Cannot save tag_list when using multi-step forms

I have a belonging model that allows user to post objects to be sold or rented on my website. I recently changed the form, making it a multi-step form: a first form asks the name of the object and if the object is for sale or to rent, a second form ask for object's details, with the fields depending on the user's choice.

I am using is_taggable, with Rails 3.0.5, and my problem is that tag_list is never saved in the database since I switched to the multi-step form (all the other fields are saved correctly).

I followed Ryan Bates Rails cast #217.

Before, I was using: @belonging.tag_list = params[:belonging][:tag_list]

Since I went from multistep, I am using: @belonging.tag_list = session[:belonging_params][:tag_list]

I am a bit of a newbie in Rails, so there might be something obvious I am missing here. I spent the whole afternoon and evening trying to understand what is wrong, any help will therefore be appreciated!

Here are the 'new' and 'create' action of my controller:

class BelongingsController < ApplicationController
   before_filter :authenticate_user!, :except => [:index, :with_tag, :remove_tag]
   after_filter :update_tag_cloud, :only => [:create, :update]
 
   def new
     @title = "Insert a new product or service"
     @user = current_user
     session[:belonging_params] ||= {}
     session[:belonging_step] = nil
     @belonging = @user.belongings.new(session[:belonging_params])
     session[:belonging_params][:tag_list] ||= []
     @belonging.current_step = session[:belonging_step]
     render 'new'
   end
 
   def create
     session[:belonging_params].deep_merge!(params[:belonging]) if params[:belonging]
     @belonging = current_user.belongings.build(session[:belonging_params])
     @belonging.current_step = session[:belonging_step]
     @belonging.tag_list=session[:belonging_params][:tag_list]
     if params[:previous_button]
       @belonging.previous_step
       render 'new'
     elsif params[:cancel_button]
       session[:belonging_step] = session[:belonging_params] = nil
       redirect_to user_path(current_user)
     elsif params[:continue_button]
       if @belonging.last_step?
         if @belonging.save!
           expire_fragment('category_list')
           flash[:success] = "New product or service created!"
           session[:belonging_step] = session[:belonging_params] = nil
           redirect_to belonging_path(@belonging)
         else
           flash[:error] = "Object could not be saved"
           render 'new'
         end
       else
         @belonging.next_step
         render 'new'
       end
     else
       render 'new'
     end
     session[:belonging_step] = @belonging.current_step
   end
 

Many many thanks for any clue !!

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