Inner Query example in active record

README.md

Inspiration: https://gist.github.com/4436444

Resulting Query:

SELECT "orders".* FROM "orders" WHERE "orders"."id" IN (SELECT id FROM "orders" GROUP BY account_id ORDER BY order_date DESC)

account.rb

class Account < ActiveRecord::Base
  attr_accessible :name
  has_many :orders
end

order.rb

class Order < ActiveRecord::Base
  belongs_to :account
  attr_accessible :amount_in_cents, :order_date

  scope :recent_by_account, ->{ where(arel_table[:id].in( 
    Arel::SqlLiteral.new( Order.group(:account_id).order("order_date DESC").select(:id).to_sql))
                                                        )
  }
end

order_test.rb

require 'test_helper'

class OrderTest < ActiveSupport::TestCase

  setup do
    Account.create!( name: "the-first")
    Account.create!( name: "the-second")
    Account.create!( name: "the-third")

    Account.all.each do |account|
      5.times do |i|
        account.orders.create!(amount_in_cents: (100..10000).to_a.sample, order_date: i.days.ago)
      end
    end
  end

  test "query should have 1 per account" do
    results = Order.recent_by_account
    assert_equal 3, results.count
  end
end

schema.rb

ActiveRecord::Schema.define(:version => 20130102181052) do

  create_table "accounts", :force => true do |t|
    t.string   "name"
    t.datetime "created_at", :null => false
    t.datetime "updated_at", :null => false
  end

  create_table "orders", :force => true do |t|
    t.integer  "account_id"
    t.datetime "order_date"
    t.integer  "amount_in_cents"
    t.datetime "created_at",      :null => false
    t.datetime "updated_at",      :null => false
  end

  add_index "orders", ["account_id"], :name => "index_orders_on_account_id"

end