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

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)

Answer 3

Try this in IRb, it should be fine:
a = 'hello'
unless a.nil?
a = a.upcase
end
a
# => "HELLO"
The second example is equivalent to the first, the expression on the right side of the = is assigned to the var on the left.
Maybe some hidden characters or a mis-spelling?
Are you sure the stacktrace points to that line, are you using becomes elsewhere in the file?