thr3ads.net - Rails - How to overcome the DoubleRender problem? [Oct 2007]

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Joshua Muheim

2007-Oct-28 23:11 UTC

How to overcome the DoubleRender problem?

Hi all

I ran into this problem more than once.

Let''s say, a user logs into my site and clicks a link that only logged
in members can see. Because he likes the content of this specific site,
he places a link into his link list.
Some days later he clicks the link from his link list. Unfortunately his
session has expired so my code forwards him to the login page, but keeps
the link he tried to open in the session so after successful login the
user is redirected straight away to the wished site.
So the user logs successfully in. But during the days that have passed
the site has been moved to another URL, but this shouldn''t be a problem
because we just forward him to the new URL - and that''s where problems
strike! You get a DoubleRenderException or something like that!

Let''s recapitulate: The redirection was from (1) the login site to (2)
the link the user entered before to (3) the new location of the site.

I guess this is quite correct semantical code, but Rails does not let me
do it.

Did I miss anything? Maybe I mis-use the redirect_to method? Or how can
I accomplish the described behavior in Rails?

Thanks
Josh
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William Pratt

2007-Oct-29 01:00 UTC

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Re: How to overcome the DoubleRender problem?

This should not create a problem at all as I do it all the time. My 
guess is that you are assuming that render and redirect_to immediately 
return from a method, but they don''t. All redirect_to does is add a 302
header to the response object that is sent back to the browser. Without 
seeing your code, all I can do is assume, but try changing the 
redirect_to line to something like the following (notice the && return):

redirect_to(:controller => ''foo'', :action =>
''bar'') && return

-Bill

Joshua Muheim wrote:> Hi all
>
> I ran into this problem more than once.
>
> Let''s say, a user logs into my site and clicks a link that only
logged
> in members can see. Because he likes the content of this specific site,
> he places a link into his link list.
> Some days later he clicks the link from his link list. Unfortunately his
> session has expired so my code forwards him to the login page, but keeps
> the link he tried to open in the session so after successful login the
> user is redirected straight away to the wished site.
> So the user logs successfully in. But during the days that have passed
> the site has been moved to another URL, but this shouldn''t be a
problem
> because we just forward him to the new URL - and that''s where
problems
> strike! You get a DoubleRenderException or something like that!
>
> Let''s recapitulate: The redirection was from (1) the login site to
(2)
> the link the user entered before to (3) the new location of the site.
>
> I guess this is quite correct semantical code, but Rails does not let me
> do it.
>
> Did I miss anything? Maybe I mis-use the redirect_to method? Or how can
> I accomplish the described behavior in Rails?
>
> Thanks
> Josh
>   
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Michael Graff

2007-Oct-29 01:08 UTC

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Re: How to overcome the DoubleRender problem?

On 10/28/07, William Pratt <billp-YbheRAKfYF4eIZ0/mPfg9Q@public.gmane.org>
wrote:
> redirect_to(:controller => ''foo'', :action =>
''bar'') && return
While this is perfectly valid syntax, I would recommend:

  redirect_to(...)
  return

because it is more clear what is going on.  The && trick works --
unless redirect_to suddenly starts returning true.

--Michael

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Rick DeNatale

2007-Oct-29 01:13 UTC

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Re: How to overcome the DoubleRender problem?

On 10/28/07, Michael Graff
<skan.gryphon-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:>
> On 10/28/07, William Pratt
<billp-YbheRAKfYF4eIZ0/mPfg9Q@public.gmane.org> wrote:
>
> > redirect_to(:controller => ''foo'', :action =>
''bar'') && return
>
> While this is perfectly valid syntax, I would recommend:
>
>   redirect_to(...)
>   return
>
> because it is more clear what is going on.  The && trick works --
> unless redirect_to suddenly starts returning true.
Actually the return will happen as long as the redirect_to returns
anything but nil or false.

-- 
Rick DeNatale

My blog on Ruby
http://talklikeaduck.denhaven2.com/

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Michael Graff

2007-Oct-29 01:29 UTC

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Re: How to overcome the DoubleRender problem?

Gah, brain fart.  :)

The point is, && is "short-circuit" in that the first in the
path that
fails causes the rest to not even run.

--Michael

On 10/28/07, Rick DeNatale
<rick.denatale-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:>
> On 10/28/07, Michael Graff
<skan.gryphon-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
> >
> > On 10/28/07, William Pratt
<billp-YbheRAKfYF4eIZ0/mPfg9Q@public.gmane.org> wrote:
> >
> > > redirect_to(:controller => ''foo'', :action
=> ''bar'') && return
> >
> > While this is perfectly valid syntax, I would recommend:
> >
> >   redirect_to(...)
> >   return
> >
> > because it is more clear what is going on.  The && trick works
--
> > unless redirect_to suddenly starts returning true.
>
> Actually the return will happen as long as the redirect_to returns
> anything but nil or false.
>
> --
> Rick DeNatale
>
> My blog on Ruby
> http://talklikeaduck.denhaven2.com/
>
> >
>
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Herryanto Siatono

2007-Oct-29 05:51 UTC

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Re: How to overcome the DoubleRender problem?

You can also try out:

return redirect_to(...)

You can avoid the "short-circuit".

- Herry

-----Original Message-----
From: rubyonrails-talk-/JYPxA39Uh5TLH3MbocFFw@public.gmane.org
[mailto:rubyonrails-talk-/JYPxA39Uh5TLH3MbocFFw@public.gmane.org] On Behalf Of
Michael Graff
Sent: 29 October 2007 09:30
To: rubyonrails-talk-/JYPxA39Uh5TLH3MbocFFw@public.gmane.org
Subject: [Rails] Re: How to overcome the DoubleRender problem?


Gah, brain fart.  :)

The point is, && is "short-circuit" in that the first in the
path that
fails causes the rest to not even run.

--Michael

On 10/28/07, Rick DeNatale
<rick.denatale-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:>
> On 10/28/07, Michael Graff
<skan.gryphon-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
> >
> > On 10/28/07, William Pratt
<billp-YbheRAKfYF4eIZ0/mPfg9Q@public.gmane.org> wrote:
> >
> > > redirect_to(:controller => ''foo'', :action
=> ''bar'') && return
> >
> > While this is perfectly valid syntax, I would recommend:
> >
> >   redirect_to(...)
> >   return
> >
> > because it is more clear what is going on.  The && trick works
--
> > unless redirect_to suddenly starts returning true.
>
> Actually the return will happen as long as the redirect_to returns
> anything but nil or false.
>
> --
> Rick DeNatale
>
> My blog on Ruby
> http://talklikeaduck.denhaven2.com/
>
> >
>


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Joshua Muheim

2007-Oct-29 15:19 UTC

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Re: How to overcome the DoubleRender problem?

Herryanto Siatono wrote:> You can also try out:
> 
> return redirect_to(...)
> 
> You can avoid the "short-circuit".
> 
> - Herry
Hrm then I guess I''m having something substantially wrong, because
I''m
always getting this error. So let''s track down what''s wrong...

The user types in the URL http://localhost:3000/admin/links/new

This routes to the controller Admin::LinksController and the action 
"new" which is handled by a module I wrote once:

class Admin::LinksController < Admin::ApplicationController
  include ActsAsManager
end

module ActsAsManager
  ...

  def new
    manage
  end

  ...
end

This calls to the quite complex method "manage" (private one) - this 
handles all the create/edit/update stuff that the normal scaffolds 
would:

  def manage
    params[:id].nil? ?
      self.model_obj = model_class.new :
      self.model_obj = model_class.find(params[:id])

    if request.get?
      member_requests(''CREATE'', model_class) do
        model_obj.attributes = params[model_obj_name]
      end
    elsif request.post?
      model_obj.attributes = params[model_obj_name]

      if model_obj.new_record?
        member_requests!(''CREATE'', model_class) do
          model_obj.creator = logged_in_member if 
model_obj.respond_to?(''creator='')
          model_obj.owner = logged_in_member if 
model_obj.respond_to?(''owner='')
        end
      else
        member_requests!(''EDIT'', model_class)
      end

      if model_obj.save
        if model_obj.new_record?
          flash[:notice] = model_obj_name.capitalize + '' was 
successfully created.''
          redirect_to :action => :list and return
        else
          flash[:notice] = model_obj_name.capitalize + '' was 
successfully updated.''
          redirect_to :action => :show, :id => model_obj and return
        end
      end
    else
      raise "Another method than GET or POST?! Woah!"
    end
    model_obj.new_record? ?
      render(:action => :new) :
      render(:action => :edit)

  # If the form contains a hidden field +lock_version+ then ActiveRecord 
throws a StaleObjectError if its value has changed since the form was 
called (positive locking).
  # TODO: Does this work anyhow anymore?!
  rescue ActiveRecord::StaleObjectError
    # TODO: Evaluate differences and add related errors
    self.model_obj.errors.add(:base, "Object is stale! 
#{self.model_obj.lock_version}:#{model_class.find_by_id(self.model_obj.id).lock_version}")
    render :action => :edit
  end

So far, we''re getting to line where the member requests permission to 
CREATE an item:

...
    if request.get?
      member_requests(''CREATE'', model_class) do
...

which calls the method member_requests():

  def member_requests(right_name, obj, options = {}, &block)
    if 
logged_in_member.has_system_right?(SystemRight.find_by_model_class_and_name(obj.to_s,
right_name))
      yield if block_given?
    else
      render :file => "#{RAILS_ROOT}/public/404.html", :layout
=> true,
:status => 404
    end
  end

And that''s where the DoubleRenderError is thrown! Anyone can see where 
the problem is? I just want to render the 404.html file, nothing else... 
No double rendering or stuff! :-P

Thanks a lot
Josh
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Frederick Cheung

2007-Oct-29 15:28 UTC

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Re: How to overcome the DoubleRender problem?

On 29 Oct 2007, at 15:19, Joshua Muheim wrote:>
>
> which calls the method member_requests():
>
>   def member_requests(right_name, obj, options = {}, &block)
>     if
> logged_in_member.has_system_right? 
> (SystemRight.find_by_model_class_and_name(obj.to_s,
> right_name))
>       yield if block_given?
>     else
>       render :file => "#{RAILS_ROOT}/public/404.html", :layout
=>
> true,
> :status => 404
>     end
>   end
>
> And that''s where the DoubleRenderError is thrown! Anyone can see
where
> the problem is? I just want to render the 404.html file, nothing  
> else...
> No double rendering or stuff! :-P
>
As far as I can see you then trickle down to> model_obj.new_record? ?
>       render(:action => :new) :
>       render(:action => :edit)
Which renders a second time, hence the error. I can also see quite a  
few other paths by which this could happen.

Fred

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Joshua Muheim

2007-Oct-29 18:31 UTC

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Re: How to overcome the DoubleRender problem?

Frederick Cheung wrote:> On 29 Oct 2007, at 15:19, Joshua Muheim wrote:
>>     else
>>
> As far as I can see you then trickle down to
>> model_obj.new_record? ?
>>       render(:action => :new) :
>>       render(:action => :edit)
> 
> Which renders a second time, hence the error.Thanks!
> I can also see quite a
> few other paths by which this could happen.What do you mean with that?
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Frederick Cheung

2007-Oct-29 19:40 UTC

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Re: How to overcome the DoubleRender problem?

On 29 Oct 2007, at 18:31, Joshua Muheim wrote:
>
> Frederick Cheung wrote:
>> On 29 Oct 2007, at 15:19, Joshua Muheim wrote:
>>>     else
>>>
>> As far as I can see you then trickle down to
>>> model_obj.new_record? ?
>>>       render(:action => :new) :
>>>       render(:action => :edit)
>>
>> Which renders a second time, hence the error.
> Thanks!
>
>> I can also see quite a
>> few other paths by which this could happen.
> What do you mean with that?
that there are other cases. eg member_requests rendering and you  
later redirecting.

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Joshua Muheim

2007-Oct-29 19:49 UTC

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Re: How to overcome the DoubleRender problem?

Frederick Cheung wrote:> On 29 Oct 2007, at 18:31, Joshua Muheim wrote:
> 
>>> Which renders a second time, hence the error.
>> Thanks!
>>
>>> I can also see quite a
>>> few other paths by which this could happen.
>> What do you mean with that?
> 
> that there are other cases. eg member_requests rendering and you
> later redirecting.
Hrm OK, so could you please tell me the key Do''s and Don''ts
how to use
redirect_to and render correctly? Thanks :-)
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Frederick Cheung

2007-Oct-29 21:50 UTC

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Re: How to overcome the DoubleRender problem?

It''s dead simple. If you''ve called render or redirect_to once
in an
action, make sure you don''t do either of them again.

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Joshua Muheim

2007-Oct-29 21:55 UTC

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Re: How to overcome the DoubleRender problem?

Frederick Cheung wrote:> It''s dead simple. If you''ve called render or redirect_to
once in an
> action, make sure you don''t do either of them again.
I like dead simple things. Thanks a lot :-)
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Rails - Oct 2007 - How to overcome the DoubleRender problem?

How to overcome the DoubleRender problem?

Re: How to overcome the DoubleRender problem?

Re: How to overcome the DoubleRender problem?

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Re: How to overcome the DoubleRender problem?

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