如何將兩個has_many關聯合併爲一個？

Question

這似乎是一個相當普遍的問題，但沒有明確的解決方案。再次重申，說我有一個模型：

def Model < ActiveRecord::Base 
    has_many :somethings, ... 
    has_many :otherthings, ... 
end 

接下來的問題是如何增加一個第三關聯:combined，結合兩個？我知道這可以通過:finder_sql來完成，而類似的結果可以通過scope來實現，但是這些都沒有給我一個實際的關聯。這樣做的意義就在於能夠與:through和事物其用於其他關聯像Model.first.combined.some_scope.count

編輯：實際的代碼

class Donation < ActiveRecord::Base 
    # either Project or Nonprofit  
    belongs_to :donatable, :polymorphic => true 
    belongs_to :account 
end 

class Project < ActiveRecord::Base 
    belongs_to :nonprofit 
end 

class Nonprofit < ActiveRecord::Base 
    has_many :projects 

    # donations can be either direct or through a project 
    # the next two associations work fine on their own 

    # has_many :donations, :as => :donatable, :through => :projects 
    # has_many :donations, :as => :donatable 

    has_many :donations, ....      # how do I get both here, 
    has_many :supporters, :through => :donations # for this to work? 
end 

感謝的相關部分。

Answer 1

如果Something和Otherthing足夠相似，使用STI：

def Model < ActiveRecord::Base 
    has_many :somethings 
    has_many :otherthings 
    has_many :genericthings 
end 

def Genericthing < Activerecord::Base 
    # put a string column named "type" in the table 
    belongs_to :model 
end 

def Something < Genericthing 
end 

def Otherthing < Genericthing 
end