如何定義has_many通過通過軌道關聯通過

Question

考慮我有軌道模型名爲A，B，C，D與以下關係。

A has_many B 
B belongs_to C 
A has_many C via B 
C belongs_to D 

現在，應該如何定義下面的關聯方式？

A has_many D via C 

Answer 1

A has_many Bs 
A has_many Cs, :through => Bs 
(i.e B has_many Cs) 
C belongs_to :D 

A has_many Ds :through => Bs 

SELECT COUNT(*) FROM `Ds` INNER JOIN `Cs` ON `Ds`.`id` = `Cs`.`D_id` INNER JOIN `Bs` ON `Cs`.`B_id` = `Bs`.`id` WHERE `Bs`.`A_i