Nested JSON aggregation in Postgres

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.