drop_receiving_element does not fire

I am trying drag and drop in RoR for the first time.

What I want to achieve, is a list of slots, and a list of entries. You
should be able to drag an entry onto a slot and slot.entry_id will
update to match the dragged entry.entry_id.

I've spent a lot of time looking in textbooks and online, but cannot
get the drop_receiving_element to fire. When I release an item over
the slot, the item simply reverts. If someone could look through the
code I've pasted and gave a hint I would appreciate it.

1. Here is the VIEW for the page:

  <% f.fields_for :slots do |builder| %>
  <% element_to_drop_id = "slot" %>
  <div id="slot" >
    <%= builder.label :slot_number, "Slot Num" %>
    <%= builder.text_field :slot_number %>
    <%= builder.label :entry_id, "Entry Num" %>
    <%= builder.text_field :entry_id %>
  <%= drop_receiving_element("slot",
     :accept => "entry",
    :hoverclass => 'hover',
    :with => "'entry=' + (element.id.split('_').last())",
    :slot_id => :slot_id,
    :url => {:action => :set_entry}
  ) %>

  <% end %>

  <div id="entry_list_to_move">
  <%= render :partial => "entry_list_for_drag" %>

2. Here is the partial entry_list_for_drag

<% @entries.each do |entry| -%>
  <% element_to_drag_id = "entry_#{entry.id}" %>
  <div class="entry" id=<%= element_to_drag_id %> style="cursor:move">
      <%= entry.title %>
      <%= entry.body %>
  <%= draggable_element(element_to_drag_id, :revert => true) %>
<% end %>

3. Entries and slots exist on pages. In the page controller I'm
starting to set up the below but I can't even get this to trigger

  def set_entry
    @slot = Slot.find(params[:slot_id])
    entry_id = params[:entry]
    @slot.entry_id = entry_id

This is now resolved.