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This module provides methods for generating asset paths and urls.

# => "/assets/rails.png"

# => "http://www.example.com/assets/rails.png"

Using asset hosts

By default, Rails links to these assets on the current host in the public folder, but you can direct Rails to link to assets from a dedicated asset server by setting ActionController::Base.asset_host in the application configuration, typically in config/environments/production.rb. For example, you'd define assets.example.com to be your asset host this way, inside the configure block of your environment-specific configuration files or config/application.rb:

config.action_controller.asset_host = "assets.example.com"

Helpers take that into account:

# => <img alt="Rails" src="http://assets.example.com/assets/rails.png" />
# => <link href="http://assets.example.com/assets/application.css" media="screen" rel="stylesheet" />

Browsers typically open at most two simultaneous connections to a single host, which means your assets often have to wait for other assets to finish downloading. You can alleviate this by using a %d wildcard in the asset_host. For example, “assets%d.example.com”. If that wildcard is present Rails distributes asset requests among the corresponding four hosts “assets0.example.com”, …, “assets3.example.com”. With this trick browsers will open eight simultaneous connections rather than two.

# => <img alt="Rails" src="http://assets0.example.com/assets/rails.png" />
# => <link href="http://assets2.example.com/assets/application.css" media="screen" rel="stylesheet" />

To do this, you can either setup four actual hosts, or you can use wildcard DNS to CNAME the wildcard to a single asset host. You can read more about setting up your DNS CNAME records from your ISP.

Note: This is purely a browser performance optimization and is not meant for server load balancing. See www.die.net/musings/page_load_time/ for background.

Alternatively, you can exert more control over the asset host by setting asset_host to a proc like this:

ActionController::Base.asset_host = Proc.new { |source|
  "http://assets#{Digest::MD5.hexdigest(source).to_i(16) % 2 + 1}.example.com"
# => <img alt="Rails" src="http://assets1.example.com/assets/rails.png" />
# => <link href="http://assets2.example.com/assets/application.css" media="screen" rel="stylesheet" />

The example above generates “assets1.example.com” and “assets2.example.com”. This option is useful for example if you need fewer/more than four hosts, custom host names, etc.

As you see the proc takes a source parameter. That's a string with the absolute path of the asset, for example “/assets/rails.png”.

 ActionController::Base.asset_host = Proc.new { |source|
   if source.ends_with?('.css')
# => <img alt="Rails" src="http://assets.example.com/assets/rails.png" />
# => <link href="http://stylesheets.example.com/assets/application.css" media="screen" rel="stylesheet" />

Alternatively you may ask for a second parameter request. That one is particularly useful for serving assets from an SSL-protected page. The example proc below disables asset hosting for HTTPS connections, while still sending assets for plain HTTP requests from asset hosts. If you don't have SSL certificates for each of the asset hosts this technique allows you to avoid warnings in the client about mixed media. Note that the request parameter might not be supplied, e.g. when the assets are precompiled via a Rake task. Make sure to use a Proc instead of a lambda, since a Proc allows missing parameters and sets them to nil.

config.action_controller.asset_host = Proc.new { |source, request|
  if request && request.ssl?

You can also implement a custom asset host object that responds to call and takes either one or two parameters just like the proc.

config.action_controller.asset_host = AssetHostingWithMinimumSsl.new(
  "http://asset%d.example.com", "https://asset1.example.com"
URI_REGEXP = %r{^[-a-z]+://|^(?:cid|data):|^//}i
ASSET_EXTENSIONS = { javascript: '.js', stylesheet: '.css' }
ASSET_PUBLIC_DIRECTORIES = { audio: '/audios', font: '/fonts', image: '/images', javascript: '/javascripts', stylesheet: '/stylesheets', video: '/videos' }

Maps asset types to public directory.

Instance Public methods
asset_path(source, options = {})

Computes the path to asset in public directory. If :type options is set, a file extension will be appended and scoped to the corresponding public directory.

All other asset *_path helpers delegate through this method.

asset_path "application.js"                     # => /assets/application.js
asset_path "application", type: :javascript     # => /assets/application.js
asset_path "application", type: :stylesheet     # => /assets/application.css
asset_path "http://www.example.com/js/xmlhr.js" # => http://www.example.com/js/xmlhr.js
Also aliased as: path_to_asset
# File actionview/lib/action_view/helpers/asset_url_helper.rb, line 123
def asset_path(source, options = {})
  source = source.to_s
  return "" unless source.present?
  return source if source =~ URI_REGEXP

  tail, source = source[/([\?#].+)$/], source.sub(/([\?#].+)$/, '')

  if extname = compute_asset_extname(source, options)
    source = "#{source}#{extname}"

  if source[0] != ?/
    source = compute_asset_path(source, options)

  relative_url_root = defined?(config.relative_url_root) && config.relative_url_root
  if relative_url_root
    source = File.join(relative_url_root, source) unless source.starts_with?("#{relative_url_root}/")

  if host = compute_asset_host(source, options)
    source = File.join(host, source)

asset_url(source, options = {})

Computes the full URL to an asset in the public directory. This will use asset_path internally, so most of their behaviors will be the same. If :host options is set, it overwrites global config.action_controller.asset_host setting.

All other options provided are forwarded to asset_path call.

asset_url "application.js"                                 # => http://example.com/assets/application.js
asset_url "application.js", host: "http://cdn.example.com" # => http://cdn.example.com/assets/application.js
Also aliased as: url_to_asset
# File actionview/lib/action_view/helpers/asset_url_helper.rb, line 161
def asset_url(source, options = {})
  path_to_asset(source, options.merge(:protocol => :request))
audio_path(source, options = {})

Computes the path to an audio asset in the public audios directory. Full paths from the document root will be passed through. Used internally by audio_tag to build the audio path.

audio_path("horse")                                            # => /audios/horse
audio_path("horse.wav")                                        # => /audios/horse.wav
audio_path("sounds/horse.wav")                                 # => /audios/sounds/horse.wav
audio_path("/sounds/horse.wav")                                # => /sounds/horse.wav
audio_path("http://www.example.com/sounds/horse.wav")          # => http://www.example.com/sounds/horse.wav
Also aliased as: path_to_audio
# File actionview/lib/action_view/helpers/asset_url_helper.rb, line 333
def audio_path(source, options = {})
  path_to_asset(source, {type: :audio}.merge!(options))
audio_url(source, options = {})

Computes the full URL to an audio asset in the public audios directory. This will use audio_path internally, so most of their behaviors will be the same.

Also aliased as: url_to_audio
# File actionview/lib/action_view/helpers/asset_url_helper.rb, line 340
def audio_url(source, options = {})
  url_to_asset(source, {type: :audio}.merge!(options))
compute_asset_extname(source, options = {})

Compute extname to append to asset path. Returns nil if nothing should be added.

# File actionview/lib/action_view/helpers/asset_url_helper.rb, line 173
def compute_asset_extname(source, options = {})
  return if options[:extname] == false
  extname = options[:extname] || ASSET_EXTENSIONS[options[:type]]
  extname if extname && File.extname(source) != extname
compute_asset_host(source = "", options = {})

Pick an asset host for this source. Returns nil if no host is set, the host if no wildcard is set, the host interpolated with the numbers 0-3 if it contains %d (the number is the source hash mod 4), or the value returned from invoking call on an object responding to call (proc or otherwise).

# File actionview/lib/action_view/helpers/asset_url_helper.rb, line 202
def compute_asset_host(source = "", options = {})
  request = self.request if respond_to?(:request)
  host = options[:host]
  host ||= config.asset_host if defined? config.asset_host

  if host.respond_to?(:call)
    arity = host.respond_to?(:arity) ? host.arity : host.method(:call).arity
    args = [source]
    args << request if request && (arity > 1 || arity < 0)
    host = host.call(*args)
  elsif host =~ /%d/
    host = host % (Zlib.crc32(source) % 4)

  host ||= request.base_url if request && options[:protocol] == :request
  return unless host

  if host =~ URI_REGEXP
    protocol = options[:protocol] || config.default_asset_host_protocol || (request ? :request : :relative)
    case protocol
    when :relative
    when :request
compute_asset_path(source, options = {})

Computes asset path to public directory. Plugins and extensions can override this method to point to custom assets or generate digested paths or query strings.

# File actionview/lib/action_view/helpers/asset_url_helper.rb, line 192
def compute_asset_path(source, options = {})
  dir = ASSET_PUBLIC_DIRECTORIES[options[:type]] || ""
  File.join(dir, source)
font_path(source, options = {})

Computes the path to a font asset. Full paths from the document root will be passed through.

font_path("font")                                           # => /fonts/font
font_path("font.ttf")                                       # => /fonts/font.ttf
font_path("dir/font.ttf")                                   # => /fonts/dir/font.ttf
font_path("/dir/font.ttf")                                  # => /dir/font.ttf
font_path("http://www.example.com/dir/font.ttf")            # => http://www.example.com/dir/font.ttf
Also aliased as: path_to_font
# File actionview/lib/action_view/helpers/asset_url_helper.rb, line 353
def font_path(source, options = {})
  path_to_asset(source, {type: :font}.merge!(options))
font_url(source, options = {})

Computes the full URL to a font asset. This will use font_path internally, so most of their behaviors will be the same.

Also aliased as: url_to_font
# File actionview/lib/action_view/helpers/asset_url_helper.rb, line 360
def font_url(source, options = {})
  url_to_asset(source, {type: :font}.merge!(options))
image_path(source, options = {})

Computes the path to an image asset. Full paths from the document root will be passed through. Used internally by image_tag to build the image path:

image_path("edit")                                         # => "/assets/edit"
image_path("edit.png")                                     # => "/assets/edit.png"
image_path("icons/edit.png")                               # => "/assets/icons/edit.png"
image_path("/icons/edit.png")                              # => "/icons/edit.png"
image_path("http://www.example.com/img/edit.png")          # => "http://www.example.com/img/edit.png"

If you have images as application resources this method may conflict with their named routes. The alias path_to_image is provided to avoid that. Rails uses the alias internally, and plugin authors are encouraged to do so.

Also aliased as: path_to_image
# File actionview/lib/action_view/helpers/asset_url_helper.rb, line 291
def image_path(source, options = {})
  path_to_asset(source, {type: :image}.merge!(options))
image_url(source, options = {})

Computes the full URL to an image asset. This will use image_path internally, so most of their behaviors will be the same.

Also aliased as: url_to_image
# File actionview/lib/action_view/helpers/asset_url_helper.rb, line 298
def image_url(source, options = {})
  url_to_asset(source, {type: :image}.merge!(options))
javascript_path(source, options = {})

Computes the path to a JavaScript asset in the public javascripts directory. If the source filename has no extension, .js will be appended (except for explicit URIs) Full paths from the document root will be passed through. Used internally by javascript_include_tag to build the script path.

javascript_path "xmlhr"                              # => /assets/xmlhr.js
javascript_path "dir/xmlhr.js"                       # => /assets/dir/xmlhr.js
javascript_path "/dir/xmlhr"                         # => /dir/xmlhr.js
javascript_path "http://www.example.com/js/xmlhr"    # => http://www.example.com/js/xmlhr
javascript_path "http://www.example.com/js/xmlhr.js" # => http://www.example.com/js/xmlhr.js
Also aliased as: path_to_javascript
# File actionview/lib/action_view/helpers/asset_url_helper.rb, line 244
def javascript_path(source, options = {})
  path_to_asset(source, {type: :javascript}.merge!(options))
javascript_url(source, options = {})

Computes the full URL to a JavaScript asset in the public javascripts directory. This will use javascript_path internally, so most of their behaviors will be the same.

Also aliased as: url_to_javascript
# File actionview/lib/action_view/helpers/asset_url_helper.rb, line 251
def javascript_url(source, options = {})
  url_to_asset(source, {type: :javascript}.merge!(options))
path_to_asset(source, options = {})
Alias for: asset_path
path_to_audio(source, options = {})
Alias for: audio_path
path_to_font(source, options = {})
Alias for: font_path
path_to_image(source, options = {})
Alias for: image_path
path_to_javascript(source, options = {})
Alias for: javascript_path
path_to_stylesheet(source, options = {})
Alias for: stylesheet_path
path_to_video(source, options = {})
Alias for: video_path
stylesheet_path(source, options = {})

Computes the path to a stylesheet asset in the public stylesheets directory. If the source filename has no extension, .css will be appended (except for explicit URIs). Full paths from the document root will be passed through. Used internally by stylesheet_link_tag to build the stylesheet path.

stylesheet_path "style"                                  # => /assets/style.css
stylesheet_path "dir/style.css"                          # => /assets/dir/style.css
stylesheet_path "/dir/style.css"                         # => /dir/style.css
stylesheet_path "http://www.example.com/css/style"       # => http://www.example.com/css/style
stylesheet_path "http://www.example.com/css/style.css"   # => http://www.example.com/css/style.css
Also aliased as: path_to_stylesheet
# File actionview/lib/action_view/helpers/asset_url_helper.rb, line 266
def stylesheet_path(source, options = {})
  path_to_asset(source, {type: :stylesheet}.merge!(options))
stylesheet_url(source, options = {})

Computes the full URL to a stylesheet asset in the public stylesheets directory. This will use stylesheet_path internally, so most of their behaviors will be the same.

Also aliased as: url_to_stylesheet
# File actionview/lib/action_view/helpers/asset_url_helper.rb, line 273
def stylesheet_url(source, options = {})
  url_to_asset(source, {type: :stylesheet}.merge!(options))
url_to_asset(source, options = {})
Alias for: asset_url
url_to_audio(source, options = {})
Alias for: audio_url
url_to_font(source, options = {})
Alias for: font_url
url_to_image(source, options = {})
Alias for: image_url
url_to_javascript(source, options = {})
Alias for: javascript_url
url_to_stylesheet(source, options = {})
Alias for: stylesheet_url
url_to_video(source, options = {})
Alias for: video_url
video_path(source, options = {})

Computes the path to a video asset in the public videos directory. Full paths from the document root will be passed through. Used internally by video_tag to build the video path.

video_path("hd")                                            # => /videos/hd
video_path("hd.avi")                                        # => /videos/hd.avi
video_path("trailers/hd.avi")                               # => /videos/trailers/hd.avi
video_path("/trailers/hd.avi")                              # => /trailers/hd.avi
video_path("http://www.example.com/vid/hd.avi")             # => http://www.example.com/vid/hd.avi
Also aliased as: path_to_video
# File actionview/lib/action_view/helpers/asset_url_helper.rb, line 312
def video_path(source, options = {})
  path_to_asset(source, {type: :video}.merge!(options))
video_url(source, options = {})

Computes the full URL to a video asset in the public videos directory. This will use video_path internally, so most of their behaviors will be the same.

Also aliased as: url_to_video
# File actionview/lib/action_view/helpers/asset_url_helper.rb, line 319
def video_url(source, options = {})
  url_to_asset(source, {type: :video}.merge!(options))