Complex Nested SUM / Sub-Select - ruby-on-rails

Question

UPDATED with sample data etc.
I am a bit over my head on this complex query. Some background: This is a rails app and I have expenditures model which has many expenditure_items which each has an amount column - these all sum up to a total for the related expenditure.
A given expenditure can be an Order which then can have multiple (or single or nil) related Invoice expenditures. I am looking for a single query that jives me all the orders which have total invoices and identify those that have invoices totalling more than a threshold (in my case 10%).
I get the idea from my searching that I need a sub-select here but I can't sort it out. I apologize as raw SQL is not my wheel house - normal Rails Active Record calls meet 99% of my needs.
Sample Data:
=> SELECT * FROM expenditures WHERE id = 17;
id | category | parent_id
-----+----------------+----------
17 | purchase_order |
=> SELECT * FROM expenditures_items WHERE expenditure_id = 17;
id | amount
-----+-------------
1 | 1000.00
2 | 2000.00
I need to obtain the SUM ( expenditures.amount ) in my result - the original order of $3,000.00.
Related Expenditures (invoices)
=> SELECT * FROM expenditures WHERE category = 'invoice', parent_id = 17;
id | category | parent_id
-----+----------------+----------
46 | invoice | 17
88 | invoice | 17
=> SELECT * FROM expenditures_items WHERE expenditure_id IN (46, 88) ;
id | amount | expenditure_id
-----+----------+---------------
23 | 500.00 | 46
24 | 1000.00 | 46
78 | 550.00 | 88
79 | 1100.00 | 88
Order 17 has two invoices (46 & 88) totalling $3,150.00 - this is the SUM of all the invoice expenditure_item amounts.
In the end I am looking for the SQL that gets me something like this:
=> SELECT * FROM expenditures WHERE category = 'purchase_order';
id | category | expenditure_total | invoice_total | percent
-----+----------------+-------------------+---------------+---------
17 | purchase_order | 3000.00 | 3150.00 | 5
45 | purchase_order | 4000.00 | 3000.00 | -25
75 | purchase_order | 7000.00 | 7000.00 | 0
99 | purchase_order | 10000.00 | 11100.00 | 11
percent is invoice_total / expenditure_total - 1.
I also need to (perhaps a HAVING clause) filter out only the results that have a percent > a threshold (say 10).
From all my searching this seems to be a sub query along with some joins but I am lost at this point.
UPDATED Further
I had another look - this is close:
SELECT DISTINCT expenditures.*, SUM( invoice_items.amount ) as invoiced_total FROM "expenditures" JOIN expenditures AS invoices ON invoices.category = 'invoice' AND expenditures.id = CAST( invoices.ancestry AS INT) JOIN expenditure_items ON expenditure_items.expenditure_id = expenditures.id JOIN expenditure_items AS invoice_items ON invoice_items.expenditure_id = invoices.id WHERE "expenditures"."category" IN ($1, $2) GROUP BY expenditures.id HAVING (( SUM( invoice_items.amount ) / SUM( expenditure_items.amount ) ) > 1.1 ) [["category", "work_order"], ["category", "purchase_order"]]
Here is the odd thing - the invoiced_total in the select works. I get the proper amounts as per my example. The issue seems to be in my HAVING where it only pulls the SUM on the first invoice.
UPDATE 3
Soooooo close:
SELECT DISTINCT
expenditures.*,
( SELECT
SUM(expenditure_items.amount)
FROM expenditure_items
WHERE expenditure_items.expenditure_id = expenditures.id ) AS order_total,
( SELECT
SUM(expenditure_items.amount)
FROM expenditure_items
JOIN expenditures invoices ON expenditure_items.expenditure_id = invoices.id
AND CAST (invoices.ancestry AS INT) = expenditures.id ) AS invoice_total
FROM "expenditures"
INNER JOIN "expenditure_items" ON "expenditure_items"."expenditure_id" = "expenditures"."id"
WHERE "expenditures"."category" IN ("work_order", "purchase_order")
The only thing I can't get is eliminate the expenditures that either have no invoices or that are over my 10% rule. The first was in my old solution with the original join - I can't seem to figure out how to sum on that join data.

Answer 1

step-by-step demo:db<>fiddle
I am sure, there is a better solution, but this one should work:
WITH cte AS (
SELECT
e.id,
e.category,
COALESCE(parent_id, e.id) AS parent_id,
ei.amount
FROM
expenditures e
JOIN
expenditures_items ei ON e.id = ei.expenditure_id
),
cte2 AS (
SELECT
id,
SUM(amount) FILTER (WHERE category = 'purchase_order') AS expentiture_total,
SUM(amount) FILTER (WHERE category = 'invoice') AS invoice_total
FROM (
SELECT
parent_id AS id,
category,
SUM(amount) AS amount
FROM cte
GROUP BY (parent_id, category)
) s
GROUP BY id
)
SELECT
*,
(invoice_total/expentiture_total - 1) * 100 AS percent
FROM
cte2
The first CTE joins the both tables. The COALESCE() function mirrors the id as parent_id if the record has none (if category = 'purchase_order'). This can be used to do one single GROUP on this id and the category.
This is done within the second CTE (most inner subquery). [Btw: I choose the CTE variant because I find it much more readable. In this case you could do all steps as subqueries of course.] This group sums up the different categories for each (parent_)id.
The outer subquery is doing a pivot. It shifts the different records per category into your expected result with the help of a GROUP BY and the FILTER clause (Have a look at this step in the fiddle to understand it). Don't worry about the SUM() function here. Because of the GROUP BY, one aggregation function is necessary, but it does nothing, because the grouping has been already done.
Last step is calculating the percent value out of the pivoted table.