如何将两个 has_many 关联合二为一？

Question

这似乎是一个相当普遍的问题，但没有明确的解决方案。再次重申，假设我有一个模型：

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

那么问题是如何添加结合两者的第三个关联:combined？我知道这可以用:finder_sql 来完成，类似的结果可以用scope 来实现，但是这些都没有给我一个实际的关联。这样做的重点是能够将其用于与:through 和Model.first.combined.some_scope.count 之类的另一个关联

编辑：实际代码的相关部分

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

谢谢。

Answer 1

如果 Something 和 Otherthing 足够相似，请使用 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