nested_has_many_through

https://github.com/romanvbabenko/nested_has_many_through

This plugin makes it possible to define has_many :through relationships that go through other has_many :through relationships, possibly through an arbitrarily deep hierarchy. This allows associations across any number of tables to be constructed, without having to resort to find_by_sql (which isn't a suitable solution if you need to do eager loading through :include as well).