Extracting mp3 metadata

MP3 data can be extracted using Paperclip and the ruby-mp3info gem.

Gemfile

gem 'ruby-mp3info', :require => 'mp3info'

Asset.rb

# Table name: assets
#
#  id                  :integer(4)      not null, primary key
#  upload_file_name    :string(255)     not null
#  upload_content_type :string(255)
#  upload_file_size    :integer(4)      not null
#  upload_updated_at   :datetime
#  metadata            :text

has_attached_file :upload
before_save :extract_metadata
serialize :metadata

# Helper method to determine whether or not an attachment is an mp3 file.
# @note Use only if you have a generic asset-type model that can handle different file types.
def audio?
  upload_content_type =~ %r{^audio/(?:mp3|mpeg|mpeg3|mpg|x-mp3|x-mpeg|x-mpeg3|x-mpegaudio|x-mpg)$}
end

# MP3-aware display name, consisting of track title and artist
# Force encoding required to deal with mixed content bugs on Ruby 1.9
# @return 'Title - Artist' if audio file
# @return 'file.ext' if not audio file
def display_name
  @display_name ||= if audio? && metadata?
    artist, title = metadata.values_at('artist', 'title')
    title.present? ? [title, artist].compact.join(' - ').force_encoding('UTF-8') : upload_file_name
  else
    upload_file_name
  end
end

private

# Retrieves metadata for MP3s
def extract_metadata
  return unless audio?
  path = upload.queued_for_write[:original].path
  open_opts = { :encoding => 'utf-8' }
  Mp3Info.open(path, open_opts) do |mp3info|
    self.metadata = mp3info.tag
  end
end

_asset.html.erb

<%= asset.upload.display_name %>