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Vahagn Hayrapetyan

2008-Sep-18 08:41 UTC

"Nil object": really?

Hello,-

I am developing a rails app with this method in it:

 # PUT /newsletters/1;send
  def sendmails
    newsletter = Newsletter.find_by_id_and_sent(params[:id], false)
    users = User.find(:all)
    raise users.to_yaml
    #debugger
    users.each do |user| #evaluates to nil.each - debug
      Notifier.deliver_newsletter(user, newsletter)
    end
    newsletter.update_attribute(''sent'', true)
    redirect_to newsletters_path
  end

I get an error saying that users.each do |user| evaluates to nil.each,
and that I probably expected an Array object there. This is true, I did
expect an Array object. Thing is, now I''ve tried to debug the code with
both debugger and raise to_yaml, and both times the users variable
evaluates to a perfect Array of 3 Users, exactly in correspondence with
the DB! I have also tried to debug the user variable and it evaluates to
a User with id 1, meaning the iterator fetches that user successfully
from the Array.

Yet after the first iteration, I get an error / crash.

I have tried to change users to @users, with the exact same result.
Debugging shows an Array of 3 user objects. The app crashes.

What is going on here?

Thanks in advance, Vahagn
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Frederick Cheung

2008-Sep-18 09:12 UTC

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Re: "Nil object": really?

On Sep 18, 9:41 am, Vahagn Hayrapetyan <rails-mailing-l...@andreas-
s.net> wrote:> Hello,-
> Yet after the first iteration, I get an error / crash.
>
> I have tried to change users to @users, with the exact same result.
> Debugging shows an Array of 3 user objects. The app crashes.
>Are you sure that the error message is on that each, not another one ?
What''s the stack trace like?

Fred


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Vahagn Hayrapetyan

2008-Sep-18 10:04 UTC

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Re: "Nil object": really?

Frederick Cheung wrote:> On Sep 18, 9:41�am, Vahagn Hayrapetyan <rails-mailing-l...@andreas-
> s.net> wrote:
>> Hello,-
> 
>> Yet after the first iteration, I get an error / crash.
>>
>> I have tried to change users to @users, with the exact same result.
>> Debugging shows an Array of 3 user objects. The app crashes.
>>
> Are you sure that the error message is on that each, not another one ?
> What''s the stack trace like?
> 
> Fred
Hi Fred,-

here''s the stack trace. Whatever it is that is causing the problem, I 
cannot glean it from there:

app/controllers/newsletters_controller.rb:60:in `sendmails''
/Library/Ruby/Gems/1.8/gems/actionpack-2.0.2/lib/action_controller/base.rb:1158:in
`send''
/Library/Ruby/Gems/1.8/gems/actionpack-2.0.2/lib/action_controller/base.rb:1158:in
`perform_action_without_filters''
/Library/Ruby/Gems/1.8/gems/actionpack-2.0.2/lib/action_controller/filters.rb:697:in
`call_filters''
/Library/Ruby/Gems/1.8/gems/actionpack-2.0.2/lib/action_controller/filters.rb:689:in
`perform_action_without_benchmark''
/Library/Ruby/Gems/1.8/gems/actionpack-2.0.2/lib/action_controller/benchmarking.rb:68:in
`perform_action_without_rescue''
/Library/Ruby/Gems/1.8/gems/actionpack-2.0.2/lib/action_controller/benchmarking.rb:68:in
`perform_action_without_rescue''
/Library/Ruby/Gems/1.8/gems/actionpack-2.0.2/lib/action_controller/rescue.rb:199:in
`perform_action_without_caching''
/Library/Ruby/Gems/1.8/gems/actionpack-2.0.2/lib/action_controller/caching.rb:678:in
`perform_action''
/Library/Ruby/Gems/1.8/gems/activerecord-2.0.2/lib/active_record/connection_adapters/abstract/query_cache.rb:33:in
`cache''
/Library/Ruby/Gems/1.8/gems/activerecord-2.0.2/lib/active_record/query_cache.rb:8:in
`cache''
/Library/Ruby/Gems/1.8/gems/actionpack-2.0.2/lib/action_controller/caching.rb:677:in
`perform_action''
/Library/Ruby/Gems/1.8/gems/actionpack-2.0.2/lib/action_controller/base.rb:524:in
`send''
/Library/Ruby/Gems/1.8/gems/actionpack-2.0.2/lib/action_controller/base.rb:524:in
`process_without_filters''
/Library/Ruby/Gems/1.8/gems/actionpack-2.0.2/lib/action_controller/filters.rb:685:in
`process_without_session_management_support''
/Library/Ruby/Gems/1.8/gems/actionpack-2.0.2/lib/action_controller/session_management.rb:123:in
`process''
/Library/Ruby/Gems/1.8/gems/actionpack-2.0.2/lib/action_controller/base.rb:388:in
`process''
/Library/Ruby/Gems/1.8/gems/activesupport-2.0.2/lib/active_support/dependencies.rb:489:in
`load''
/Library/Ruby/Gems/1.8/gems/activesupport-2.0.2/lib/active_support/dependencies.rb:489:in
`load''
/Library/Ruby/Gems/1.8/gems/activesupport-2.0.2/lib/active_support/dependencies.rb:342:in
`new_constants_in''
/Library/Ruby/Gems/1.8/gems/activesupport-2.0.2/lib/active_support/dependencies.rb:489:in
`load''
/Library/Ruby/Gems/1.8/gems/rails-2.0.2/lib/commands/servers/mongrel.rb:64
/Library/Ruby/Gems/1.8/gems/activesupport-2.0.2/lib/active_support/dependencies.rb:496:in
`require''
/Library/Ruby/Gems/1.8/gems/activesupport-2.0.2/lib/active_support/dependencies.rb:342:in
`new_constants_in''
/Library/Ruby/Gems/1.8/gems/activesupport-2.0.2/lib/active_support/dependencies.rb:496:in
`require''
/Library/Ruby/Gems/1.8/gems/rails-2.0.2/lib/commands/server.rb:39


/ V.

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Vahagn Hayrapetyan

2008-Sep-18 10:07 UTC

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PS Re: "Nil object": really?

And here''s the output of raise users.to_yaml:

---
- !ruby/object:User
  attributes:
    last_login_at:
    updated_at: 2008-09-01 12:47:57
    hashed_password: 
8c6976e5b5410415bde908bd4dee15dfb167a9c873fc4bb8a81f6f2ab448a918
    profile: Site Administrator
    username: Admin
    id: "1"
    enabled: "1"
    email: admin-EoFj0TZMsd0@public.gmane.org
    created_at: 2008-09-01 12:47:57
  attributes_cache: {}

- !ruby/object:User
  attributes:
    last_login_at:
    updated_at: 2008-09-01 19:18:14
    hashed_password: 
a1da7f927134c10ec6bec60000dd8f9308410a4efcf8c8799280e8ea37a1b2cb
    profile: Vahagn
    username: Vahagn
    id: "2"
    enabled: "1"
    email: webmaster-EoFj0TZMsd0@public.gmane.org
    created_at: 2008-09-01 19:18:14
  attributes_cache: {}

- !ruby/object:User
  attributes:
    last_login_at:
    updated_at: 2008-09-12 15:32:14
    hashed_password: 
9f86d081884c7d659a2feaa0c55ad015a3bf4f1b2b0b822cd15d6c15b0f00a08
    profile: <ul></ul>
    username: "*kjbvbv*"
    id: "3"
    enabled: "1"
    email: test-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org
    created_at: 2008-09-01 19:22:35
  attributes_cache: {}
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Phlip

2008-Sep-18 11:39 UTC

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Re: "Nil object": really?

Vahagn Hayrapetyan wrote:
>     users.each do |user| #evaluates to nil.each - debug
In general, you need to cut your action up into tiny methods and write unit 
tests on each one. They incredibly reduce the odds of exactly this kind of 
drama! Then the methods should run in the model layer, and the controller should
only call into the model.

Specifically, check if ''users'' is aliased anywhere else, such
as to a method.
Then try this:

  (users || []).each

That will force a nil users to behave like an empty array. .find(:all) should 
naturally return an empty array if you have no users, so you should not need 
this tweak. But it lets you manually debug more:

   (users || (p ''still nil!''; []).each

-- 
   Phlip


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Vahagn Hayrapetyan

2008-Sep-18 12:18 UTC

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Re: "Nil object": really?

Hi Phlip, I appreciate the feedback. Firstly, I renamed users to 
all_users and the error is still there; therefore, no aliasing occurs.

Secondly, I''ve tried as you wrote:

  (all_users || (p ''still nil!''; []).each
      Notifier.deliver_newsletter(user, newsletter)
   end

but that gives a syntax error ("Unexpected kEND"). I''m new to
Ruby!

Lastly, do you mean to say that this method is best moved to the 
Newsletter model instead of being in the controller?

/ Vahagn



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Bharat Ruparel

2008-Sep-18 13:28 UTC

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Re: "Nil object": really?

Hello Vahagn,
I suggest you do it as follows:
a) old-fashioned way by using puts liberally &
b) take baby steps
until it is debugged.

Your code  with puts statements inserted; just an example:

  def sendmails
    newsletter = Newsletter.find_by_id_and_sent(params[:id], false)
    users = User.find(:all)
    # do not raise anything...  confuses Ruby interpretor
    puts users
    users.each do |user|
      puts user
      #Notifier.deliver_newsletter(user, newsletter)
    end
    #newsletter.update_attribute(''sent'', true)
    #redirect_to newsletters_path
  end

Make sure first that the each iterator is printing your object instances 
correctly.  You can use "pp"  short for pretty-print if you want but 
then you will have to require it.  puts is built into the language and 
most of the times, it is sufficient.

This way, you can be absolutely sure that nothing unexpected is 
happening with your basic loop.  Once you are sure then you can 
uncomment each statement one by one, e.g., uncomment Notifier.... first 
and test.  If that works then go to newslettr.update...

Hope this helps.

Bharat
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Vahagn Hayrapetyan

2008-Sep-19 18:57 UTC

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Re: "Nil object": really?

Thanks Bharat, that was excellent advice. It turns out the real trouble 
is being caused by this line:

Notifier.deliver_newsletter(user, newsletter)

Now, the stack trace looks like this:


NameError in Newsletters#sendmails

Showing notifier/newsletter.text.plain.rhtml where line #3 raised:

undefined local variable or method `user'' for 
#<ActionView::Base:0x23df2a0>

Extracted source (around line #3):

1: My Newsletter
2:
3: <%= render :inline => @body %>



I have a Notifier class that looks like this:

class Notifier < ActionMailer::ARMailer
  def newsletter(user, newsletter)
    recipients user.email
    from "MyApp <system-EoFj0TZMsd0@public.gmane.org>"
    subject newsletter.subject
    body :body => newsletter.body, :user => user
  end
end


...and this is the file the stack trace refers to:

/app/views/notifier/newsletter.text.plain.rhtml:

My Newsletter

<%= render :inline => @body %>


Still - user is not being referenced from this file at all, so why the 
error?

Thanks again,
/ Vahagn
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Bharat

2008-Sep-20 15:06 UTC

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Re: "Nil object": really?

Glad to be of help.  I am no expert on ActiveRecord Mailer, but when I
run into problems like this, I turn to:
1.  Agile Web Development book -- Excellent discussion on this topic
2.  Rails Way by Obie Fernandez -- Anyone doing any serious
development should have this book
I am sure you can figure this out by a bit of reading and research.

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Vahagn Hayrapetyan

2008-Sep-22 19:49 UTC

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Re: "Nil object": really?

Hi Bharat,-

there''s hardly a Rails book that is not near the top of my Amazon wish 
list - so you''re right on ;-)

Cheers,
Vahagn

Bharat Ruparel wrote:> Glad to be of help.  I am no expert on ActiveRecord Mailer, but when I
> run into problems like this, I turn to:
> 1.  Agile Web Development book -- Excellent discussion on this topic
> 2.  Rails Way by Obie Fernandez -- Anyone doing any serious
> development should have this book
> I am sure you can figure this out by a bit of reading and research.
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Rails - Sep 2008 - "Nil object": really?

"Nil object": really?

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