without :child_key

require 'rubygems'
gem 'dm-core', "0.10"
require 'dm-core'

gem 'data_objects', "0.10"
gem 'do_sqlite3', "0.10"

DataMapper.setup(:sqlite3, 'sqlite3::memory:')
DataMapper::Repository.adapters[:default] =  DataMapper::Repository.adapters[:sqlite3]

class Post
  include DataMapper::Resource

  property :id, Serial
  property :comments_count, Integer, :default => 0
  has n, :comments
end

class Comment
  include DataMapper::Resource

  property :id, Serial
  belongs_to :post

end

class User
  include DataMapper::Resource

  property :id, Serial
  property :groups_count, Integer, :default => 0      

  has n, :group_memberships
  has n, :groups, :through => :group_memberships, :class_name => "Group", :via => :group, :parent_key => [:id]
end

class Group
  include DataMapper::Resource

  property :id, Serial
  property :members_count, Integer, :default => 0
  has n, :group_memberships
  has n, :members, :through => :group_memberships, :class_name => "User", :via => :member, :parent_key => [:id]
end

class GroupMembership
  include DataMapper::Resource

  property :id, Serial

  belongs_to :group
  belongs_to :member, :class_name => "User", :child_key => [:user_id]
end    


GroupMembership.auto_migrate!
User.auto_migrate!
Group.auto_migrate!
Comment.auto_migrate!
Post.auto_migrate!