ActiveRecord query to sum over joins - sql

Question

I want to get the sum of the receipt items that are in a particular budget (same title) and from the current query I'm getting to many record and obvious wrong sum of amounts from the receipt items.
My current attempt is looking like that in ActiveRecord (AR):
ReceiptItem.includes(donation: [:budgets]).joins(:donation, :receipt).where(budgets: {title: "Some title 2015"}).sum(:amount)
and my SQL attempt was looking like that (its also wrong):
-- want to test just the outcome its not actually not summing up the amounts
SELECT "receipt_items"."amount"
FROM
"receipt_items" INNER JOIN "donations" ON "donations"."id" = "receipt_items"."donation_id"
RIGHT JOIN "receipts" ON "receipts"."receipt_id" = "receipt_items"."receipt_id"
LEFT OUTER JOIN "budgets" ON "budgets"."donation_id" = "donations"."id"
WHERE "budgets"."title" = 'Some title 2015';
Why I'm getting double records although I've joined the tables and set also the condition?
Here is the ER modell to understand the problem.
And here's the AR Assoziations:
class Budget < ActiveRecord::Base
belongs_to :donation
class Donation < ActiveRecord::Base
has_many :receipt_items
has_many :budgets
class ReceiptItem < ActiveRecord::Base
belongs_to :donation

Answer 1

Because a budget can be linked to a reciept item multiple times, via different donations, it's appearing in the big join table multiple times, and thus being counted several times.
Let's try to think this through a step at a time. If you wanted to do it without worrying about eager loading, you would do:
Budget.where(title: "some title").all.collect(&:donation).collect(&:receipt_items).flatten.uniq.collect(&:amount).sum
is that right?
If so, you can tailor the eager loading to fit this chain of method calls:
Budget.where(title: "some title", include: {:donation => [:receipt_items]}).all.collect(&:donation).collect(&:receipt_items).uniq.collect(&:amount).sum
try that?