Model methods and joins...

Hello, this is a repost, something seemed to go wrong last time I
tried to post...

Anyway I got this model:
class Command < ActiveRecord::Base
  has_many :executions

  def with_extension(ext)
    Extension.all(:include => :command,
                       :conditions => { :suffix => ext})
  end
end

And this code in a controller method
      @commands = Command.with_extension('txt')
But I get this error then
undefined method `with_extension' for #<Class:0xb6eaecbc>

I'm obviously doing something wrong, but since I'm not very
experienced with ruby, I don't know what it is...

Another problem I have is that I'm not sure the method code is right.
This is the extensions schema:
  create_table "extensions", :id => false, :force => true do |t|
    t.string "suffix", :null => false
    t.integer "command_id", :null => false
  end

I want to select all commands which is mapped to a specific suffix.