Nested JSON aggregation in Postgres - sql

Question

I have a need to run a query over a Postgres database and aggregate it and export it as a json object using native Postgres tooling.
I can't quite get the aggregation working correctly and I'm a bit stumped.
Below is an example of some of the data
| msgserial | object_type | payload_key | payload | user_id |
+-----------+---------------+-------------+-----------------------------------------------------------+---------+
| 1696962 | CampaignEmail | a8901b2c | {"id": "ff7221da", "brand": "MAGIC", "eventType": "SENT"} | 001 |
| 1696963 | OtherType | b8901b2c | {"id": "ff7221db", "brand": "MAGIC", "eventType": "SENT"} | 001 |
| 1696964 | OtherType | c8901b2c | {"id": "ff7221dc", "brand": "MAGIC", "eventType": "SENT"} | 002 |
| 1696965 | OtherType | d8901b2c | {"id": "ff7221dd", "brand": "MAGIC", "eventType": "SENT"} | 001 |
| 1696966 | CampaignEmail | e8901b2c | {"id": "ff7221de", "brand": "MAGIC", "eventType": "SENT"} | 001 |
| 1696967 | CampaignEmail | f8901b2c | {"id": "ff7221df", "brand": "MAGIC", "eventType": "SENT"} | 002 |
| 1696968 | SomethingElse | g8901b2c | {"id": "ff7221dg", "brand": "MAGIC", "eventType": "SENT"} | 001 |
+-----------+---------------+-------------+-----------------------------------------------------------+---------+
I need to output a JSON object like this grouped by user_id
{
"user_id": 001,
"brand": "MAGIC",
"campaignEmails": [
{"id": "ff7221da", "brand": "MAGIC", "eventType": "SENT"},
{"id": "ff7221de", "brand": "MAGIC", "eventType": "SENT"},
{"id": "ff7221de", "brand": "MAGIC", "eventType": "SENT"}
],
"OtherTypes": [
{"id": "ff7221db", "brand": "MAGIC", "eventType": "SENT"},
{"id": "ff7221dd", "brand": "MAGIC", "eventType": "SENT"}
],
"Somethingelses": [
{"id": "ff7221dg", "brand": "MAGIC", "eventType": "SENT"}
]
},
{
"user_id": 002,
"campaignEmails": [
],
"OtherTypes": [
],
"Somethingelses": [
]
}
Essentially need to group al the payloads into arrays by their type grouped by the user_id
I started with JSONB_BUILD_OBJECT getting one of the object_types grouped together into an array but then got stumped.
Am I trying to achieve the impossible in raw PSQL? I'm really stumped and I keep hitting errors like X needs to be included in the GROUP BY clause etc...
I can group one of the object_types into an array grouped by user_id but can't seem to do all 3
My other thinking was to do have 3 subqueries but I'm not sure how to do that either.

Answer 1

You need two aggregations, first one in groups by user_id, object_type and the other by user_id only:
select
jsonb_build_object('user_id', user_id)
|| jsonb_object_agg(object_type, payload) as result
from (
select user_id, object_type, jsonb_agg(payload) as payload
from my_table
group by user_id, object_type
) s
group by user_id
Db<>Fiddle.