Question

• Live site: http://iatidata.heroku.com
• Github: https://github.com/markbrough/IATI-Data
• Based on aid information released through the IATI Registry: iatiregistry.org

I’m a bit of a Rails n00b so sorry if this is a really stupid question.

There are two key Models in this app:

• Activity – which contains details
  such as recipient country, funding
  organisation
• Transaction – which contains details such as how much money (value) was committed or disbursed (transaction_type), when, to whom, etc.

All Transactions nest under an Activity. Each Activity has multiple Transactions. They are connected together by activity_id. has_many :transactions and belongs_to :activity are defined in the Activity and Transaction Models respectively.

So: all of this works great when I’m trying to get details of transactions for a single activity – either when looking at a single activity (activity->show) or looping through activities on the all activities page (activity->index). I just call

@activities.each do |activity|
    activity.transactions.each do |transaction|
        transaction.value # do something like display it
    end
end

But what I now really want to do is to get the sum of all transactions for all activities (subject to :conditions for the activity).

What’s the best way to do this? I guess I could do something like:

@totalvalue = 0
@activities.each do |activity|
    activity.transactions.each do |transaction|
        @totalvalue = @totalvalue + transaction.value
    end 
end

… but that doesn’t seem very clean and making the server do unnecessary work. I figure it might be something to do with the model…?! sum() is another option maybe?

This has partly come about because I want to show the total amount going to each country for the nice bubbles on the front page 🙂

Thanks very much for any help!

Update:

Thanks for all the responses! So, this works now:

@thiscountry_activities.each do |a|
    @thiscountry_value = @thiscountry_value + a.transactions.sum(:value)
end

But this doesn’t work:

@thiscountry_value = @thiscountry_activities.transactions.sum(:value)

It gives this error:

undefined method `transactions' for #<Array:0xb5670038>

Looks like I have some sort of association problem. This is how the models are set up:

class Transaction < ActiveRecord::Base
    belongs_to :activity
end

class Activity < ActiveRecord::Base
    has_and_belongs_to_many :policy_markers
    has_and_belongs_to_many :sectors
    has_many :transactions
end

I think this is probably quite a simple problem, but I can’t work out what’s going on. The two models are connected together via id (in Activity) and activity_id (in Transactions).

Thanks again!

Answer 1

Use Active Record’s awesome sum method, available for classes:

Transaction.sum(:value)

Or, like you want, associations:

activity.transactions.sum(:value)