Iteration for insert into values - postgresql

Question

I will try and present the current setup as an abstract view, the focus being on the logical approach to batch insert.
CREATE TABLE factv (id, ..other columns..);
CREATE TABLE facto (id, ..other columns..);
CREATE TABLE dims (id serial, dimensions jsonb);
The 2 fact tables share the same dimensions, but they've got different columns.
There's an event stream which send messages to a table and there's function which is executed for each row, the logic is similar to:
CREATE OR REPLACE FUNCTION insert_event() RETURNS TRIGGER AS $$
IF event_json ->> 'type' = 'someEvent' THEN
WITH factv_insert AS (
INSERT INTO factv VALUES (id,..other columns..)
fn_createDimId,
NEW.event_json->>..,
...
RETURNING id
)
INSERT INTO facto VALUES (id,..other columns..)
(select id from factv_insert),
NEW.event_json->>..
ELSE DoSomethingElse...
END IF;
RETURN NEW;
END;
$$ LANGUAGE PLPGSQL;
The function called in here fn_createDimId just looks up the dims table and if these dimension are not found, they're inserted. If those are already in there, just give me that id for those dimensions as the id for this fact insert.
I do have some new events coming through and I need to grab some information which breaks the rules around insert into..values with ERROR: more than one row returned by a subquery used as an expression
The event structure is similar, but not limited to
{
"type": "someEvent",
"instruction": {
"contains": {
"id": "containerid",
"map": {
"50561:null:null": {
"productid": "50561",
"quantity": 3
},
"50562:null:null": {
"productid": "50562",
"quantity": 8
},
"50559:null:null": {
"productid": "50559",
"quantity": 5
}
}
},
"target": {
"50561": "Random",
"50562": "Random",
"50559": "Mix",
}
}
}
What is causing the problems here is the information around target and the respective quantities for those ids. From the event shown above, I need to aggregate and insert into the fact table:
-------|-----
target | qty
-------|-----
Random | 11
Mixed | 5
-------------
If I was to query to grab the information, I would run the following:
WITH meta_data as (
SELECT
json_object_keys(event_json -> 'instruction' ->'target') as prodid
,event_json -> 'instruction' ->'target'->>json_object_keys(event_json -> 'instruction' ->'target') as target
,event_json -> 'instruction' ->'contains'->'map'->json_object_keys(event_json -> 'instruction' ->'contains'->'map')->>'quantity' as qty
FROM event_table )
select
target,
sum(qty::int)
from meta_data
group by target;
I am looking to find a solution which makes it possible to do the same logical operations, but overcomes the error on the multiple returned rows, ideally an iteration for each event which returns more than 1 row.