如何對未知類型的對象上的對象實例使用define_method？

Question

所以，我很想做類似於rspec/mocha的mock，但只適用於兩個對象，而不是全部。這是我到目前爲止有：

def mock(obj, method_to_mock, value) 
    obj.class << obj do 
     define_method(method_to_mock) do 
      return value 
     end 
    end 
end 

我得到了主意，寫這樣的，從這個帖子：https://stackoverflow.com/a/185969/356849

於是我可以做這樣的事情：

mock(self.instantiated, :sections, sections) 

，它將使用我的數組Section對象sections覆蓋我已存儲在self.instantiated的sections中的對象。

我之所以這樣做，是因爲我存儲了一個對象的序列化和加密版本，我希望能夠解密和反序列化對象，然後還原所有關係，例如我可以在我的視圖中查看該對象，就好像它正在從數據庫中讀取一樣。但這並不重要，大部分工作已經完成。

所以，我希望能夠做到這一點：

mock(<Instance of object>, :<method of object that is going to be overridden, to avoid db access>, <the stuff to return when the overridden method is invoked)

目前，我得到一個錯誤的obj.class << obj do符合這樣的：

NoMethodError: undefined method `obj' for #<MyObject::Encrypted:0x7f190eebcd18> 

想法？

---

UPDATE

改變第二行class << obj現在無限循環。

from /home/me/.rvm/gems/ruby-1.8.7-p371@project/gems/activerecord-2.3.15/lib/active_record/connection_adapters/abstract/connection_pool.rb:351:in `retrieve_connection_pool' 
from /home/me/.rvm/gems/ruby-1.8.7-p371@project/gems/activerecord-2.3.15/lib/active_record/connection_adapters/abstract/connection_pool.rb:351:in `retrieve_connection_pool' 
from /home/me/.rvm/gems/ruby-1.8.7-p371@project/gems/activerecord-2.3.15/lib/active_record/connection_adapters/abstract/connection_pool.rb:325:in `retrieve_connection' 
from /home/me/.rvm/gems/ruby-1.8.7-p371@project/gems/activerecord-2.3.15/lib/active_record/connection_adapters/abstract/connection_specification.rb:123:in `retrieve_connection' 
from /home/me/.rvm/gems/ruby-1.8.7-p371@project/gems/activerecord-2.3.15/lib/active_record/connection_adapters/abstract/connection_specification.rb:115:in `connection' 
from /home/me/.rvm/gems/ruby-1.8.7-p371@project/gems/activerecord-2.3.15/lib/active_record/base.rb:1305:in `columns' 
from /home/me/.rvm/gems/ruby-1.8.7-p371@project/gems/activerecord-2.3.15/lib/active_record/base.rb:1318:in `column_names' 
from /home/me/.rvm/gems/ruby-1.8.7-p371@project/gems/searchlogic-2.4.28/lib/searchlogic/named_scopes/ordering.rb:35:in `ordering_condition_details' 
from /home/me/.rvm/gems/ruby-1.8.7-p371@project/gems/searchlogic-2.4.28/lib/searchlogic/named_scopes/ordering.rb:26:in `method_missing' 
from /home/me/.rvm/gems/ruby-1.8.7-p371@project/gems/searchlogic-2.4.28/lib/searchlogic/named_scopes/or_conditions.rb:28:in `method_missing' 
from /home/me/.rvm/gems/ruby-1.8.7-p371@project/gems/activerecord-2.3.15/lib/active_record/base.rb:2002:in `method_missing_without_paginate' 
from /home/me/.rvm/gems/ruby-1.8.7-p371@project/gems/will_paginate-2.3.16/lib/will_paginate/finder.rb:170:in `method_missing_without_attr_encrypted' 
from /home/me/.rvm/gems/ruby-1.8.7-p371@project/bundler/gems/attr_encrypted-a4b25f01d137/lib/attr_encrypted/adapters/active_record.rb:50:in `method_missing' 
from /home/me/Work/GravityLabs/project/app/models/proposal/encrypted.rb:119:in `mock' 
from /home/me/Work/GravityLabs/project/app/models/proposal/encrypted.rb:79:in `instantiate' 
from /home/me/Work/GravityLabs/project/app/models/proposal/encrypted.rb:58:in `each' 
from /home/me/Work/GravityLabs/project/app/models/proposal/encrypted.rb:58:in `instantiate' 

Answer 1

obj.class << obj do是沒有意義的。

什麼你可能想要說的是

def mock(obj, method_to_mock, value) 
    (class << obj; self; end).class_eval do 
    define_method(method_to_mock) do 
     return value 
    end 
    end 
end 

的(class << obj; self; end).class_eval語法打開單例類的obj返回該單例類，然後調用上傳遞塊單例類class_eval。

在你的語法，obj.class發送:class消息obj作爲接收器返回一個參考的obj的類（而不是它的單例類），在其上，然後嘗試調用<<方法經過評估obj do...end作爲的結果ARG。由於obj不是self（MyObject :: Encrypted：0x7f190eebcd1）的方法，因此您將得到NoM​​ethodError。

在現代的紅寶石，而不是說比較神祕，(class << obj; self; end)獲得單例類，你可以使用singleton_class方法，像這樣：obj.singleton_class.class_eval do ... end

Answer 2

def mock(obj, method_to_mock, value=nil) 
    obj.define_singleton_method(method_to_mock) do value end 
end