Rails/SQL: finding invoices by checking two sums - sql

Question

I have an Invoice model that has_many lines and has_many payments.
Invoice:
id
ref
Line:
invoice_id:
total (decimal)
Payment:
invoice_id:
total(decimal)
I need to find all paid invoices. So I'm doing the following:
Invoice.joins(:lines, :payments).having(' sum(lines.total) = sum(payments.total').group('invoices.id')
Which queries:
SELECT *
FROM "invoices"
INNER JOIN "lines" ON "lines"."invoice_id" = "invoices"."id"
INNER JOIN "payments" ON "payments"."invoice_id" = "invoices"."id"
GROUP BY invoices.id
HAVING sum(lines.total) = sum(payments.total)
But it always return empty array even if there are invoices fully paid.
Is something wrong with my code?

Answer 1

If you join to more than one table with a 1:n relationship, the joined rows can multiply each other.
This related answer has more detailed explanation for the problem:
Two SQL LEFT JOINS produce incorrect result
To avoid that, sum the totals before you join. This way you join to exactly 1 (or 0) rows, and nothing is multiplied. Not only correct, also considerably faster.
SELECT i.*, l.sum_total
FROM invoices i
JOIN (
SELECT invoice_id, sum(total) AS sum_total
FROM lines
GROUP BY 1
) l ON l.invoice_id = i.id
JOIN (
SELECT invoice_id, sum(total) AS sum_total
FROM payments
GROUP BY 1
) p ON p.invoice_id = i.id
WHERE l.sum_total = p.sum_total;
Using [INNER] JOIN, not LEFT [OUTER] JOIN on purpose. Invoices that do not have any lines or payments are not of interest to begin with. Since we want "paid" invoices. For lack of definition and by the looks of the provided query, I am assuming that means invoices with actual lines and payments, both totaling the same.

Answer 2

If one invoice have a line and two payments fully paid like this:
lines:
id total invoice_id
1 30 1
payments:
id total invoice_id
1 10 1
2 20 1
Then join lines and payments to invoice with invoce_id will get 2 rows like this:
payment_id payment_total line_id line_total invoice_id
1 10 1 30 1
2 20 1 30 1
So the sum of line_total will not equal to sum of payment_total.
To get all paid invoice could use exists instead of joins:
Invoice.where(
"exists
(select 1 from
(select invoice_id
from (select invoice_id,sum(total) as line_total
from lines
group by invoice_id) as l
inner join (select invoice_id,sum(total) as payment_total
from payments
group by invoice_id) as p
on l.invoice_id = p.invoice_id
where payment_total = line_total) as paid
where invoices.id = paid.id) ")
The sub_query paid will get all paid invoice_ids.