Dynamic checkbox with Ajax in Rails

I have a Form with some checkboxes loaded from the Database, and an option to add other items manually to the items table. This is done by Ajax in the code below...

item_controller.rb

   def manual_item_add
      @manual_item = Item.find(params[:id_item].to_i)     
      respond_to do |format|
        format.js
      end
   end

manual_item_add.js.erb

$("#manual_items_table").append("<%= escape_javascript(render 'manual_item_add', :manual_item => @manual_item) %>")

_manual_item_add.html.erb

...
<td><%= check_box_tag("manual_items[]", item.id, true) %></td>
...

So... the problem here is that though I see the new checkboxes i am adding to the table (they are being created normally), the "manual_items[]" array is not being passed to the controller when I submit the resulting form (by the way, the "manual_items_table" is inside the form definition).

Does anybody know what I am doing wrong? Sorry for the newbie question, I'm starting to work with Ruby + Rails.