如何在未知类型对象上的对象实例上使用 define_method？

Question

所以，我有点想做类似于 rspec / mocha 的 mock 的事情，但只针对两个对象，而不是全部。这是我目前所拥有的：

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

我从这篇文章中得到了这样写的想法：https://stackoverflow.com/a/185969/356849

那么我可以做这样的事情：

mock(self.instantiated, :sections, sections)

它会用我的 Section 对象数组sections 覆盖我存储在self.instantiated 的sections 中的对象。

我这样做的原因是因为我正在存储对象的序列化和加密版本，并且我希望能够对对象进行解密和反序列化，然后恢复所有关系，以便我可以在我的视图中查看该对象，就好像它是从数据库中读取的一样。但这并不重要，而且大部分都已经完成了。

所以，我希望能够做到这一点：

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，我在obj.class << obj do 行遇到错误：

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

想法？

---

更新

将第二行改为class << obj，现在无限循环。

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 毫无意义。

你可能想说的是

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

(class << obj; self; end).class_eval 语法是打开 obj 的单例类返回该单例类，然后在该单例类上调用 class_eval 传递块。

在您的语法中，obj.class 将:class 消息作为接收器发送到obj，该接收器返回对 obj 类（而不是其单例类）的引用，然后您尝试在其上调用 << 方法传递将 obj do...end 评估为 arg 的结果。因为obj 不是self (MyObject::Encrypted:0x7f190eebcd1) 的方法，所以你会得到NoM​​ethodError。

在现代 ruby​​ 中，不要说相对晦涩的 (class << obj; self; end) 来获取单例类，您可以使用 singleton_class 方法，如下所示：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