snippet.rb

class Book
  belongs_to :group
  has n, :subscriptions
  has n, :subscribers, :through => :subscriptions, :class_name => 'User', 
                       :child_key => [:book_id], :remote_name => :user
end

class Group
  has n, :books
  has n, :users, :through => Resource
end

class User
  has n, :groups, :through => Resource
  has n, :books
end

class Subscription
  belongs_to :book
  belongs_to :user
end

Book.all 'group.users.user_id' => 1 # => SQL Error

# SELECT ... FROM `books` INNER JOIN `groups` ON (`groups`.`id` = `books`.`group_id`) 
# INNER JOIN `groups` ON (`groups`.`id` = `groups_users`.`group_id`) 
# WHERE (`groups_users`.`user_id` = 1) ORDER BY `books`.`id`

# it should look like:

# SELECT ... FROM `books` INNER JOIN `groups` ON (`groups`.`id` = `books`.`group_id`) 
# INNER JOIN `groups_users` ON (`groups`.`id` = `groups_users`.`group_id`) 
# WHERE (`groups_users`.`user_id` = 1) ORDER BY `books`.`id`