"undefined method 'becomes' for nil:NilClass" where it shouldn't

Question

I was testing a snippet today

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

this raises an error

undefined method `becomes' for nil:NilClass

if I do this

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

Everything goes right.

What is the difference if the right part of the =operator is executed first ?

EDIT :

Something nasty is happening, the last line under if false is being executed, but "ALOHA" is never printed.

<%
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}"
%>

it prints

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

BBBB nil

but if I do this

<%
res = resource

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

it print correctly

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: ""

I already have tried to restart the server. This snippet is at devise/registrations/new.html.erb. The resourcevariable is supposed to be an instance of User, created by devise's RegistrationController

I have checked the text for hidden characters, the snippets I pasted here are the whole text for the file being tested.

This is the content of the controller at ~/.rvm/gems/ruby-2.3.3@mygem/gems/devise-4.2.0/app/controllers/devise/registrations_controller.rb

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 rails (4.2.7.1) devise (4.2.0)

Answer 1

Look at this ruby snippet:

if true
  foo = "hello"
end
puts foo

#=> hello

And this one:

if false
  foo = "hello"
end
puts foo

#=> nil

In many languages, if-statements have their own scope, but in ruby they share the scope of the surrounding function. This means variables declared inside if-statements are accessible outside of if-statements.

The issue here is that variables are declared at compile-time, before ruby knows whether the if-statement is true or false. So in ruby, all local variables are declared and initialized as nil, even if they're in a conditional statement.

This code:

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

Causes problems because of another rule in ruby which gives local variables priority over methods. So when you say resource = resource you're actually calling the method resource and saving its value to the local variable resource, which then overshadows the method by the same name.

Ultimately you're getting the error:

undefined method `becomes' for nil:NilClass

because at compile-time, the local variable resource is being created, overshadowing the method. Then, at run-time, the condition is being executed because resource is not yet nil. But at the line where you create the local variable, it instantly comes into scope, making resource = nil and causing this the error.

The error can be reproduced in this generalized example:

def blah
  "foo"
end

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

And the fix for it is to specify the method itself:

def blah
  "foo"
end

def blah= value
  #do nothing
end

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

puts blah

That being said, I'm not sure whether devise actually implements resource=(). If it doesn't, then your best solution is the one you already came up with- use a local variable:

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

---

After doing some research, I found that local variables in ruby are defined from top to bottom and left to right, based on position in the file, not their position in program flow. For example:

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

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

This is part of the ruby spec, according to matz.

Answer 2

If using ruby Ruby 2.3.0 and above use Safe Navigation Operator (&.)

resource = resource&.becomes(Accounts::Admin)