dm-preload.rb

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

DataMapper::Logger.new($stdout, :debug)
DataMapper.setup(:default, 'postgres://localhost/test') # createdb test

class Group
  include DataMapper::Resource

  property :id, Serial

  has n, :users
end

class User
  include DataMapper::Resource

  property :id, Serial

  belongs_to :group

  has n, :posts
end

class Post
  include DataMapper::Resource

  property :id, Serial
  property :title, String
  
  belongs_to :user
end

DataMapper.finalize
DataMapper.auto_migrate!

group = Group.create(
  :users => [ 
    {:posts => [{:title => "Post A"}]},
    {:posts => [{:title => "Post B"}]}
  ])
group.reload
group.users.to_a.each do |user| # The .to_a cannot be avoided
  user.posts.each do |post|     # How to eager load these posts?
    puts post.title
  end
end