Question

My entry model has many counters:

class Entry < ActiveRecord::Base
  has_many :counters
end

Every counter has a number, and the total represents the sum of the numbers:

class Counter < ActiveRecord::Base
  scope :total, sum(:number)
end

I need to get the sum of all the numbers of the counters which belong to a specific entry.

In SQL it would be:

SELECT SUM(`number`) AS `total` FROM `counters` WHERE `entry_id` = entry.id

I tried:

entry.counters.total

But it returns:

NoMethodError: undefined method `default_scoped?’ for 0:Fixnum

Is there any “Rails way” to do this nicely with ActiveRecord associations and scopes?

Answer 1

In your example the call to sum happens straightaway, ie your code is equivalent to

scope :total, 0

(assuming that the sum is 0 at the moment that your class is 0), which isn’t valid

Fundamentally scopes are about scoping a result set : adding conditions, order, limit or options such as joins, but with the constant that the result is a collection of active record objects.

What you want to do is best expressed as a class method:

def self.total
  sum(:number)
end

You can still chain this onto a scope, for example

some_entry.counters.total