# Has\_one, belongs\_to - I am just not getting it

**URL:** <https://www.ruby-forum.com/t/has-one-belongs-to-i-am-just-not-getting-it/97290>\
**Category:** Rails\
**Created:** [July 6, 2006, 5:17am UTC](https://www.ruby-forum.com/t/has-one-belongs-to-i-am-just-not-getting-it/97290 "2006-07-06T05:17:45Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![superwick](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/46a35a/32.png) [@superwick](https://www.ruby-forum.com/u/superwick)\
**Post date:** [July 6, 2006, 5:17am UTC](https://www.ruby-forum.com/t/has-one-belongs-to-i-am-just-not-getting-it/97290/1 "2006-07-06T05:17:45Z")

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Let me preface by saying i’m a total noob at ruby/rails. 🙂  
For some reason, I am just not grasping the concept of has\_one &  
belongs\_to…

I’m working on a project that includes user authentication, and 3  
different types of user profiles.

User profiles are either “fan”, “band”, or “venue” objects, and user  
logins are “user” objects.

Just working with the “fan” profiles now.

I have the following tables:  
table fans:  
id  
name  
url  
…  
(etc)  
…  
user\_id (foreign key)

table users:  
id  
login  
hashed\_password  
email  
salt  
created\_at

In User model:  
class User \< ActiveRecord::Base  
has\_one :fan

In Fan model:  
class Fan \< ActiveRecord::Base  
belongs\_to :user

in user\_controller.rb:  
def signup  
@user = User.new(@params[:user])  
if request.post?  
if @user.save  
session[:user] = User.authenticate(@user.login,  
@user.password)  
flash[:message] = “Signup successful”  
redirect\_to :controller =\> ‘fan’, :action =\> “new”  
else  
flash[:warning] = “Signup unsuccessful”  
end  
end  
end

In fan\_controller.rb:  
def new  
@fan = Fan.new  
end

def create  
@fan = Fan.new(params[:id])  
@user.fan \<\< @fan  
if @fan.save  
flash[:notice] = ‘Your Fan Profile was successfully created.’  
redirect\_to :action =\> ‘list’  
else  
render :action =\> ‘new’  
end  
end

The goal is, when a new user signs up, after creating their login info,  
they get to create their “fan” profile, and when it is saved, the  
profile is saved in the fans table, with the appropriate user\_id  
assigned to it.

As I currently have it, the user signup works, then takes me to the new  
profile form, but when I try to save, I get:  
NoMethodError in FanController#create

You have a nil object when you didn’t expect it!  
The error occured while evaluating nil.fan

RAILS\_ROOT: script/…/config/…  
Application Trace | Framework Trace | Full Trace

#{RAILS\_ROOT}/app/controllers/fan\_controller.rb:28:in `create’

line 28 is ‘@user.fan \<\< @fan’

i’ve tried @user.fan = @fan, that doesn’t work…

I keep reading and reading about this, and I just can’t seem to get a  
handle on it yet. I’m in dire need of help!

much obliged,  
Chad

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<div class="post-metadata">

**Author:** ![superwick](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/46a35a/32.png) [@superwick](https://www.ruby-forum.com/u/superwick)\
**Post date:** [July 6, 2006, 8:04am UTC](https://www.ruby-forum.com/t/has-one-belongs-to-i-am-just-not-getting-it/97290/2 "2006-07-06T08:04:35Z")

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> def create  
> @fan = Fan.new(params[:id])  
> @user.fan \<\< @fan  
> if @fan.save  
> flash[:notice] = ‘Your Fan Profile was successfully created.’  
> redirect\_to :action =\> ‘list’  
> else  
> render :action =\> ‘new’  
> end  
> end

you didn’t define @user.

def create  
@user = User.find(…whicheveroneyouwant…) OR  
@user = User.find(params[:id]) OR  
@user = User.find\_by\_some\_column(value) OR  
…

@fan = Fan.new…  
…  
end

i reccomended you should do some tutorial to help you out; have you  
tried four days on rails? or the pragmatic store self-tutorial? these  
are great books and will give you a great base to jump off of.

anyway, i hope that helps you out.

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**Author:** ![superwick](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/46a35a/32.png) [@superwick](https://www.ruby-forum.com/u/superwick)\
**Post date:** [July 6, 2006, 11:32am UTC](https://www.ruby-forum.com/t/has-one-belongs-to-i-am-just-not-getting-it/97290/3 "2006-07-06T11:32:02Z")

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On Thu, 6 Jul 2006, Chad Wells wrote:

> ```
> redirect_to :action => 'list'
> 
> ```
> 
> As I currently have it, the user signup works, then takes me to the new  
> profile form, but when I try to save, I get:  
> NoMethodError in FanController#create
> 
> You have a nil object when you didn’t expect it!  
> The error occured while evaluating nil.fan

Instance variables only survive for the duration of one action, which  
may include a “render” but not a “redirect” (that’s a new action). So  
in create, you never initialize @user – so it’s nil.

I have a feeling that what you really want is something like:

def create  
@user = User.find(params[:id])  
@fan = Fan.create(params[:fan])  
@user.fan = @fan

# etc.

Doing Fan.new(params[:id]) doesn’t make sense, since if it’s new, it  
will get assigned an id automatically, and if you’re looking it up by  
id, you’d use “find”.

David

–  
“To fully realize the potential of Rails, it’s crucial that you take  
the time to fully understand Ruby–and with “Ruby for Rails” David  
has provided just what you need to help you achieve that goal.”  
– DAVID HEINEMEIER HANSSON, in the foreword to RUBY FOR RAILS.  
Complete foreword & sample chapters at [Ruby for Rails](http://www.manning.com/black)!

---

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**Author:** ![superwick](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/46a35a/32.png) [@superwick](https://www.ruby-forum.com/u/superwick)\
**Post date:** [July 6, 2006, 3:16pm UTC](https://www.ruby-forum.com/t/has-one-belongs-to-i-am-just-not-getting-it/97290/4 "2006-07-06T15:16:07Z")

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unknown wrote:

> Instance variables only survive for the duration of one action, which  
> may include a “render” but not a “redirect” (that’s a new action). So  
> in create, you never initialize @user – so it’s nil.

I think this is my main problem! I guess I just didn’t realize this  
fact. This makes sense as to why I was having so many other little  
problems.

Thank you so much!

Another question, is there a way to keep an instance variable alive for  
a total session, instead of having to intialize it over and over?

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**Author:** ![superwick](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/46a35a/32.png) [@superwick](https://www.ruby-forum.com/u/superwick)\
**Post date:** [July 6, 2006, 6:36pm UTC](https://www.ruby-forum.com/t/has-one-belongs-to-i-am-just-not-getting-it/97290/5 "2006-07-06T18:36:55Z")

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On Jul 6, 2006, at 6:16 AM, Chad Wells wrote:

> Another question, is there a way to keep an instance variable alive  
> for  
> a total session, instead of having to intialize it over and over?

That’s what session is for:

session[:key] = @blah

and in a subsequent request:

@blah = session[:key]

On caveat: Don’t store an ActiveRecord in the session. Instead store  
it’s  
id and refetch it the next time, unless it’s you absolutely KNOW that it  
won’t change.

–  
– Tom M.