Rails3, Datamapper and select

Hi all,
I'm testing rails3 with Datamapper on a simple project.

I've 2 classes :
class Societe
  include DataMapper::Resource

  property :id, Serial
  property :nom, String
  property :adresse, Text

  has n, :clients

class Client
  include DataMapper::Resource

  property :id, Serial
  property :prenom, String
  property :nom, String
  property :telephone, String
  property :courriel, String
  property :adresse, Text
  property :view_only, Boolean

  belongs_to :societe

I created them using rails generate scaffold (except belongs/ has of

I've then have in the folder "view" 5 erb templates per class, and
especially one (_form) used by "new" and "edit" templates.
In order to select the society, I have added in client's _form a

<%= form_for(@client) do |f| %>
  <%= f.error_messages %>
<div class="field">
    <%= f.label :societe %><br />
    <%= f.select(:societe_id, @societes.map{|s| [s.nom, s.id]}) %>
  <div class="actions">
    <%= f.submit %>
<% end %>

In creation mode, everything is allright and the customer is created
with the link to its society
But when I edit, I have the following error:

Showing /home/.../app/views/clients/_form.html.erb where line #30

undefined method `societe_id' for #<Client:0xa51833c>

Extracted source (around line #30):

27: </div>
28: <div class="field">
29: <%= f.label :societe %><br />
30: <%= f.select(:societe_id, @societes.map{|s| [s.nom, s.id]}) %>
31: </div>
32: <div class="actions">
33: <%= f.submit %>

Any idea why it's working with empty "client" and not with existing

Thanks in advance,