Rails, Ransack, HABTM - Empty result when multiple select - ruby-on-rails

Question

I have a form with :
<%= f.collection_select :carburants_id_eq_all, Carburant.order(:name), :id, :name, {}, { multiple: true } %>
When I select only 1 Carburant, it's render me the result normaly, but if I select more than 1 Carburant, the result is empty.
Here's my form in 'index.html.erb' :
<%= search_form_for #search do |f| %>
<%= f.label :title_cont, "Name contains" %>
<%= f.text_field :title_cont %>
<%= f.collection_select :carburants_id_eq_all, Carburant.order(:name), :id, :name, {}, { multiple: true } %>
<%= f.submit "Search" %>
<% end %>
My index action in my 'post_controller.rb':
def index
#search = Post.search(params[:q])
#posts = #search.result(distinct: true)
end
My post.rb, carburant.rb and category.rb model :
class Category < ActiveRecord::Base
attr_accessible :carburant_id, :post_id
belongs_to :carburant
belongs_to :post
end
class Post < ActiveRecord::Base
attr_accessible :content, :title, :carburant_ids
has_many :categories
has_many :carburants, through: :categories
end
class Carburant < ActiveRecord::Base
attr_accessible :name
has_many :categories
has_many :posts, through: :categories
end
My DB shema :
ActiveRecord::Schema.define(:version => 20130114004613) do
create_table "carburants", :force => true do |t|
t.string "name", :limit => nil
t.datetime "created_at"
t.datetime "updated_at"
end
create_table "categories", :force => true do |t|
t.integer "post_id"
t.integer "carburant_id"
t.datetime "created_at", :null => false
t.datetime "updated_at", :null => false
end
create_table "posts", :force => true do |t|
t.string "title"
t.text "content"
t.datetime "created_at"
t.datetime "updated_at"
end
create_table "recherches", :force => true do |t|
t.string "title"
t.integer "carburant_id"
t.datetime "created_at", :null => false
t.datetime "updated_at", :null => false
end
end
Thank you by advance.
EDIT 1:
I tried replacing :carburants_id_eq_all by :carburants_id_eq_any in my form just to test, and it work perfecly with the EQ_ANY predicate, but still not with the EQ_ALL.