如何通过Rails中的via关联定义has_many

Question

consider i have rails models named A, B, C, D with following relationship.

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

Now, what should be the rails way to define below association ?

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