How to update groupings via Mailchimp API? - ruby-on-rails

Question

I'm trying to subscribe and update members of a Mailchimp list from my Rails 3.2 app via the API.
Everything is working fine, except that I am unable to set interest groups.
What is the correct format for setting groupings.
I've tried
merge_vars: 'GROUPINGS' => [
[
'id' => group_id,
'groups' => ['array', 'of', 'groups']
]
]
and
merge_vars: 'GROUPINGS' => [
[
'id' => group_id,
'groups' => "comma,separated,groups"
]
]
and
merge_vars: 'GROUPINGS' => [
0 => [
'id' => group_id,
'groups' => "comma,separated,groups"
]
]
and several other variations.
Nothing seems to work and the docs are unclear

Answer 1

Took me some time but I finnaly figured out the correct format :
#gb.lists.subscribe({:id => list_id,
:email => {:email => self.email},
:merge_vars => {:FNAME => self.firstname,
:groupings => [{:id => grouping_id,
:groups => ["name","of","groups"]
}]
},
:double_optin => false,
:send_welcome => false,
:update_existing => true})
I'm using rails 3.1.11 and gibbon 0.5.0
Hope this will help !