How I'd do https://www.youtube.com/watch?v=UfALIVZc87c

Invoice.total_outstanding.spec.rb

require 'active_record'

ActiveRecord::Base.establish_connection adapter: 'sqlite3', database: ':memory:'

ActiveRecord::Schema.define do
  self.verbose = false
  create_table(:invoices) { |t| t.float :amount }
end

class Invoice < ActiveRecord::Base
  def self.total_outstanding
    sum :amount
  end
end


describe Invoice do
  specify '.total_outstanding is the sum of the invoice amounts' do
    expect(Invoice.total_outstanding).to eq 0
    Invoice.create! amount: 1
    expect(Invoice.total_outstanding).to eq 1
    Invoice.create! amount: 2.2
    expect(Invoice.total_outstanding).to eq 3.2
  end
end