gistfile1.rb

require 'rubygems'
require 'dm-core'

class User
  include DataMapper::Resource
  
  property :id, Serial
end

class Paper
  include DataMapper::Resource
  
  property :id, Serial
  
  has n, :author_joins
  has n, :authors,        User,     :through => :author_joins
end

class AuthorJoin
  include DataMapper::Resource
  
  property :id,       Serial
  property :user_id,  Integer
  property :paper_id, Integer
  
  belongs_to :user
  belongs_to :paper
end

DataMapper.setup(:default, "sqlite3::memory:")
User.auto_migrate!
AuthorJoin.auto_migrate!
Paper.auto_migrate!

user = User.create!
paper = Paper.create!
paper.authors << user   # => NameError: Cannot find target relationship authors or author in AuthorJoin within the :default repository