Calculate sum for items in nested jsonb array - postgresql

Question

So I have Postgres DB table 'purchases' with columns 'id' and 'receipt'. 'id' is primary int column, the value in column 'receipt' is jsonb and can look like this:
{
"shop_name":"some_shop_name",
"items": [
{
"name": "foo",
"spent": 49,
"quantity": 1
},
{
"name": "bar",
"price": 99,
"quantity": 2
},
{
"name": "abc",
"price": 999,
"quantity": 1
},
...
]
}
There can be varied amount of items in receipt.
In the end I need to write a query so the resulting table contains purchase id and amount spent on all bar for every purchase in table as such:
id
spent
1
198
..
...
My issue:
I can't figure out how to work with jsonb inside select query along regular columns in the resulting table, I guess query should be structured as this:
SELECT p.id, %jsonb_parsing_result_here% AS spent
FROM purchases p
It's blocking me from moving further with iterating through items in FOR cycle (or maybe using another way).

Answer 1

You can unnest the items from the receipt column with a lateral join and the jsonb_array_elements function like:
SELECT p.id, SUM((item->>'price')::NUMERIC)
FROM purchases p,
LATERAL jsonb_array_elements(p.receipt->'items') r (item)
GROUP BY p.id
It's also possible to add a where condition, for example:
WHERE item->>'name' = 'bar