Prawn 0.11.1 doesn't work with rails 3.0.9

I have a Rails 3 app, and I'm using Prawn to generate PDF files.

I have defined this in my controller:

Class DocumentosController < ApplicationController
    def show
        @documento = Documento.find(params[:id])
        prawnto :prawn => {
           :left_margin => 20,
           :right_margin => 20
        }
    end
end

In my view I have this:

<p><%= link_to "Download", documento_path(@docmento, :format => "pdf") %></p>

In application.rb I have this:

config.gem "prawn"

And I get:

NoMethodError in DocumentosController#show
undefined method 'prawnto' for #<DocumentosController:0xb6e99358>

I try to reinstall the gem, but the problem continuem, and if I try /documentos/6.pdf e.g. the gem generate the PDF but there is an error with the file.

Any idea, which i开发者_如何学Cs the problem?

Thanks for your help.

Rails 3 uses bundler in order to require gems. So you have to add prawn to your Gemfile and then run bundle install