understanding group by statements in rails - sql

Question

Given a invoices table like this:
invoice_date customer total
2012/01/01 A 780
2013/05/01 A 3800
2013/12/01 A 1500
2012/07/01 B 15
2013/03/01 B 21
Say that i want both:
the count of invoices of each customer of each year
the sum of the amounts of all the invoices of each customer of each year
the max amount among all the invoices of each customer of each year
That is, in SQL, very easily:
SELECT CUSTOMER, YEAR(invoice_date) as INVOICE_YEAR, MAX(total) AS MAX_TOTAL, SUM(total) AS SUM_AMOUNTS, count(*) AS INVOICES_NUM AS SUM_TOTAL FROM invoices GROUP BY YEAR(invoice_date), CUSTOMER;
(the function to extract the year of a date may be YEAR(date) or something else depending on the database server, on sqllite is strftime('%y', invoice_date))
Ok, i've tryed to translate this in rails/ActiveRecord:
Invoice.count(:group => 'customer')
This works, but how can i get both count and sum and max?
The idea i'm familiar with is that (in SQL) a group by generates the rows (well, to be correct, determines which rows should exist in the result table), and then you pass an arbitrary number of aggregation functions that are applyed on every disaggregate set of rows that are behind a single result row. E.G: group by customer means: one row for Customer A, one row for customer B; then I can pass how many aggregation function i want: count(*), max(total), max(date), min(total) just to list the most common.
Looking at the rails ActiveRecord API it seems that you're supposed to do just one function at a time, because the group is an argument of the count. And if i want a multiple aggregation functions, say max, sum etc?
Second attempt
irb> i = Invoice.select('customer, sum(total)').group('customer')
Invoice Load (0.3ms) SELECT customer, sum(total) AS TOTAL_GROUP FROM "invoices" GROUP BY customer
=> [#, #]
That is: it doesn't give back the field with the sum...

Answer 1

Well it does, it just doesn't get printed out.
Say you query is i = Invoice.select('customer, sum(total) as sum_total').group('customer')
So i is an array(technically it's not an array, but not important here) containing all the result. So i[0].sum_total will give you the sum of the first customer, but of course you should iterate it to get everything you want.