# Understanding methods

**URL:** <https://www.ruby-forum.com/t/understanding-methods/180202>\
**Category:** Rails\
**Created:** [December 14, 2009, 5:38pm UTC](https://www.ruby-forum.com/t/understanding-methods/180202 "2009-12-14T17:38:35Z")\
**Posts on this page:** 9\
**Page:** 1

<div class="post-metadata">

**Author:** ![mike343](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/58f4c7/32.png) [@mike343](https://www.ruby-forum.com/u/mike343)\
**Post date:** [December 14, 2009, 5:38pm UTC](https://www.ruby-forum.com/t/understanding-methods/180202/1 "2009-12-14T17:38:35Z")

</div>

Hi there, I am new to Ruby and to Rails and am having some trouble  
understanding when a method has been defined or not.

My current understanding, and I am not sure if I am correct, is that  
methods are defined only for a class unless inherited by subclasses  
(designated by the SubClass \< Class in the head of the controller  
document for that Class).

So I am wondering why I keep getting a an error of undefined\_method for  
a method I am defining in my application controller. I have tried  
bouncing it around into different controllers but I feel like it should  
either be recognized in the application or in my voting controller.  
This is what the method looks like:

def voted\_by\_user?(user)  
return = false  
if user  
self.votes.each { |v|  
return = true if user.id == v.user\_id  
}  
end  
return  
end

I am trying to call this method like this:

unless @link.voted\_by\_user?(@current\_user)  
@link.votes.create(:user =\> @current\_user)  
end

I should mention that I am new to Object Oriented Programming so am  
still wrapping my mind around inheritance, instance methods and class  
methods. I have some background in C++ and I may be trying to think  
about methods too strictly as functions, not sure.

Any help or leads on how to make sense of this would be very helpful.  
Thank you.

---

<div class="post-metadata">

**Author:** ![mike343](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/58f4c7/32.png) [@mike343](https://www.ruby-forum.com/u/mike343)\
**Post date:** [December 14, 2009, 6:32pm UTC](https://www.ruby-forum.com/t/understanding-methods/180202/2 "2009-12-14T18:32:36Z")

</div>

Michael M. wrote:

> Hi there, I am new to Ruby and to Rails and am having some trouble  
> understanding when a method has been defined or not.
> 
> My current understanding, and I am not sure if I am correct, is that  
> methods are defined only for a class unless inherited by subclasses

Mostly right.

> (designated by the SubClass \< Class in the head of the controller  
> document for that Class).

Controller document? What do you think you mean by that? It’s  
certainly not a standard term, so I’d like to hear your definition…

> So I am wondering why I keep getting a an error of undefined\_method for  
> a method I am defining in my application controller. I have tried  
> bouncing it around into different controllers but I feel like it should  
> either be recognized in the application or in my voting controller.  
> This is what the method looks like:
> 
> def voted\_by\_user?(user)  
> return = false  
> if user  
> self.votes.each { |v|  
> return = true if user.id == v.user\_id  
> }  
> end  
> return  
> end
> 
> I am trying to call this method like this:
> 
> unless @link.voted\_by\_user?(@current\_user)  
> @link.votes.create(:user =\> @current\_user)  
> end

Unless @link is an instance of ApplicationController (most unlikely!),  
this will fail.

> I should mention that I am new to Object Oriented Programming so am  
> still wrapping my mind around inheritance, instance methods and class  
> methods. I have some background in C++ and I may be trying to think  
> about methods too strictly as functions, not sure.

Possibly. Remember that unlike in C++, every Ruby function is a  
method – that is, it belongs to an object. There is no concept of a  
global function as in C++ – even something like puts is actually a  
method of Object.

> Any help or leads on how to make sense of this would be very helpful.  
> Thank you.

Reread the chapters about Ruby’s object model in the Pickaxe Book.  
Repeat till understood. 🙂

Best,  
–Â  
Marnen Laibow-Koser  
[http://www.marnen.org](http://www.marnen.org)  
removed\_email\_address@domain.invalid

---

<div class="post-metadata">

**Author:** ![mike343](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/58f4c7/32.png) [@mike343](https://www.ruby-forum.com/u/mike343)\
**Post date:** [December 14, 2009, 6:56pm UTC](https://www.ruby-forum.com/t/understanding-methods/180202/3 "2009-12-14T18:56:10Z")

</div>

Michael M. wrote:

> class VotesController \< ApplicationController  
> before\_filter :login\_required
> 
> def create  
> @link = Link.find(params[:link\_id])  
> # make sure that the current user can’t vote for their own links  
> if !(@current\_user.id == @link.user.id)  
> #user can only vote once for a link that isn’t their own  
> unless @link.voted\_by\_user?(@current\_user)  
> @link.votes.create(:user =\> @current\_user)  
> end  
> end  
> end

@link is an object of class Link so the method voted\_by\_user? must be  
defined in app/models/link.rb which is where class Link is defined.

Is it there? From what I read your first post it seems you put it into  
some controller, which is very surprising.

Paolo

---

<div class="post-metadata">

**Author:** ![mike343](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/58f4c7/32.png) [@mike343](https://www.ruby-forum.com/u/mike343)\
**Post date:** [December 14, 2009, 6:46pm UTC](https://www.ruby-forum.com/t/understanding-methods/180202/4 "2009-12-14T18:46:41Z")

</div>

Thanks for the reply Marnen.

What I meant by controller document is simply the “controller”.rb file.  
Document was probably the wrong word there and I apologize for the  
confusion.

@link is an instance of the votes\_controller.rb file but even when I  
place the voted\_by\_user? method in this controller, it’s still deemed  
“undefined.”

I think I should pick up your suggested book up as I am having a hard  
time wrapping my mind around the object model.

If you are curious, here’s my votes\_controller.rb contents.

class VotesController \< ApplicationController  
before\_filter :login\_required

def create  
@link = Link.find(params[:link\_id])  
# make sure that the current user can’t vote for their own links  
if !(@current\_user.id == @link.user.id)  
#user can only vote once for a link that isn’t their own  
unless @link.voted\_by\_user?(@current\_user)  
@link.votes.create(:user =\> @current\_user)  
end  
end  
end

Thank you again for your help.

---

<div class="post-metadata">

**Author:** ![mike343](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/58f4c7/32.png) [@mike343](https://www.ruby-forum.com/u/mike343)\
**Post date:** [December 14, 2009, 6:57pm UTC](https://www.ruby-forum.com/t/understanding-methods/180202/5 "2009-12-14T18:57:18Z")

</div>

Michael M. wrote:

> My current understanding, and I am not sure if I am correct, is that  
> methods are defined only for a class unless inherited by subclasses  
> (designated by the SubClass \< Class in the head of the controller  
> document for that Class).

I don’t follow you on this. I’m not sure what you mean by “the head of  
the controller document for that Class.”

Take a look at this little contrived example:

[http://pastie.textmate.org/private/pvwaf4z21dt0fchsvqbw](http://pastie.textmate.org/private/pvwaf4z21dt0fchsvqbw)

This example illustrates that a rectangle and a square share some common  
behavior, but that a square is a more specific example of a rectangle.  
Notice how Square overrides certain behavior inherited from Rectangle.

Also notice how Square overrides the behavior of three methods also  
defined by Rectangle (width=, height=, and area). Technically, there is  
no reason for Square to override Rectangle’s area method, but I did so  
in the example for illustration.

In case you’re wondering Ruby allows you to define setter/getter pairs  
of methods with the shortcut attr\_accessor.

> So I am wondering why I keep getting a an error of undefined\_method for  
> a method I am defining in my application controller. I have tried  
> bouncing it around into different controllers but I feel like it should  
> either be recognized in the application or in my voting controller.
> 
> I am trying to call this method like this:
> 
> unless @link.voted\_by\_user?(@current\_user)  
> @link.votes.create(:user =\> @current\_user)  
> end

You’re saying that voted\_by\_user? is defined in your application  
controller. So unless @link is a reference to an instance of your  
ApplicationController class then “@link.voted\_by\_user?” will throw an  
undefined method exception. The object referenced by @link, or one of  
it’s superclasses must implement the method.

---

<div class="post-metadata">

**Author:** ![mike343](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/58f4c7/32.png) [@mike343](https://www.ruby-forum.com/u/mike343)\
**Post date:** [December 14, 2009, 7:23pm UTC](https://www.ruby-forum.com/t/understanding-methods/180202/6 "2009-12-14T19:23:43Z")

</div>

Michael M. wrote:

> Whoah, this worked!? Now I am a bit confused and I will have to go back  
> and investigate why this works. I thought that the model was  
> responsible for setting relationships between different classes in the  
> database like “belongs\_to” etc. I guess there’s more there than I  
> realized or at least there are some large gaps in my understanding.

It sounds to me like it’s the Model-View-Controller (MVC) design pattern  
that’s confusing you. That’s where you need to start your research. I  
would recommend you read up on MVC in general (not specific to Rails).  
Once you understand that, then go back and look at how Rails implements  
MVC.

---

<div class="post-metadata">

**Author:** ![mike343](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/58f4c7/32.png) [@mike343](https://www.ruby-forum.com/u/mike343)\
**Post date:** [December 14, 2009, 7:12pm UTC](https://www.ruby-forum.com/t/understanding-methods/180202/7 "2009-12-14T19:12:21Z")

</div>

> @link is an object of class Link so the method voted\_by\_user? must be  
> defined in app/models/link.rb which is where class Link is defined.
> 
> Is it there? From what I read your first post it seems you put it into  
> some controller, which is very surprising.
> 
> Paolo

Whoah, this worked!? Now I am a bit confused and I will have to go back  
and investigate why this works. I thought that the model was  
responsible for setting relationships between different classes in the  
database like “belongs\_to” etc. I guess there’s more there than I  
realized or at least there are some large gaps in my understanding.

I have been following the book “Simply Rails 2,” and I think it served  
me well as a large overview but I will need to go into greater depth on  
some of these relationships.

---

<div class="post-metadata">

**Author:** ![mike343](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/58f4c7/32.png) [@mike343](https://www.ruby-forum.com/u/mike343)\
**Post date:** [December 14, 2009, 7:26pm UTC](https://www.ruby-forum.com/t/understanding-methods/180202/8 "2009-12-14T19:26:56Z")

</div>

Robert W. wrote:

> It sounds to me like it’s the Model-View-Controller (MVC) design pattern  
> that’s confusing you. That’s where you need to start your research. I  
> would recommend you read up on MVC in general (not specific to Rails).  
> Once you understand that, then go back and look at how Rails implements  
> MVC.

That’s a good suggestion. Thank you.

---

<div class="post-metadata">

**Author:** ![mike343](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/58f4c7/32.png) [@mike343](https://www.ruby-forum.com/u/mike343)\
**Post date:** [December 14, 2009, 7:52pm UTC](https://www.ruby-forum.com/t/understanding-methods/180202/9 "2009-12-14T19:52:33Z")

</div>

Michael M. wrote:

> Robert W. wrote:
> 
> > It sounds to me like it’s the Model-View-Controller (MVC) design pattern  
> > that’s confusing you. That’s where you need to start your research. I  
> > would recommend you read up on MVC in general (not specific to Rails).  
> > Once you understand that, then go back and look at how Rails implements  
> > MVC.
> 
> That’s a good suggestion. Thank you.

My suggestion would be a bit different. Put aside Rails for a few days,  
and learn about Ruby’s object model itself. Once you understand that,  
look at your Rails code again and understand it in light of what you’ve  
learned.

Bonus tip: in Rails MVC design, Link and LinksController are completely  
separate classes.

Best,  
–Â  
Marnen Laibow-Koser  
[http://www.marnen.org](http://www.marnen.org)  
removed\_email\_address@domain.invalid