gistfile1.rb

saved = 0
not_saved = 0

Product.all.to_a.select do |p|
  p.image && p.image.url
end.each do |p|
  begin
    temp = Tempfile.new("foo")
    temp.puts \
      Mongo::GridFileSystem.new(Mongoid.database).
        open(p.image.url.sub("/images/", ""), "r").
        read.
        force_encoding("UTF-8")

    p.image.remove!
    p.image = temp
    p.save

    saved += 1
  rescue Exception => e
    not_saved += 1
    pp e.message
    pp e.class
  end
end

puts "salvos: #{saved}\nnao salvos #{not_saved}"