habtm many-to-many relationship problem attribute_methods.rb

Hi,

I have a many-to-many relationship between: registrations and groups
(properly setup on each side with has_and_belongs_to_many)

so I do this in the console:

reg = Registration.find(:first)
grp = Group.find(:first)

grp.registrations list me the registrations' attributes linked with
the group in question

reg.groups returns me:

NoMethodError: undefined method `groups' for #<Registration:0x3164718>
        from /usr/local/lib/ruby/gems/1.8/gems/activerecord-2.1.0/lib/
active_record/attribute_methods.rb:256:in `method_missing'
        from (irb):XX

where XX started around 10 I think and kept augmenting each time I ran
"reg.groups"

I can't figure out what is happening to get this message...
registrations and groups are identically setup to what I can see...

thanks if anyone can bring me any clue!

PS. I'm using restful_authentication and restful_acl, but it shouldn't
give any problem here...

Actually, some model names are in french so I translated, so it could
make more sens ... but registration is inscription and groups are
periodes in the above example...

some code in the models are temporary, just so that I can debug more
easily.
Here are models and migrations