undefined method `scoped' for Model:Class with Formtastic and MongoMapper - ruby-on-rails-3

Question

I'm having a hard time making MongoMapper & Formtastic work together.
In an effort to isolate the problem, I've created a dummy application with the bare minimum of code.
I have 2 main models (Cabinet & Device), with "many/belongs_to" relationship between them and a scaffolded controller for the Cabinet ressource.
When I load a very basic formtastic form (see below) with anything other than a simple "key" field declaration, I get a "NoMethodError" exception in the controller, with the following error "undefined method `scoped' for Device:Class" refering to the line #4 of the _form.html.haml partial.
Any idea on this one? Didn't find anything useful through Google or StackOverflow...
app/models/cabinet.rb
class Cabinet
include MongoMapper::Document
safe
key :label, String, :required => true
many :devices
end
app/models/device.rb
class Device
include MongoMapper::Document
safe
key :label, String, :required => true
belongs_to :cabinet
end
app/views/cabinet/_form.html.haml
= semantic_form_for #cabinet do |f|
= f.inputs do
= f.input :label
= f.input :devices
= f.actions
I did't include the scaffolded Controller, as I didn't touched the scaffolded code in any way.
Gem versions used :
Gems included by the bundle:
* actionmailer (3.2.7)
* actionpack (3.2.7)
* activemodel (3.2.7)
* activerecord (3.2.7)
* activeresource (3.2.7)
* activesupport (3.2.7)
* arel (3.0.2)
* bson (1.6.4)
* bson_ext (1.6.4)
* builder (3.0.0)
* bundler (1.1.5)
* coffee-rails (3.2.2)
* coffee-script (2.2.0)
* coffee-script-source (1.3.3)
* erubis (2.7.0)
* execjs (1.4.0)
* formtastic (2.2.1)
* haml (3.1.6)
* haml-rails (0.3.4)
* hike (1.2.1)
* hpricot (0.8.6)
* i18n (0.6.0)
* journey (1.0.4)
* jquery-rails (2.0.2)
* json (1.7.4)
* libv8 (3.3.10.4)
* mail (2.4.4)
* mime-types (1.19)
* mongo (1.6.4)
* mongo_mapper (0.11.2)
* multi_json (1.3.6)
* plucky (0.5.1)
* polyglot (0.3.3)
* rack (1.4.1)
* rack-cache (1.2)
* rack-ssl (1.3.2)
* rack-test (0.6.1)
* rails (3.2.7)
* railties (3.2.7)
* rake (0.9.2.2)
* rdoc (3.12)
* ruby_parser (2.3.1)
* sass (3.1.20)
* sass-rails (3.2.5)
* sexp_processor (3.2.0)
* sprockets (2.1.3)
* therubyracer (0.10.1)
* thor (0.15.4)
* tilt (1.3.3)
* treetop (1.4.10)
* tzinfo (0.3.33)
* uglifier (1.2.7)

Answer 1

mongomapper doesn't have the "scoped" class method. It's a feature of ActiveRecord and Mongoid.