how to create an association through a Drag & Drop

I have 3 tables A, B and C

Table A has_many Cs
Table B has_many Cs
so C contains 2 foreign keys, A_id and B_id)

I have a screen with 2 lists : A list and B list
I would like to be able to create a C record by simply dropping an A
element on a B element

I tried this :

<ul id="A_list">
  <% for A in @As %>
  <% draggableA_id = "draggableA_#{}" %>
  <li id=<%= draggableA_id %> class="draggableAs">
     <% %>
        <%= draggable_element(draggableA_id, :revert=>true) %>
  <% end %>

<ul id="B_list">
  <% for B in @Bs %>
       <% droppableB_id = "droppableB_#{}" %>
       <li id=<%= droppableB_id %> class="droppableBs">
           <%= %>
      <%= drop_receiving_element(droppableB_id,
      :accept => "draggableAs",
      :with => "'draggableA=' + ('_').last())",
      :url => {:action=>:create_C}
  <% end %>

It drops OK, but :
1) The problem is, the ":with" parameter takes only one key. How do I
convey the second one ? It doesn't seem to accept a hash...
2) When I debug, I see that the draggableA_id that I drop is always
the last draggableA_id, whichever draggableA I drag...

There's some examples of the usage of :with here:

(it's mostly about link_to_remote but that doesn't matter)