Average values by group - ruby-on-rails

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!

Answer 1

The simplest syntax to do this is:
#donor.donations.group(:donatedate).average(:donateamount)
This will return a hash in the format { 'donatedate' => 'average donateamount' }.
Naturally, this assumes you have a has_many :donations association on your Donor model. A reusable method would look like:
def average_donation_by_date
donations.group(:donatedate).average(:donateamount)
end
And you can now simply call:
#donor.average_donation_by_date
To see this visually, you can call this in your controller:
#average_donations = #donor.average_donation_by_date.to_a.sort_by(&:first)
And your view might contain something like this:
<table>
<thead>
<tr>
<th>Date</th>
<th>Average Donation</th>
</tr>
</thead>
<tbody>
<% #average_donations.each do |date, amount| %>
<tr>
<td><%= date.strftime('MM/dd/yyyy') %></td>
<td><%= amount %></td>
</tr>
<% end %>
</tbody>
</table>
Reference
Rails api - calculate grouped values