While developing an application with Sebastián that allow users to upload videos with some file name restrictions, meaning that it must contain only A-Z and 0-9 digits, underscores (_) as a valid component as well, and also the name must be preceded by it’s own #id, we came up with the need of applying this custom filter to each uploaded video. After doing some research on paperclip source code and internet tutorials, we suggest the following solution:

class Video < ActiveRecord::Base
  has_attached_file :video,
    :path => ":rails_root/public/system/:attachment/:id/:style/:normalized_video_file_name",
    :url => "/system/:attachment/:id/:style/:normalized_video_file_name"

  Paperclip.interpolates :normalized_video_file_name do |attachment, style|
    attachment.instance.normalized_video_file_name
  end

  def normalized_video_file_name
    "#{self.id}-#{self.video_file_name.gsub( /[^a-zA-Z0-9_\.]/, '_')}"
  end
end

What are we doing here? Easy, in has_attached_file we edit the way paperclip returns the path and url by default, the most relevant components when saving and loading the file in order to display it. Paperclip default values are:

path default => ":rails_root/public/system/:attachment/:id/:style/:filename"
url default => "/system/:attachment/:id/:style/:filename"

Values preceded by ’:’ are the standard interpolations paperclip has. For further information on this visit http://wiki.github.com/thoughtbot/paperclip/interpolations(Interpolations).

What we did was change :filename with :normalized_video_file_name in both path and url, being the second a custom interpolation and then added the normalized_video_file_name method to video.rb.

By doing this we not only achieve a way for paperclip to handle the file by this normalized way, but also have a method to access the normalized file name, plus being able to access the original file name through paperclip video_file_name method.

So remember on video_file_name you have the uploaded filename and on normalized_video_file_name you have the server filename.