gistfile1.txt

_form.html.erb (Product)
 <% f.fields_for :product_images do |builder| %>
      <%= builder.text_field :image %>
    <% end %>

-----------
class Product < ActiveRecord::Base
	has_many :product_images, dependent: :destroy
	accepts_nested_attributes_for :product_images
end

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class ProductImage < ActiveRecord::Base
	has_attached_file :image, :stytles => {:medium => "250x250", :thumb => "140x140"}, :default_url => "/images/:style/missing.png"
	validates_attachment_content_type :image, :content_type => /\Aimage\/.*\Z/
	belongs_to :product
end
-----------
ProductController
def new
    @product = Product.new
    3.times {@product.product_images.build}
  end