ruby-on-rails - Rails 猴子补丁仅在某些列上

Question

我试图仅在子类具有特定列时才包含模块，我正在初始化程序中执行此操作:

 class ActiveRecord::Base
      def self.inherited(subclass)
        subclass.include(MultiTenancy) if subclass.new.respond_to?(:tenant_id)
      end
    end

我一直收到这个错误:

NoMethodError: undefined method `[]' for nil:NilClass

这是我要导入的模块:

module MultiTenancy
  class TenantNotSetError < StandardError ; end

  def self.included(model)
    model.class_eval do
      belongs_to :tenant
      validates :tenant_id, presence: true
      default_scope -> {
        raise TenantNotSetError.new unless Tenant.current_tenant
        where(tenant_id: Tenant.current_tenant.id)
      }

      def multi_tenanted?
        true
      end
    end
  end
end

我做错了什么。我可以将模块包含在任何单独的子类中，例如。 Klass.include(MultiTenancy) 成功，所以问题出在初始化程序上。

下面是堆栈跟踪:

from -e:1:in `<main>'2.1.1 :002 > Klass.all
NoMethodError: undefined method `[]' for nil:NilClass
    from /home/lee/.rvm/gems/ruby-2.1.1/gems/activerecord-4.1.4/lib/active_record/relation/delegation.rb:9:in `relation_delegate_class'
    from /home/lee/.rvm/gems/ruby-2.1.1/gems/activerecord-4.1.4/lib/active_record/relation/delegation.rb:112:in `relation_class_for'
    from /home/lee/.rvm/gems/ruby-2.1.1/gems/activerecord-4.1.4/lib/active_record/relation/delegation.rb:106:in `create'
    from /home/lee/.rvm/gems/ruby-2.1.1/gems/activerecord-4.1.4/lib/active_record/model_schema.rb:133:in `table_name='
    from /home/lee/Code/mobifit/app/models/klass.rb:25:in `<class:Klass>'
    from /home/lee/Code/mobifit/app/models/klass.rb:22:in `<top (required)>'
    from /home/lee/.rvm/gems/ruby-2.1.1/gems/activesupport-4.1.4/lib/active_support/dependencies.rb:443:in `load'
    from /home/lee/.rvm/gems/ruby-2.1.1/gems/activesupport-4.1.4/lib/active_support/dependencies.rb:443:in `block in load_file'
    from /home/lee/.rvm/gems/ruby-2.1.1/gems/activesupport-4.1.4/lib/active_support/dependencies.rb:633:in `new_constants_in'
    from /home/lee/.rvm/gems/ruby-2.1.1/gems/activesupport-4.1.4/lib/active_support/dependencies.rb:442:in `load_file'
    from /home/lee/.rvm/gems/ruby-2.1.1/gems/activesupport-4.1.4/lib/active_support/dependencies.rb:342:in `require_or_load'
    from /home/lee/.rvm/gems/ruby-2.1.1/gems/activesupport-4.1.4/lib/active_support/dependencies.rb:480:in `load_missing_constant'
    from /home/lee/.rvm/gems/ruby-2.1.1/gems/activesupport-4.1.4/lib/active_support/dependencies.rb:180:in `const_missing'
    from (irb):2
    from /home/lee/.rvm/gems/ruby-2.1.1/gems/railties-4.1.4/lib/rails/commands/console.rb:90:in `start'
    from /home/lee/.rvm/gems/ruby-2.1.1/gems/railties-4.1.4/lib/rails/commands/console.rb:9:in `start'
    from /home/lee/.rvm/gems/ruby-2.1.1/gems/railties-4.1.4/lib/rails/commands/commands_tasks.rb:69:in `console'
    from /home/lee/.rvm/gems/ruby-2.1.1/gems/railties-4.1.4/lib/rails/commands/commands_tasks.rb:40:in `run_command!'
    from /home/lee/.rvm/gems/ruby-2.1.1/gems/railties-4.1.4/lib/rails/commands.rb:17:in `<top (required)>'
    from /home/lee/.rvm/gems/ruby-2.1.1/gems/activesupport-4.1.4/lib/active_support/dependencies.rb:247:in `require'
    from /home/lee/.rvm/gems/ruby-2.1.1/gems/activesupport-4.1.4/lib/active_support/dependencies.rb:247:in `block in require'
    from /home/lee/.rvm/gems/ruby-2.1.1/gems/activesupport-4.1.4/lib/active_support/dependencies.rb:232:in `load_dependency'
    from /home/lee/.rvm/gems/ruby-2.1.1/gems/activesupport-4.1.4/lib/active_support/dependencies.rb:247:in `require'
    from /home/lee/Code/mobifit/bin/rails:8:in `<top (required)>'
    from /home/lee/.rvm/gems/ruby-2.1.1/gems/activesupport-4.1.4/lib/active_support/dependencies.rb:241:in `load'
    from /home/lee/.rvm/gems/ruby-2.1.1/gems/activesupport-4.1.4/lib/active_support/dependencies.rb:241:in `block in load'
    from /home/lee/.rvm/gems/ruby-2.1.1/gems/activesupport-4.1.4/lib/active_support/dependencies.rb:232:in `load_dependency'
    from /home/lee/.rvm/gems/ruby-2.1.1/gems/activesupport-4.1.4/lib/active_support/dependencies.rb:241:in `load'
    from /home/lee/.rvm/rubies/ruby-2.1.1/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require'
    from /home/lee/.rvm/rubies/ruby-2.1.1/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require'
    from -e:1:in `<main>'2.1.1 :003 > 

Answer 1

如果您稍微跟踪一下堆栈跟踪，您会发现不应该是 nil 的是类的 @relation_delegate_cache。看着the source ，您可以看到这通常在 initialize_relation_delegate_cache 方法中初始化，该方法在 ActiveRecord::Delegation::DelegateCache 的 inherited Hook 中调用.

ActiveRecord::Base 扩展了这个模块，所以通常一个继承自 ActiveRecord::Base 的类会调用这个继承的钩子(Hook)。所以你的代码的问题是你在 ActiveRecord::Base 上猴子修补 inherited，这意味着 ActiveRecord::Delegation::中的钩子(Hook)DelegateCache 不再被调用。

两条评论:

1. 这就是为什么猴子补丁不是一个好主意的原因，尤其是像事件记录这样基础和复杂的东西。有没有可能以更直接的方式做您想做的事情？
2. 如果不是，你应该记下 ActiveRecord::Delegation::DelegateCache 是如何工作的，并重写你的猴子补丁以混合到你自己的模块中，而不是仅仅覆盖 ActiveRecord::Base的inherited方法。

(2) 中的建议看起来像这样:

module MultitenancyConcern

  def self.inherited(subclass)
    subclass.include(MultiTenancy) if subclass.column_names.include?("tenant_id")
    super #this is critical for avoiding the error you're getting
  end

end

然后你的猴子补丁就变成了

ActiveRecord::Base.extend(MultitenancyConcern)