dm_eager_load.rb

#!/usr/bin/env ruby

require 'rubygems'
require 'dm-core'
require 'dm-migrations'

DataMapper::Logger.new(STDOUT, :debug)
DataMapper.setup(:default, 'sqlite::memory:')

class Customer
  include DataMapper::Resource

  property :id,   Serial
  property :name, String, :required => true, :length => 1..100

  has n, :orders
  has n, :items, :through => :orders
end

class Order
  include DataMapper::Resource

  property :id,        Serial
  property :reference, String, :required => true, :length => 1..20

  belongs_to :customer

  has n, :order_lines
  has n, :items, :through => :order_lines
end

class OrderLine
  include DataMapper::Resource

  property :id,         Serial
  property :quantity,   Integer, :required => true, :default => 1, :min => 1
  property :unit_price, Decimal, :required => true, :default => lambda { |r, p| r.item.unit_price }

  belongs_to :order
  belongs_to :item
end

class Item
  include DataMapper::Resource

  property :id,         Serial
  property :sku,        String,  :required => true, :length => 1..20
  property :unit_price, Decimal, :required => true, :min => 0

  has n, :order_lines
end

DataMapper.finalize
DataMapper.auto_migrate!

# create some objects
5.times do
  c = { :name => rand(1000).to_s, :orders => [] }
  5.times do
    o = { :reference => rand(1000).to_s, :order_lines => [] }
    5.times do
      ol = { :quantity => rand(10).to_i,
             :item => {
               :sku => rand(1000).to_s,
               :unit_price => rand(1000)
             }
           }
      o[:order_lines] << ol
    end
    c[:orders] << o
  end
  Customer.create(c)
end

# now print all orders,
# with eager loading it should fire 4 queries (Customers, Orders, OrderLines, Items)
Customer.all(:limit => 3).each do |c|
  c.orders.each do |o|
    o.order_lines.each do |ol|
      a = "#{c.name}: #{o.reference} #{ol.quantity} x #{ol.item.sku}"
    end
  end
end

# But it does:
# ~ (0.000028) SELECT "id", "name" FROM "customers" ORDER BY "id" LIMIT 3
# ~ (0.000031) SELECT "id", "reference", "customer_id" FROM "orders" WHERE "customer_id" IN (1, 2, 3) ORDER BY "id"
# ~ (0.000031) SELECT "id", "quantity", "unit_price", "order_id", "item_id" FROM "order_lines" WHERE "order_id" IN (1, 2, 3, 4, 5) ORDER BY "id"
# ~ (0.000031) SELECT "id", "sku", "unit_price" FROM "items" WHERE "id" IN (1, 2, 3, 4, 5) ORDER BY "id"
# ~ (0.000027) SELECT "id", "sku", "unit_price" FROM "items" WHERE "id" IN (6, 7, 8, 9, 10) ORDER BY "id"
# ~ (0.000025) SELECT "id", "sku", "unit_price" FROM "items" WHERE "id" IN (11, 12, 13, 14, 15) ORDER BY "id"
# ~ (0.000033) SELECT "id", "sku", "unit_price" FROM "items" WHERE "id" IN (16, 17, 18, 19, 20) ORDER BY "id"
# ~ (0.000028) SELECT "id", "sku", "unit_price" FROM "items" WHERE "id" IN (21, 22, 23, 24, 25) ORDER BY "id"
# ~ (0.000032) SELECT "id", "quantity", "unit_price", "order_id", "item_id" FROM "order_lines" WHERE "order_id" IN (6, 7, 8, 9, 10) ORDER BY "id"
# ~ (0.000026) SELECT "id", "sku", "unit_price" FROM "items" WHERE "id" IN (26, 27, 28, 29, 30) ORDER BY "id"
# ~ (0.000026) SELECT "id", "sku", "unit_price" FROM "items" WHERE "id" IN (31, 32, 33, 34, 35) ORDER BY "id"
# ~ (0.000027) SELECT "id", "sku", "unit_price" FROM "items" WHERE "id" IN (36, 37, 38, 39, 40) ORDER BY "id"
# ~ (0.000025) SELECT "id", "sku", "unit_price" FROM "items" WHERE "id" IN (41, 42, 43, 44, 45) ORDER BY "id"
# ~ (0.000025) SELECT "id", "sku", "unit_price" FROM "items" WHERE "id" IN (46, 47, 48, 49, 50) ORDER BY "id"
# ~ (0.000030) SELECT "id", "quantity", "unit_price", "order_id", "item_id" FROM "order_lines" WHERE "order_id" IN (11, 12, 13, 14, 15) ORDER BY "id"
# ~ (0.000056) SELECT "id", "sku", "unit_price" FROM "items" WHERE "id" IN (51, 52, 53, 54, 55) ORDER BY "id"
# ~ (0.000026) SELECT "id", "sku", "unit_price" FROM "items" WHERE "id" IN (56, 57, 58, 59, 60) ORDER BY "id"
# ~ (0.000026) SELECT "id", "sku", "unit_price" FROM "items" WHERE "id" IN (61, 62, 63, 64, 65) ORDER BY "id"
# ~ (0.000045) SELECT "id", "sku", "unit_price" FROM "items" WHERE "id" IN (66, 67, 68, 69, 70) ORDER BY "id"
# ~ (0.000030) SELECT "id", "sku", "unit_price" FROM "items" WHERE "id" IN (71, 72, 73, 74, 75) ORDER BY "id"