Is there a way to reference a Rails ActiveRecord object before it is saved?

I am trying to update a div on a form based on user input (using jQuery). Works fine on edit because object.id exists, so I can pass it to the controller action for the jQuery update. In new there is no object.id yet, so how can I find the correct object in my controller action? CONTROLLER...

  def update_category_select
    @organization = Organization.find(params[:id])
    categories = Category.where(:type => params[:master_type]).order(:name)
      render :partial => "categories", :locals => {:collection => categories, :organization => @organization}
  end

APPLICATION.JS

$(document).ready(function() { 
  $('#master_t开发者_如何学Cype').change(function () {
    var master_type = $('select#master_type :selected').val();
    var id = $('#organization_id').val();
    jQuery.get('/organizations/update_category_select/'+ id + '?master_type='+ master_type, function(data){
      $("#categories").html(data);
    })
    return false;
  });
});

Thanks in advance for any suggestions!

Just in case anyone else is looking. Ryan Bates shared this with me today via Twitter...

"@patrick_connor Basically you can't. You have to save it first or pass all new attributes around."