readme.md

I have moved the method to customer model.

joins - for inner join to avoid customers witout order

includes - for eager loading

then sum to return array

order.rb

class Order < ApplicationRecord
  belongs_to :customer
end

class Customer < ApplicationRecord
  has_many :orders
  def totals_by_customer
    Student.joins(:orders).includes(:orders).map { |student|
      [ student , student.orders.sum(:total) ]
    }
  end
end