require 'dm-core'
require 'dm-migrations'
require 'dm-sqlite-adapter'

DataMapper.setup :default, "sqlite::memory:"

class Comment
  include DataMapper::Resource
  property :id, Serial
  belongs_to :post
end

class Post
  include DataMapper::Resource
  property :id, Serial
  has n, :comments
  belongs_to :user
end

class User
  include DataMapper::Resource
  property :id, Serial
  has n, :posts
end

DataMapper.auto_migrate!

user = User.create
3.times { Post.create(user: user) }

repository do
  posts = Post.all.entries

  DataObjects::Sqlite3.logger = DataMapper::Logger.new($stdout, :debug)
  posts.each { |post| post.comments.entries }
end

# ~ (0.000040) SELECT "id", "post_id" FROM "comments" WHERE "post_id" = 1 ORDER BY "id"
# ~ (0.000039) SELECT "id", "post_id" FROM "comments" WHERE "post_id" = 2 ORDER BY "id"
# ~ (0.000031) SELECT "id", "post_id" FROM "comments" WHERE "post_id" = 3 ORDER BY "id"