具有单表继承（STI）和has_and_belongs_to_many（HABTM）关联的Bug

class User < ActiveRecord::Base
end

class Administrator < User
  has_and_belongs_to_many :clients
end

class Client < ActiveRecord::Base
  has_and_belongs_to_many :administrators
  has_and_belongs_to_many :meetings
end

class Meeting < ActiveRecord::Base
  has_and_belongs_to_many :clients

  def self.foo
    self.joins(:clients => :administrators)
  end
end

Meeting.foo.to_sql

SELECT meetings.*
FROM meetings
INNER JOIN clients_meetings ON clients_meetings.event_id = meetings.id
INNER JOIN clients ON clients.id = clients_meetings.child_id
INNER JOIN clients_administrators ON clients_administrators.child_id = clients.id
INNER JOIN users ON users.type = \'Administrator\'

INNER JOIN users ON users.id = clients_administrators.user_id
WHERE users.type = \'Administrator\'