Q:

rails 3, paperclip (& formtastic) - deleting image attachments

I can't seem to find an example that is complete in all the components. I am having a hard time deleting image attachments

Classes
  class Product
    has_many :product_images, :dependent => :destroy
    accepts_nested_attributes_for :product_images
  end

  class ProductImage
    belongs_to :product
    has_attached_file :image #(etc)
  end

View
  <%= semantic_form_for [:admin, @product], :html => {:multipart => true} do |f| %>
    <%= f.inputs "Images" do %>
      <%= f.semantic_fields_for :product_images do |product_image| %>
        <% unless product_image.object.new_record? %>
          <%= product_image.input :_destroy, :as => :boolean, 
             :label => image_tag(product_image.object.image.url(:thumb)) %>
        <% else %>
          <%= product_image.input :image, :as => :file, :name => "Add Image" %>
        <% end %>
      <% end %>
    <% end %>
  <% end %>

Controller
  class Admin::ProductsController < AdminsController
   def edit
     @product = Product.find_by_permalink(params[:id])
     3.times {@product.product_images.build} # added this to create add slots
   end

   def update
      @product = Product.find_by_permalink(params[:id])

      if @product.update_attributes(params[:product])
        flash[:notice] = "Successfully updated product."
        redirect_to [:admin, @product]
      else
        flash[:error] = @product.errors.full_messages
        render :action => 'edit'
      end
    end
  end

Looks good, but, literally nothing happens when I check the checkbox.
In the request I see:
      "product"=>{"manufacturer_id"=>"2", "size"=>"", "cost"=>"5995.0", 
         "product_images_attributes"=>{"0"=>{"id"=>"2", "_destroy"=>"1"}}

But nothing gets updated and the product image is not saved. 
Am I missing something fundamental about how 'accepts_nested_attributes_for' works?

A:

From the API docs for ActiveRecord::NestedAttributes::ClassMethods

:allow_destroy

If true, destroys any members from the attributes hash with a _destroy key and a value that evaluates to true (eg. 1, ‘1’, true, or ‘true’). This option is off by default.

So:
accepts_nested_attributes_for :product_images, allow_destroy: true

