Question

How does one sum all of the “total” columns in an association?

My SQL-fu sucks, so I would like to learn how to do so using Active Record for my rails 2.3.5 app (so no fancy syntax just yet please 😉 And I’m on MySQL.

Let’s say I have:

Shop 

has_many :customers
has_many :transactions, :through => :customers

So normal stuff.

shop = Shop.first
shop.transactions
=> 100 

Ok, all that background for the question:

I want to SUM the total column in the transactions from the past year (Jan 1st 2010..Dec 31 2010) and display them by customer.

Whilst I know how to group transactions and find with conditions, it’s the sum part where my lack of SQL lets me down.

first = Date.new(2010, 01, 01)
last = Date.new(2010, 12, 31)

shop.transactions(:conditions => {:created_at => first..last}, :group => :customer_id, :include => sum(:total))

I just took a stab, am i on the right track?

Answer 1

shop.transactions.sum(:total, :conditions => {:created_at => first..last}, :group => :customer_id)

This looks like an easier way. I now know that sum can take AR attributes too. cool.