Multi-level hierarchy selections

Im not sure what the best subject line for this is… I am trying to
understand more about what parts of Rails have access to what

I have one type (Interest) organized into a self-referential hierarchy.
A user can select any leaf node in that hierarchy. This is working

I want to present the user with all of the levels in the hierarchy,
however, despite recording only the leaf node ID. For an existing
selection, he should see multiple drop-downs, one for each level of the
hierarchy. For a new selection, he should see only the first level,
with a new select widget created when he has selected a non-leaf node in
the previous one.

Pointers to recipes or resources would be most welcome. I only need
this to work in the editing interface (for which I am using formtastic);
the show view can just display the leaf node.

Below are some relevant code snippets; the comment in
_user_interest_fields.html.erb is where I want the change to manifest.

I think that I need (a) to create some pseudo-attributes for the user or
interest to give the hierarchy, so that the view can act on them, but
Im not sure how to do this for arbitrary depth; and (b) to provide some
JavaScript so that the view knows how to dynamically create new select
lists for each hierarchy level. However, Ive been thrashing around
trying to figure out where to start, and would really love some

Thanks in advance,

=-=-=-= app/models/interest.rb =-=-=-=

class Interest < ActiveRecord::Base
attr_accessible :name

validates_presence_of :name
validates_uniqueness_of :name

has_many :parent_relationships,
:class_name => “InterestParentage”,
:foreign_key => :child_id,
:dependent => :destroy
has_many :parents,
:through => :parent_relationships,
:source => :parent

has_many :child_relationships,
:class_name => “InterestParentage”,
:foreign_key => :parent_id,
:dependent => :destroy
has_many :children,
:through => :child_relationships,
:source => :child

=-=-=-= app/models/user_interest.rb =-=-=-=

class UserInterest < ActiveRecord::Base
attr_accessible :user_id, :interest_id

validates_presence_of :user_id, :interest_id
validates_uniqueness_of :interest_id, :scope => :user_id

belongs_to :user
belongs_to :interest

=-=-=-= app/controllers/user_controller.rb =-=-=-=

GET /users/1/edit

def edit
@user = User.where(:user_id => current_user)

=-=-=-= app/views/users/_form.html.erb =-=-=-=

<%= semantic_form_for( @user ) do |f| %>

Interests <%= f.semantic_fields_for :user_interests do |builder| %> <%= render( 'user_interest_fields', f: builder ) %> <% end %>

<%= link_to_add_fields( "Add Interest", f, :user_interests ) %>

<%= f.submit %>
<% end %>

=-=-=-= app/views/users/_user_interest_fields.html.erb =-=-=-=

    <%= f.input( :interest, :as => :select, :collection => Interest.all.collect { |s| [,] }, :label => true ) %> <%= f.hidden_field :_destroy %> <%= link_to_remove_fields( "remove", :user_interests ) %>

=-=-=-= end =-=-=-=