(
_json_output,
_temprow.jsonpath,
_temprow.jsonobject
)
INTO _json_output;
END LOOP;
RETURN _json_output;
END;
$$ LANGUAGE plpgsql;
Please notice: The ltree structure is not a really good choice for this use case because you need to calculate the subpaths again and again. A simple reference to the parent would be more helpful and faster.
Edit: The db<>fiddle admin is great and installed the ltree extension, so there is a new fiddle