多態的has_many：通過NoMethodError：對於無未定義的方法`克拉斯：NilClass

Question

於是我開始設置一個的has_many：通過多態關聯在我的應用程序。該模型如下所示：

class Song < ActiveRecord::Base 
    has_many :collections, through: :collectionitems 
    has_many :collectionitems, through: :collectable 
end 

class Album < ActiveRecord::Base 
    has_many :collections, through: :collectionitems 
    has_many :collectionitems, through: :collectable 
end 

class Collection < ActiveRecord::Base 
    has_many :albums, through: :collectionitems, source: :collectable, source_type: "Album" 
    has_many :songs, through: :collectionitems, source: :collectable, source_type: "Song" 
    has_many :collectionitems 
end 

class Collectionitem < ActiveRecord::Base 
    belongs_to :collection 
    belongs_to :collectable, polymorphic: true 
end 

這讓我做以下電話：

Collection.first.songs =>返回歌曲的第一個集合數組 Collection.first.albums =>返回專輯的數組第一個集合 Collectionitem.first.collection =>返回此Collectionitem所屬的集合 Collectionitem.first.collectable =>返回此Collectionitem所屬的唱片（歌曲或專輯）

談到我的問題，當我嘗試查找特定專輯或歌曲所屬的所有集合。

Song.first.collections和Album.first.collections在rails控制檯中調用時會返回以下錯誤。

Song.first.collections 
Song Load (0.2ms) SELECT "songs".* FROM "songs" ORDER BY "songs"."id" ASC LIMIT 1 
NoMethodError: undefined method `klass' for nil:NilClass 
    from /Users/jonathan/.rvm/gems/ruby-1.9.3-p392/gems/activerecord-4.0.0/lib/active_record/reflection.rb:420:in `block in source_reflection' 
    from /Users/jonathan/.rvm/gems/ruby-1.9.3-p392/gems/activerecord-4.0.0/lib/active_record/reflection.rb:420:in `collect' 
    from /Users/jonathan/.rvm/gems/ruby-1.9.3-p392/gems/activerecord-4.0.0/lib/active_record/reflection.rb:420:in `source_reflection' 
    from /Users/jonathan/.rvm/gems/ruby-1.9.3-p392/gems/activerecord-4.0.0/lib/active_record/reflection.rb:579:in `derive_class_name' 
    from /Users/jonathan/.rvm/gems/ruby-1.9.3-p392/gems/activerecord-4.0.0/lib/active_record/reflection.rb:133:in `class_name' 
    from /Users/jonathan/.rvm/gems/ruby-1.9.3-p392/gems/activerecord-4.0.0/lib/active_record/reflection.rb:178:in `klass' 
    from /Users/jonathan/.rvm/gems/ruby-1.9.3-p392/gems/activerecord-4.0.0/lib/active_record/reflection.rb:420:in `block in source_reflection' 
    from /Users/jonathan/.rvm/gems/ruby-1.9.3-p392/gems/activerecord-4.0.0/lib/active_record/reflection.rb:420:in `collect' 
    from /Users/jonathan/.rvm/gems/ruby-1.9.3-p392/gems/activerecord-4.0.0/lib/active_record/reflection.rb:420:in `source_reflection' 
    from /Users/jonathan/.rvm/gems/ruby-1.9.3-p392/gems/activerecord-4.0.0/lib/active_record/reflection.rb:557:in `check_validity!' 
    from /Users/jonathan/.rvm/gems/ruby-1.9.3-p392/gems/activerecord-4.0.0/lib/active_record/associations/association.rb:25:in `initialize' 
    from /Users/jonathan/.rvm/gems/ruby-1.9.3-p392/gems/activerecord-4.0.0/lib/active_record/associations/has_many_through_association.rb:9:in `initialize' 
    from /Users/jonathan/.rvm/gems/ruby-1.9.3-p392/gems/activerecord-4.0.0/lib/active_record/associations.rb:157:in `new' 
    from /Users/jonathan/.rvm/gems/ruby-1.9.3-p392/gems/activerecord-4.0.0/lib/active_record/associations.rb:157:in `association' 
    from /Users/jonathan/.rvm/gems/ruby-1.9.3-p392/gems/activerecord-4.0.0/lib/active_record/associations/builder/association.rb:70:in `collections' 
    from (irb):3 
    from /Users/jonathan/.rvm/gems/ruby-1.9.3-p392/gems/railties-4.0.0/lib/rails/commands/console.rb:90:in `start' 
    from /Users/jonathan/.rvm/gems/ruby-1.9.3-p392/gems/railties-4.0.0/lib/rails/commands/console.rb:9:in `start' 
    from /Users/jonathan/.rvm/gems/ruby-1.9.3-p392/gems/railties-4.0.0/lib/rails/commands.rb:64:in `<top (required)>' 
    from bin/rails:4:in `require' 

有人能告訴我我在這裏弄錯了什麼。它好像沒有看到歌曲與收藏的關係？我只是不知道我做錯了什麼。

Answer 1

由於@damien在評論中指出我需要改變：

has_many :collectionitems, through: :collectable 

到：

has_many :collectionitems, as: :collectable 

一旦這已經到位everythign發揮預期。

再次感謝@damien！