Variable scope in rails controller

Hi,

I need to access one variable in entire controller i.e it is accessible
to all actions in controller.

I had tried with session and class variable but for example

def create
document_id = get_document
@@document_id = session[:document_id] = document_id
end

def validate
if document_id == (@@document_id || session[:document_id])
true
else
false
end
end

it works some time , some time uninitialized constant error for class
variable and session token is null when I refresh the browser.

I know session variable stored in server side but when I request
multiple times it returns null value.

i am also using before filter for that task, but no use.

please suggest me.