双重嵌套形式提交多个记录时出现未知属性错误

class Climb < ActiveRecord::Base
  has_one :route_ascent
  accepts_nested_attributes_for :route_ascent
end

class RouteAscent < ActiveRecord::Base
  has_many :ascent_steps, :class_name => \'RouteStep\', :as => :steppable
  accepts_nested_attributes_for :ascent_steps, :allow_destroy => true
end

class RouteStep < ActiveRecord::Base
  belongs_to :steppable, :polymorphic => true
end

f.fields_for :route_ascent

<%= f.fields_for :ascent_steps %>
<p><%= f.link_to_add \"Add A Step\", :ascent_steps %></p>

<div class=\"field\">
<%= f.label :order %>
<%= f.text_field :position %><br>
<%= f.label :description %>
<%= f.text_area :description %>
<%= f.link_to_remove \"Remove Step\" %>
</div>

\"route_ascent_attributes\"=>{\"ascent_steps_attributes\"=>{\"0\"=>{\"position\"=>\"1\",
 \"description\"=>\"this will also work\",
 \"_destroy\"=>\"false\",
 \"id\"=>\"66\"}},
 \"0\"=>{\"new_1307386880995\"=>{\"position\"=>\"2\",
 \"description\"=>\"broken!\",
 \"_destroy\"=>\"false\"}},
 \"id\"=>\"4\"},