Add new fields to nested JSON array in JSONB - json

Question

I have a nested JSON structure stored in a PostgreSQL table.
Table users:
id | content [JSON]
JSON:
{
"purchases": [
{
"id": 1,
"costs": [
{
"amount": 23
},
{
"amount": 34
}
]
},
{
"id": 2,
"costs": [
{
"amount": 42
}
]
}
]
}
I would like to add a field "jsonClass": "Static" to all the objects within the costs array so I have following in the end:
{
"purchases": [
{
"id": 1,
"costs": [
{
"jsonClass": "Static",
"amount": 23
},
{
"jsonClass": "Static",
"amount": 34
}
]
},
{
"id": 2,
"costs": [
{
"jsonClass": "Static",
"amount": 42
}
]
}
]
}
I couldn't figure out how to add values to such a nested structure. Anyone knows how to achieve such thing? The only way I found was to make it a text and do string replace which is not very performant and I have a lot of such entries.

Answer 1

Unfortunately, due to having to change multiple sub-objects, I don't know of a better way than to deconstruct and then reconstruct the object. It gets pretty hairy.
UPDATE users
SET content=(
SELECT jsonb_agg(purchase)
FROM (
SELECT jsonb_build_object('id', pid, 'purchases', jsonb_agg(cost)) AS purchase
FROM (
SELECT pid, cost || '{"jsonClass":"static"}'::jsonb AS cost
FROM (
SELECT purchase->'id' AS pid, jsonb_array_elements(purchase->'costs') AS cost
FROM jsonb_array_elements(content::jsonb->'purchases') AS purchase
) AS Q
) AS R
GROUP BY pid
) AS S
);
Fiddle
EDIT: Sorry about all the edits, forgot to test for multiple rows. Should be good now. It might be possible to simplify it a bit more, not sure.