# Instance variable not holding between get? and post

**URL:** <https://www.ruby-forum.com/t/instance-variable-not-holding-between-get-and-post/74387>\
**Category:** Rails\
**Created:** [November 6, 2006, 4:34am UTC](https://www.ruby-forum.com/t/instance-variable-not-holding-between-get-and-post/74387 "2006-11-06T04:34:31Z")\
**Posts on this page:** 5\
**Page:** 1

<div class="post-metadata">

**Author:** ![Flippper](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/35a633/32.png) [@Flippper](https://www.ruby-forum.com/u/Flippper)\
**Post date:** [November 6, 2006, 4:34am UTC](https://www.ruby-forum.com/t/instance-variable-not-holding-between-get-and-post/74387/1 "2006-11-06T04:34:31Z")

</div>

I am having trouble with this bit of code

if request.get?  
session[:user\_id] = nil  
@user = User.new

```
  logger.error("get:#{@user.object_id}")
else
  logger.error("post:#{@user.object_id}")

  logged_in_user = @user.try_to_login

  if logged_in_user
    session[:user] = logged_in_user.id
    redirect_to(:action => "index")
  else
    flash[:notice] = "Invalid user/password combo"
  end

```

For some reason, when posting back the object\_id of the @user variable  
is 4 but when the get? is executed, the object\_id is 28208630.

The error message is

NoMethodError (You have a nil object when you didn’t expect it!  
The error occured while evaluating nil.try\_to\_login):

This happens if i enter a user name and password or leave them both  
blank, and yes there is a method called try\_to\_login. Im trying to learn  
rails and the code is exactly as is written in the book. Could something  
have changed in rails since the book was published?

---

<div class="post-metadata">

**Author:** ![Flippper](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/35a633/32.png) [@Flippper](https://www.ruby-forum.com/u/Flippper)\
**Post date:** [November 6, 2006, 4:47am UTC](https://www.ruby-forum.com/t/instance-variable-not-holding-between-get-and-post/74387/2 "2006-11-06T04:47:36Z")

</div>

Hey Flipper -

Which book?

When you get to the line:

logged\_in\_user = @user.try\_to\_login

on the postback, you have not yet defined @user, thus the  
“nil.try\_to\_login” error. Somewhere before that line you need to create  
the @user instance variable as a new user object, probably by passing in  
the parameters from the form, something like:

@user = User.new(params[:user])

Or even better, you don’t really need the @user object at that point, so  
you could just do something like this:

logged\_in\_user = User.new(params[:user]).try\_to\_login

Check the code in your book again - they’ve got to be creating the @user  
somewhere in the code before you get to that line. It doesn’t just come  
back in fully-formed from the login view form without you doing anything  
in the controller to create it.

c.

Flippper wrote:

> I am having trouble with this bit of code
> 
> if request.get?  
> session[:user\_id] = nil  
> @user = User.new
> 
> ```
> logger.error("get:#{@user.object_id}")
> else
> logger.error("post:#{@user.object_id}")
> 
> logged_in_user = @user.try_to_login
> 
> if logged_in_user
> session[:user] = logged_in_user.id
> redirect_to(:action => "index")
> else
> flash[:notice] = "Invalid user/password combo"
> end
> 
> ```
> 
> For some reason, when posting back the object\_id of the @user variable  
> is 4 but when the get? is executed, the object\_id is 28208630.
> 
> The error message is
> 
> NoMethodError (You have a nil object when you didn’t expect it!  
> The error occured while evaluating nil.try\_to\_login):
> 
> This happens if i enter a user name and password or leave them both  
> blank, and yes there is a method called try\_to\_login. Im trying to learn  
> rails and the code is exactly as is written in the book. Could something  
> have changed in rails since the book was published?

---

<div class="post-metadata">

**Author:** ![Flippper](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/35a633/32.png) [@Flippper](https://www.ruby-forum.com/u/Flippper)\
**Post date:** [November 6, 2006, 5:34am UTC](https://www.ruby-forum.com/t/instance-variable-not-holding-between-get-and-post/74387/3 "2006-11-06T05:34:33Z")

</div>

Cayce B. wrote:

> Hey Flipper -
> 
> Which book?
> 
> When you get to the line:
> 
> logged\_in\_user = @user.try\_to\_login
> 
> on the postback, you have not yet defined @user, thus the  
> “nil.try\_to\_login” error. Somewhere before that line you need to create  
> the @user instance variable as a new user object, probably by passing in  
> the parameters from the form, something like:
> 
> @user = User.new(params[:user])
> 
> Or even better, you don’t really need the @user object at that point, so  
> you could just do something like this:
> 
> logged\_in\_user = User.new(params[:user]).try\_to\_login
> 
> Check the code in your book again - they’ve got to be creating the @user  
> somewhere in the code before you get to that line. It doesn’t just come  
> back in fully-formed from the login view form without you doing anything  
> in the controller to create it.
> 
> c.

Thanks very much for that, it worked.

Although I dont understand why it needs to be reassigned.

Why would I @user = User.new in the get? and then recreate the User  
object again at post?

The book mentioned that form data will be stored in the @user object so  
woulden’t recreating it lose the information?

---

<div class="post-metadata">

**Author:** ![Flippper](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/35a633/32.png) [@Flippper](https://www.ruby-forum.com/u/Flippper)\
**Post date:** [November 6, 2006, 6:30am UTC](https://www.ruby-forum.com/t/instance-variable-not-holding-between-get-and-post/74387/4 "2006-11-06T06:30:30Z")

</div>

CatLady [] wrote:

> Thanks very much for that, it worked.
> 
> Although I dont understand why it needs to be reassigned.
> 
> Why would I @user = User.new in the get? and then recreate the User  
> object again at post?
> 
> The book mentioned that form data will be stored in the @user object so  
> woulden’t recreating it lose the information?

I believe what you want is:

@user = User.new(params[:user])

This creates a new user object, but from the parameters passd in from  
the form that just submitted it. The form doesn;t store the object  
itself, just its attributes. And instance variables in controllers are  
only alive for the length of a single request. Once a second request  
comes in a new instane of the controller is created so all instance  
variables are lost.

---

<div class="post-metadata">

**Author:** ![Flippper](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/35a633/32.png) [@Flippper](https://www.ruby-forum.com/u/Flippper)\
**Post date:** [November 6, 2006, 2:46pm UTC](https://www.ruby-forum.com/t/instance-variable-not-holding-between-get-and-post/74387/5 "2006-11-06T14:46:15Z")

</div>

Because HTTP is a ‘stateless’ protocol. Between hitting the page for the  
first time when rails does @user = User.new and the user submitting the  
form with data, the server (and rails) completely ‘forgets’ what’s going  
on. Each request, whether it is the first hit on the page (the GET) or  
the submission of data (the POST), is a separate and distinct request to  
the server. The server does not maintain any state data in between -  
from its perspective, that @user variable is destroyed once the request  
completes.

When the form is submitted, the data you filled in are part of the POST  
header in the request. Rails picks this data out and populates the  
params[] hash with it, giving you easy access to the data, but rails  
really has no idea what kind of data that is or what you want to do with  
it - that’s up to you. In this case, you have need of an object of a  
particular class, so you create it. In other cases, maybe you just need  
a particular piece of submitted data, so you’d access it in params[].

c.

> Thanks very much for that, it worked.
> 
> Although I dont understand why it needs to be reassigned.
> 
> Why would I @user = User.new in the get? and then recreate the User  
> object again at post?
> 
> The book mentioned that form data will be stored in the @user object so  
> woulden’t recreating it lose the information?