I got this error in my Rails app after updating to Ruby 1.8.7
patchlevel 63. After reverting to 1.8.6 patchlevel 279, all is well
again.

NoMethodError in PhotosController#random

undefined method `[]' for #<Enumerable::Enumerator:0x24abaa8>

/opt/local/lib/ruby/gems/1.8/gems/activesupport-2.0.2/lib/
active_support/core_ext/string/access.rb:43:in `first'
vendor/plugins/attachment_fu/lib/technoweenie/attachment_fu.rb:70:in
`has_attachment'
app/models/thumbnail.rb:5
app/models/photo.rb:15
app/controllers/photos_controller.rb:23:in `random'

Hello, I had the same problem after upgrading ruby. In ruby 1.8.7 some
methods of the String class were removed, so you have to modify the
attachment_fu.rb file. Navigate to line 70 of the file vendor/plugins/
attachment_fu/lib/technoweenie/attachment_fu.rb and change the the
line

attachment_options[:path_prefix]   = attachment_options[:path_prefix]
[1..-1] if options[:path_prefix].first == '/'

to

attachment_options[:path_prefix]   = attachment_options[:path_prefix]
[1..-1] if options[:path_prefix].starts_with('/')

Now, my application works after this simple modification. Hope this
helps you too.