Forum: Ruby on Rails newb confusion: instance variable accessible from outside class, but nil from inside instance metho

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16516a73442e55fbfa3174e5c9e473d0?d=identicon&s=25 Brian (Guest)
on 2008-11-19 17:49
(Received via mailing list)
I've got a simple class, defined by the following table in a
migration:

create_table :buzzusers do |t|
   t.integer :user_id
   t.string  :name
   t.integer :level, :default => 0

   t.timestamps
end

I usually create it with the following

Buzzuser.create(:name => "Bob")

which works fine.  It appears to save and load fine.  I've written the
following method within the Buzzuser class model:

def promote
  @level += 1
end

This blows up with a nil error when called.  More precisely, I get:

NoMethodError: You have a nil object when you didn't expect it!
You might have expected an instance of Array.
The error occurred while evaluating nil.+

However, if I'm outside the class, I can access the level variable
just fine.  If I do

bu = Buzzuser.find(1)

I can then do bu.level and it returns the correct value for level.

What's wrong?  I'm assuming I'm missing something basic, but it's
pretty confusing.

Thanks
81b61875e41eaa58887543635d556fca?d=identicon&s=25 Frederick Cheung (Guest)
on 2008-11-19 17:56
(Received via mailing list)
On 19 Nov 2008, at 16:48, Brian wrote:
> What's wrong?  I'm assuming I'm missing something basic, but it's
> pretty confusing.
>
attributes aren't instance variables

Fred
16516a73442e55fbfa3174e5c9e473d0?d=identicon&s=25 Brian (Guest)
on 2008-11-19 19:20
(Received via mailing list)
thanks for the response, but can anyone provide a simple example of
the proper way to access and change an attribute from within an
instance method?

I've worked through AWDwR, re-read the ActiveRecord and Ruby sections,
am currently going through wiki.rubyonrails.org and the Reference
Docs, and still haven't found it.

On Nov 19, 11:55 am, Frederick Cheung <frederick.che...@gmail.com>
81b61875e41eaa58887543635d556fca?d=identicon&s=25 Frederick Cheung (Guest)
on 2008-11-19 20:06
(Received via mailing list)
On Nov 19, 6:19 pm, Brian <brifo...@gmail.com> wrote:
> thanks for the response, but can anyone provide a simple example of
> the proper way to access and change an attribute from within an
> instance method?
>

You just need to call the accessor methods, same as for people outside
the class. the only subtlety is that to set it you need to do
self.attribute_name = value (as attribute_name = value would be
interpreted as you trying to set a local variable)

Fred
16516a73442e55fbfa3174e5c9e473d0?d=identicon&s=25 Brian (Guest)
on 2008-11-19 20:49
(Received via mailing list)
ok thanks that got it working.  I think I had this working before like
that, but then changed it because of confusion over the usage of
self.  I thought self was used to define class-level methods.
Apparently the usage is different in instance methods?  It is partly
addressed here

http://wiki.rubyonrails.org/rails/pages/RubyHeadScratching

On Nov 19, 1:56 pm, Frederick Cheung <frederick.che...@gmail.com>
81b61875e41eaa58887543635d556fca?d=identicon&s=25 Frederick Cheung (Guest)
on 2008-11-19 21:06
(Received via mailing list)
On Nov 19, 7:49 pm, Brian <brifo...@gmail.com> wrote:
> ok thanks that got it working.  I think I had this working before like
> that, but then changed it because of confusion over the usage of
> self.  I thought self was used to define class-level methods.
> Apparently the usage is different in instance methods?  It is partly
> addressed here

Self is simple when you get it right. It just means 'the current
object'. The only reason it is relevant here is that when ruby sees
self.foo = bar it can be damn sure you want to call the method foo= on
the current object, whereas with foo= bar you might just want to
create a local variable.

in the same way
def self.foo
end

creates a class method (in the context of a class) because it says
"define a method on the object self" (as opposed to inside the
appropriate class), given that at this point the self object is the
class, this creates a method on that class object.

so for example

class Foo
  def self.bar
    puts "hi"
  end
end

is the same as
class Foo
  def Foo.bar
    puts "hi"
  end
end

is the same (from the point of view of method definition - constant
scopes and so on would be different) as

class Foo
end

def Foo.bar
    puts "hi"
end

In the first example self is at that point Foo, which is why it works.
It's more convenient that typing Foo.bar partly because class names
are usually longer, partly because it stands out more and partly
because if you were to rename the Foo class you can leave all of your
def self.bar alone whereas with the second example you would have to
change def Foo.bar to def NewClassName.bar

Fred
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