How to reverse query for a record, returning all of its parent and their parents? - postgresql

Question

Suppose the following,
create schema if not exists my_schema;
CREATE TABLE IF NOT EXISTS my_schema.category (
id serial PRIMARY KEY,
category_name VARCHAR (255) NOT NULL,
subcategories BIGINT[] DEFAULT ARRAY[]::BIGINT[]
);
INSERT INTO my_schema.category VALUES
(
1,
'Pickup/dropoff',
'{}'
),
(
2,
'Electrician',
'{}'
),
(
3,
'Around the house',
'{2}'
),
(
4,
'Personal',
'{3}'
);
I'm trying to create breadcrumbs on the frontend. For instance, if Electrician is the selected category, I'd like to render something like Personal > Around the house > Electrician.
Ideally, I'd like the query to return something like,
{
"id": 2,
"breadcrumbs": "Personal.Around the house.Electrician"
}
This way, I could just do breadcrumbs.split('.').map(. . .).
Another result that would work is something like,
{
"id": 2,
"category_name": "Electrician"
"breadcrumbs": [
{
"id": 4,
"category_name": "Personal"
},
{
"id": 3,
"category_name": "Around the house"
},
]
}
Or even,
{
"breadcrumbs": [
{
"id": 4,
"category_name": "Personal"
},
{
"id": 3,
"category_name": "Around the house"
},
{
"id": 2,
"category_name": "Electrician"
}
]
}
How can I attain such a result?

Answer 1

You can do it using WITH RECURSIVE as follows :
to get any breadcrumbs just put its id on the first select where id = 2
WITH RECURSIVE cte(n, id, selectedCat) AS
(
SELECT 1, id, id::text
from category
where id = 2
UNION ALL
SELECT n+1, e.id, ep.selectedCat
FROM cte AS ep JOIN category AS e
ON ep.id = ANY (e.subcategories)
)
SELECT selectedCat, string_agg(category_name, ',') as breadcrumbs FROM (
SELECT selectedCat, category_name
FROM cte
inner join category c on c.id = cte.id
order by n desc
) AS s
GROUP BY selectedCat
demo here