Speed Test

models.rb

class Order < ApplicationRecord
  belongs_to :customer
  
  def totals_by_customer
    Order.all.pluck(:customer).distinct.map do |customer|
      [customer, customer.orders.sum(:total))]
    end
  end
end

class Customer < ApplicationRecord
  has_many :orders
end

schema.sql

CREATE TABLE `orders` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `customer_id` int(11) DEFAULT NULL,
  `total` int(11) DEFAULT NULL,
);

CREATE TABLE `customers` (
  `id` int(11) NOT NULL AUTO_INCREMENT
);