Aggregating subquery keys to build a complex object - json

Question

I have a table that describes a list of products made for a given order, in this every row on this table has to have a an id of the product and the reason why it was purchased,
I would like to build a json response that amounts to an array of locations the ordered product is for, within that, an array of unique product codes and with that an array of reasons why that unique product was ordered.
I've only been able to get the topmost part of my query defined but the join and sub-select nature of the request is actually getting me in a bit of a fiddle. Is this kind of thing actually possible in plpgsql?
Additionally I'd like to join product_order.reason on product_order_reason.id and retrieve the longform_text inside the table associated with this row but I figure the bigger thing is to get the return at all and that's where i've been left stumped.
product
id |name |cost |cost_rate|
--------|------------------------|------|---------|
WALLC |Wall Clock | 15.00|SINGLE |
MIRR |Mirror | 25.00|SINGLE |
KEY |Door Keys | 5.00|SINGLE |
KEYFOB |Key Fob | 40.00|SINGLE |
product_order
product_id|quantity|location |quote_detail_quote_id |is_primary_order|reason|
----------|--------|----------|------------------------------------|----------------|------|
MIRR | 2|floor_0 |C7D33FED-CB15-5796-DC7D-A7BCEA8923C5|true | 1|
KEYF | 3|floor_0 |C7D33FED-CB15-5796-DC7D-A7BCEA8923C5|true | 2|
WALLC | 3|floor_1 |C7D33FED-CB15-5796-DC7D-A7BCEA8923C5|true | 1|
WALLC | 3|floor_1 |C7D33FED-CB15-5796-DC7D-A7BCEA8923C5|true | 3|
product_order_reason
------------------------------------------------
id (varchar, pk) | shortform_text(varchar) | longform_text(varchar)
------------------------------------------------
id|shortform_text |longform_text |
--|-------------------------------------|-----------------------------------------------------|
1|Employee Room |Standard employee room with no window |
2|Meeting Room |Standard Meeting Room |
3|Mirror |Additional Mirror Request |
create
or replace
function get_breakdown_v1_0_0(p_quote_id character varying,
p_location character varying,
p_product_code character varying) returns json language plpgsql as $function$ declare row_count smallint := 0;
begin
raise notice 'Location: %',
p_location;
raise notice 'Product: %',
p_product_code;
-- Perform santiy check on quote_id so that the json does not include a null result.
select
count(*) into
strict row_count
from
quote_detail
where
quote_id = p_quote_id;
if row_count = 0 then raise 'Quote ID % not found',
p_quote_id
using ERRCODE = '02000';
-- SQL standard no_data
elseif row_count > 1 then raise 'Too many rows returned for ID %',
p_quote_id
using ERRCODE = 'P0003';
-- PL/pgSQL too_many_rows
end if;
-- Returns an object comprised of unique values for locations, where not null and their associated products
return (
select
jsonb_build_object ('locations',jsonb_agg( jsonb_build_object( 'area', location, 'items', items)))
from
(
select
location,
jsonb_agg(jsonb_build_object ('code', product_id, 'reasons', reason)) as items
from
product_order
where
(quote_detail_quote_id = p_quote_id)
and (location = p_location
or p_location is null)
and (product_id = p_product_code
or p_product_code is null)
group by
location) a );
end $function$ ;
Desired response;
{
"area": "floor_0",
"items": [
{
"code": "WALLC",
"reasons": [
{
"quantity": 2,
"reason_code": "Standard Employee Room"
},
{
"quantity": 2,
"reason_code": "Standard Cubicle"
}
]
},
{
"code": "MIRR",
"reasons": [
{
"quantity": 3,
"reason_code": "Meeting Room"
}
]
}
]
}]

Answer 1

Alright I think I have something for you. The idea is to build one of the arrays at a time and carry the necessary remaining info to the outer queries for further array building. You can add your constraints for quote_detail_quote_id , location, and product_id to the innermost query's WHERE clause.
SQLFiddle to show it in action.
This may take some studying:
SELECT json_build_object('area', t3.location, 'items', t3.code_json)
FROM
(
SELECT t2.location
, array_to_json(array_agg(jsonb_build_object('code', t2.product_id, 'reasons', t2.qty_reason_json))) AS code_json
FROM
(
SELECT t.location
, t.product_id
, array_to_json(array_agg(jsonb_build_object('quantity', t.quantity, 'reason_code', t.longform_text))) AS qty_reason_json
FROM
(
SELECT po.product_id
, po.quantity
, po.location
, po.reason
, por.longform_text
FROM product_order po
JOIN product_order_reason por ON (por.id = po.reason)
WHERE quote_detail_quote_id = 'C7D33FED'
) t
GROUP BY t.location, t.product_id
) t2
GROUP BY t2.location
) t3
;