Average values by group

Question

I have two tables in my web app: one is for Donors (called "donors") and the other is for Donation Amounts (called "donations). When you click on a donor, you can see all of their donations.

I'm trying to average values associated with a particular date, for a particular charity. For example, if these records exist for Donor A:

        *Donor A*

   Date       Donation Amount
05/04/2013          30
05/04/2013          40
05/05/2013          15
05/05/2013          75

I'd like the system to also calculate and display that the average donation amount for 05/04/2013 was 35 and the average donation amount for 05/05/2013 was 45.

Currently I've tried using the group attribute in my donor model:

def self.average_donateperdate
    Donation.average(:donateamount, conditions: ["donor_id = ?", @donor], group: "donatedate")
end

This doesn't seem to work. Since I'm new to Rails, I'm not entirely sure whether this is the right way to do it or not. There are a few other posts that touch on this topic, but none that have helped me solve the issue. Any help would be greatly appreciated!