ruby - "undefined method ' 变成 ' for nil:NilClass"不应该的地方

Question

我今天在测试一个片段

unless resource.nil?
  resource = resource.becomes(Accounts::Admin)
end

这会引发错误

undefined method `becomes' for nil:NilClass

如果我这样做

unless resource.nil?
  a = resource.becomes(Accounts::Admin)
  resource = a
end

一切顺利。

如果先执行=运算符右边的部分有什么区别？

编辑:

发生了一些令人讨厌的事情，正在执行 if false 下的最后一行，但从未打印“ALOHA”。

<%
puts "AAAA #{resource.inspect}"
if false
    puts "ALOHA"

    # this line is being executed !
    # if I comment it out the BBBB output is correct
    resource = nil
end 
puts "BBBB #{resource.inspect}"
%>

它打印

AAAA User id: nil, nome: nil, endereco_id: nil, created_at: nil, updated_at: nil, email: ""

BBBB nil

但如果我这样做

<%
res = resource

puts "AAAA #{res.inspect}"
if false
  puts "ALOHA"
  res = nil
end 
puts "BBBB #{res.inspect}"
%>

打印正确

AAAA User id: nil, nome: nil, endereco_id: nil, created_at: nil, updated_at: nil, email: ""

BBBB User id: nil, nome: nil, endereco_id: nil, created_at: nil, updated_at: nil, email: ""

我已经尝试重启服务器了。此代码段位于 devise/registrations/new.html.erb。 resource 变量应该是 User 的一个实例，由 devise 的 RegistrationController 创建

我已经检查了隐藏字符的文本，我在此处粘贴的片段是正在测试的文件的完整文本。

这是 ~/.rvm/gems/ruby-2.3.3@mygem/gems/devise-4.2.0/app/controllers/devise/registrations_controller.rb 中 Controller 的内容

class Devise::RegistrationsController < DeviseController
  prepend_before_action :require_no_authentication, only: [:new, :create, :cancel]
  prepend_before_action :authenticate_scope!, only: [:edit, :update, :destroy]
  prepend_before_action :set_minimum_password_length, only: [:new, :edit]

  # GET /resource/sign_up
  def new
    build_resource({})
    yield resource if block_given?
    respond_with resource
  end

  # POST /resource
  def create
    build_resource(sign_up_params)

    resource.save
    yield resource if block_given?
    if resource.persisted?
      if resource.active_for_authentication?
        set_flash_message! :notice, :signed_up
        sign_up(resource_name, resource)
        respond_with resource, location: after_sign_up_path_for(resource)
      else
        set_flash_message! :notice, :"signed_up_but_#{resource.inactive_message}"
        expire_data_after_sign_in!
        respond_with resource, location: after_inactive_sign_up_path_for(resource)
      end
    else
      clean_up_passwords resource
      set_minimum_password_length
      respond_with resource
    end
  end

  # GET /resource/edit
  def edit
    render :edit
  end

  # PUT /resource
  # We need to use a copy of the resource because we don't want to change
  # the current user in place.
  def update
    self.resource = resource_class.to_adapter.get!(send(:"current_#{resource_name}").to_key)
    prev_unconfirmed_email = resource.unconfirmed_email if resource.respond_to?(:unconfirmed_email)

    resource_updated = update_resource(resource, account_update_params)
    yield resource if block_given?
    if resource_updated
      if is_flashing_format?
        flash_key = update_needs_confirmation?(resource, prev_unconfirmed_email) ?
          :update_needs_confirmation : :updated
        set_flash_message :notice, flash_key
      end
      bypass_sign_in resource, scope: resource_name
      respond_with resource, location: after_update_path_for(resource)
    else
      clean_up_passwords resource
      respond_with resource
    end
  end

  # DELETE /resource
  def destroy
    resource.destroy
    Devise.sign_out_all_scopes ? sign_out : sign_out(resource_name)
    set_flash_message! :notice, :destroyed
    yield resource if block_given?
    respond_with_navigational(resource){ redirect_to after_sign_out_path_for(resource_name) }
  end

  # GET /resource/cancel
  # Forces the session data which is usually expired after sign
  # in to be expired now. This is useful if the user wants to
  # cancel oauth signing in/up in the middle of the process,
  # removing all OAuth session data.
  def cancel
    expire_data_after_sign_in!
    redirect_to new_registration_path(resource_name)
  end

  protected

  def update_needs_confirmation?(resource, previous)
    resource.respond_to?(:pending_reconfirmation?) &&
      resource.pending_reconfirmation? &&
      previous != resource.unconfirmed_email
  end

  # By default we want to require a password checks on update.
  # You can overwrite this method in your own RegistrationsController.
  def update_resource(resource, params)
    resource.update_with_password(params)
  end

  # Build a devise resource passing in the session. Useful to move
  # temporary session data to the newly created user.
  def build_resource(hash=nil)
    self.resource = resource_class.new_with_session(hash || {}, session)
  end

  # Signs in a user on sign up. You can overwrite this method in your own
  # RegistrationsController.
  def sign_up(resource_name, resource)
    sign_in(resource_name, resource)
  end

  # The path used after sign up. You need to overwrite this method
  # in your own RegistrationsController.
  def after_sign_up_path_for(resource)
    after_sign_in_path_for(resource)
  end

  # The path used after sign up for inactive accounts. You need to overwrite
  # this method in your own RegistrationsController.
  def after_inactive_sign_up_path_for(resource)
    scope = Devise::Mapping.find_scope!(resource)
    router_name = Devise.mappings[scope].router_name
    context = router_name ? send(router_name) : self
    context.respond_to?(:root_path) ? context.root_path : "/"
  end

  # The default url to be used after updating a resource. You need to overwrite
  # this method in your own RegistrationsController.
  def after_update_path_for(resource)
    signed_in_root_path(resource)
  end

  # Authenticates the current scope and gets the current resource from the session.
  def authenticate_scope!
    send(:"authenticate_#{resource_name}!", force: true)
    self.resource = send(:"current_#{resource_name}")
  end

  def sign_up_params
    devise_parameter_sanitizer.sanitize(:sign_up)
  end

  def account_update_params
    devise_parameter_sanitizer.sanitize(:account_update)
  end

  def translation_scope
    'devise.registrations'
  end
end

ruby-2.3.3导轨 (4.2.7.1)设计(4.2.0)

Answer 1

看看这个 ruby 片段:

if true
  foo = "hello"
end
puts foo

#=> hello

还有这个:

if false
  foo = "hello"
end
puts foo

#=> nil

在许多语言中，if 语句都有自己的作用域，但在 ruby​​ 中它们共享周围函数的作用域。这意味着在 if 语句内声明的变量可以在 if 语句外访问。

这里的问题是变量是在编译时声明的，在 ruby​​ 知道 if 语句是 true 还是 false 之前。所以在 ruby​​ 中，所有局部变量都声明并初始化为 nil，即使它们在条件语句中也是如此。

这段代码:

unless resource.nil?
  resource = resource.becomes(Accounts::Admin)
end

由于 ruby​​ 中的另一条规则导致局部变量优先于方法而导致问题。因此，当您说 resource = resource 时，您实际上是在调用方法 resource 并将其值保存到局部变量 resource 中，这会掩盖方法同名。

最终你得到了错误:

undefined method `becomes' for nil:NilClass

因为在编译时，局部变量 resource 被创建，掩盖了方法。然后，在运行时，条件被执行，因为 resource 还不是 nil。但是在您创建局部变量的那一行，它立即进入范围，使 resource = nil 并导致此错误。

错误可以在这个通用示例中重现:

def blah
  "foo"
end

unless blah.nil?
  blah = blah.size
end
puts blah

解决方法是指定方法本身:

def blah
  "foo"
end

def blah= value
  #do nothing
end

unless blah.nil?
  self.blah = blah.size
end

puts blah

也就是说，我不确定设计是否真的实现了 resource=()。如果没有，那么最好的解决方案就是您已经想到的解决方案 - 使用局部变量:

unless resource.nil?
  res = resource.becomes(Accounts::Admin)
end
puts res

---

经过一些研究，我发现 ruby​​ 中的局部变量是根据文件中的位置从上到下和从左到右定义的，而不是它们在程序流中的位置。例如:

if x="foo"
  puts x
end
#=> "foo"

puts y if y="foo"
#NameError: undefined variable or method 'y'

这是 ruby​​ 规范的一部分，according to matz .