Merge remote-tracking branch 'upstream/master'

1 files changed, 356 insertions, 111 deletions

diff --git a/railties/guides/source/asset_pipeline.textile b/railties/guides/source/asset_pipeline.textile
index 51cb332e38..3681501293 100644
--- a/railties/guides/source/asset_pipeline.textile
+++ b/railties/guides/source/asset_pipeline.textile
@@ -1,6 +1,6 @@
 h2. Asset Pipeline
 
-This guide will cover the ideology of the asset pipeline introduced in Rails 3.1.
+This guide covers the asset pipeline introduced in Rails 3.1.
 By referring to this guide you will be able to:
 
 * Understand what the asset pipeline is and what it does
@@ -13,16 +13,22 @@ endprologue.
 
 h3. What is the Asset Pipeline?
 
-The asset pipeline provides a framework to concatenate and minify or compress JavaScript and CSS assets. It also adds the ability to write these assets in other languages such as CoffeeScript, SCSS and ERB.
+The asset pipeline provides a framework to concatenate and minify or compress JavaScript and CSS assets. It also adds the ability to write these assets in other languages such as CoffeeScript, Sass and ERB.
 
-Prior to Rails 3.1 these features were added through third-party Ruby libraries such as Jammit and Sprockets. Rails 3.1 includes the +sprockets-rails+ gem, which depends on the +sprockets+ gem, by default.
+Prior to Rails 3.1 these features were added through third-party Ruby libraries such as Jammit and Sprockets. Rails 3.1 is integrated with Sprockets through Action Pack which depends on the +sprockets+ gem, by default.
 
-By having this as a core feature of Rails, all developers can benefit from the power of having their assets pre-processed, compressed and minified by one central library, Sprockets. This is part of Rails' "Fast by default" strategy as outlined by DHH in his 2011 keynote at Railsconf.
+By having this as a core feature of Rails, all developers can benefit from the power of having their assets pre-processed, compressed and minified by one central library, Sprockets. This is part of Rails' "fast by default" strategy as outlined by DHH in his keynote at RailsConf 2011.
 
-In new Rails 3.1 application the asset pipeline is enable by default. It can be disabled in +application.rb+ by putting this line inside the +Application+ class definition:
+In Rails 3.1, the asset pipeline is enabled by default. It can be disabled in +config/application.rb+ by putting this line inside the application class definition:
 
-<plain>
+<ruby>
 config.assets.enabled = false
+</ruby>
+
+You can also disable it while creating a new application by passing the <tt>--skip-sprockets</tt> option.
+
+<plain>
+rails new appname --skip-sprockets
 </plain>
 
 It is recommended that you use the defaults for all new apps.
@@ -30,29 +36,31 @@ It is recommended that you use the defaults for all new apps.
 
 h4. Main Features
 
-The first feature of the pipeline is to concatenate assets. This is important in a production environment, as it reduces the number of requests that a browser needs to make to render a web page. While Rails already has a feature to concatenate these types of asset--by placing +:cache => true+ at the end of tags such as +javascript_include_tag+ and +stylesheet_link_tag+--, many people do not use it.
+The first feature of the pipeline is to concatenate assets. This is important in a production environment, as it reduces the number of requests that a browser must make to render a web page.
+
+While Rails already has a feature to concatenate these types of assets by placing +:cache => true+ at the end of tags such as +javascript_include_tag+ and +stylesheet_link_tag+, it has a series of limitations. For example, it cannot generate the caches in advance, and it is not able to transparently include assets provided by third-party libraries.
 
-The default behavior in Rails 3.1 and onward is to concatenate all files into one master file each for JS and CSS. However, you can separate files or groups of files if required (see below). In production an MD5 fingerprint is inserted into each filename so that the file is cached by the web browser but can be invalidated if the fingerprint is altered.
+The default behavior in Rails 3.1 and onward is to concatenate all files into one master file each for JS and CSS. However, you can separate files or groups of files if required (see below). In production, an MD5 fingerprint is inserted into each filename so that the file is cached by the web browser but can be invalidated if the fingerprint is altered.
 
-The second feature is to minify or compress. For CSS, this usually involves removing whitespace and comments. For JavaScript, more complex processes can be applied. You can choose from a set of built in options or specify your own.
+The second feature is to minify or compress assets. For CSS, this usually involves removing whitespace and comments. For JavaScript, more complex processes can be applied. You can choose from a set of built in options or specify your own.
 
-The third feature is the ability to code these assets using another language, or language extension. These include SCSS or Sass for CSS, CoffeeScript for JavaScript, and ERB for both.
+The third feature is the ability to code these assets using another language, or language extension. These include Sass for CSS, CoffeeScript for JavaScript, and ERB for both.
 
 h4. What is Fingerprinting and Why Should I Care?
 
-Fingerprinting is a technique where the filenames of content that is static or infrequently updated is altered to be unique to the content contained in the file.
+Fingerprinting is a technique whereby the filenames of content that is static or infrequently updated are altered to be unique to the content contained in the file.
 
-When a filename is unique and based on its content, HTTP headers can be set to encourage caches everywhere (at ISPs, in browsers) to keep their own copy of the content. When the content is updated, the fingerprint will change and the remote clients will request the new file. This is generally known as _cachebusting_.
+When a filename is unique and based on its content, HTTP headers can be set to encourage caches everywhere (at ISPs, in browsers) to keep their own copy of the content. When the content is updated, the fingerprint will change and the remote clients will request the new file. This is generally known as _cache busting_.
 
-The most effective technique is to insert a hash of the content into the name, usually at the end. For example a CSS file +global.css+ is hashed and the filename is updated to incorporate the hash.
+The most effective technique is to insert a hash of the content into the name, usually at the end. For example a CSS file +global.css+ is hashed and the filename is updated to incorporate the digest, for example becoming:
 
 <plain>
-global.css => global-908e25f4bf641868d8683022a5b62f54.css
+global-908e25f4bf641868d8683022a5b62f54.css
 </plain>
 
 This is the strategy adopted by the Rails asset pipeline.
 
-Rails old strategy was to append a query string to every asset linked with a built-in helper. In the source the generated code looked like this:
+Rails' old strategy was to append a query string to every asset linked with a built-in helper. In the source the generated code looked like this:
 
 <plain>
 /stylesheets/global.css?1309495796
@@ -62,8 +70,8 @@ This has several disadvantages:
 
 <ol>
   <li>
-    <strong>Not all caches will cache content with a query string</strong><br>
-    "Steve Souders recommends":http://www.stevesouders.com/blog/2008/08/23/revving-filenames-dont-use-querystring/, "...avoiding a querystring for cacheable resources". He found that in these case 5-20% of requests will not be cached.
+    <strong>Not all caches will cache content with a query string</strong>.<br>
+    "Steve Souders recommends":http://www.stevesouders.com/blog/2008/08/23/revving-filenames-dont-use-querystring/, "...avoiding a querystring for cacheable resources". He found that in this case 5-20% of requests will not be cached. Query strings in particular do not work at all with some CDNs for cache invalidation.
   </li>
   <li>
     <strong>The file name can change between nodes in multi-server environments.</strong><br>
@@ -71,9 +79,11 @@ This has several disadvantages:
   </li>
 </ol>
 
-The other problem is that when static assets are deployed with each new release of code, the mtime of *all* these files changes, forcing all remote clients to fetch them again, even when the content of those assets has not changed.
+The other problem is that when static assets are deployed with each new release of code, the mtime of _all_ these files changes, forcing all remote clients to fetch them again, even when the content of those assets has not changed.
+
+Fingerprinting fixes these problems by avoiding query strings, and by ensuring filenames are consistent based on their content.
 
-Fingerprinting avoids all these problems by ensuring filenames are consistent based on the content.
+Fingerprinting is enabled by default for production and disabled for all the others environments. You can enable or disable it in your configuration through the +config.assets.digest+ option.
 
 More reading:
 
@@ -83,14 +93,18 @@ More reading:
 
 h3. How to Use the Asset Pipeline
 
-In previous versions of Rails, all assets were located in subdirectories of +public+ such as +images+, +javascripts+ and +stylesheets+. With the asset pipeline, the preferred location for these assets is now the +app/assets+ directory. Files in this directory will be served by the Sprockets middleware included in the sprockets gem.
+In previous versions of Rails, all assets were located in subdirectories of +public+ such as +images+, +javascripts+ and +stylesheets+. With the asset pipeline, the preferred location for these assets is now the +app/assets+ directory. Files in this directory are served by the Sprockets middleware included in the sprockets gem.
 
 This is not to say that assets can (or should) no longer be placed in +public+; they still can be and will be served as static files by the application or web server. You would only use +app/assets+ if you wish your files to undergo some pre-processing before they are served.
 
-When a scaffold or controller is generated for the application, Rails will also generate a JavaScript file (or CoffeeScript if the +coffee-script+ gem is in the +Gemfile+) and a Cascading Style Sheet file (or SCSS if +sass-rails+ is in the +Gemfile+) file for that controller.
+In production, the default is to precompile these files to +public/assets+ so that they can be more efficiently delivered by the web server.
+
+When a scaffold or controller is generated for the application, Rails also generates a JavaScript file (or CoffeeScript file if the +coffee-rails+ gem is in the +Gemfile+) and a Cascading Style Sheet file (or SCSS file if +sass-rails+ is in the +Gemfile+) for that controller.
 
 For example, if a +ProjectsController+ is generated, there will be a new file at +app/assets/javascripts/projects.js.coffee+ and another at +app/assets/stylesheets/projects.css.scss+. You should put any JavaScript or CSS unique to a controller inside their respective asset files, as these files can then be loaded just for these controllers with lines such as +<%= javascript_include_tag params[:controller] %>+ or +<%= stylesheet_link_tag params[:controller] %>+.
 
+NOTE: You will need a "ExecJS":https://github.com/sstephenson/execjs#readme supported runtime in order to use CoffeeScript. If you are using Mac OS X or Windows you have a JavaScript runtime installed in your operating system. Check "ExecJS":https://github.com/sstephenson/execjs#readme documentation to know all supported JavaScript runtimes.
+
 h4. Asset Organization
 
 Assets can be placed inside an application in one of three locations: +app/assets+, +lib/assets+ or +vendor/assets+.
@@ -101,13 +115,17 @@ Assets can be placed inside an application in one of three locations: +app/asset
 
 +vendor/assets+ is for assets that are owned by outside entities, such as code for JavaScript plugins.
 
-All subdirectories that exist within these three locations will be added to the search path for Sprockets (visible by calling +Rails.application.config.assets.paths+ in a console). When an asset is requested, these paths will be looked through to see if they contain an asset matching the name specified. Once an asset has been found, it's processed by Sprockets and served.
+All subdirectories that exist within these three locations are added to the search path for Sprockets (visible by calling +Rails.application.config.assets.paths+ in a console). When an asset is requested, these paths are traversed to see if they contain an asset matching the name specified. Once an asset has been found, it's processed by Sprockets and served.
 
-h4. Coding Links to Assets
+You can add additional (fully qualified) paths to the pipeline in +config/application.rb+. For example:
 
-To access assets, we can use the same tags that we are generally familiar with:
+<ruby>
+config.assets.paths << Rails.root.join("app", "assets", "flash")
+</ruby>
+
+h4. Coding Links to Assets
 
-Sprockets does not add any new methods to require your assets, we still use the familiar +javascript_include_tag+ and +stylesheet_link_tag+.
+Sprockets does not add any new methods to access your assets - you still use the familiar +javascript_include_tag+ and +stylesheet_link_tag+.
 
 <erb>
 <%= stylesheet_link_tag "application" %>
@@ -120,65 +138,88 @@ In regular views you can access images in the +assets/images+ directory like thi
 <%= image_tag "rails.png" %>
 </erb>
 
-Images can be organized into directories if required, and they can be accessed by specifying the directory's name in the tag:
+Provided that the pipeline is enabled within your application (and not disabled in the current environment context), this file is served by Sprockets. If a file exists at +public/assets/rails.png+ it is served by the webserver.
+
+Alternatively, a request for a file with an MD5 hash such as +public/assets/rails-af27b6a414e6da00003503148be9b409.png+ is treated the same way. How these hashes are generated is covered in the "In Production":#in-production section later on in this guide.
+
+Sprockets will also look through the paths specified in +config.assets.paths+ which includes the standard application paths and any path added by Rails engines.
+
+Images can also be organized into subdirectories if required, and they can be accessed by specifying the directory's name in the tag:
 
 <erb>
 <%= image_tag "icons/rails.png" %>
 </erb>
 
-Providing that assets are enabled within our application (+config.assets.enabled+ in the current environment's file is not set to +false+), this file will be served by Sprockets unless a file at +public/assets/rails.png+ exists, in which case that file will be served.
-
-Alternatively, a file with an MD5 hash after its name such as +public/assets/rails-af27b6a414e6da00003503148be9b409.png+ will also be picked up by Sprockets. How these hashes are generated is covered in the "Production Assets":#production_assets section later on in this guide.
-
-Otherwise, Sprockets will look through the available paths until it finds a file that matches the name and then will serve it, first looking in the application's assets directories and then falling back to the various engines of the application.
+h5. CSS and ERB
 
-If you want to use a "css data URI":http://en.wikipedia.org/wiki/Data_URI_scheme - a method of embedding the image data directly into the CSS file - you can use the +asset_data_uri+ helper.
+If you add an +erb+ extension to a CSS asset, making it something such as +application.css.erb+, then helpers like +asset_path+ are available in your CSS rules:
 
 <plain>
-#logo { background: url(<%= asset_data_uri 'logo.png' %>)
+.class { background-image: url(<%= asset_path 'image.png' %>) }
 </plain>
 
-This will insert a correctly formatted data URI into the CSS source.
+This writes the path to the particular asset being referenced. In this example, it would make sense to have an image in one of the asset load paths, such as +app/assets/images/image.png+, which would be referenced here. If this image is already available in +public/assets+ as a fingerprinted file, then that path is referenced.
 
-h5. CSS and ERB
-
-If you add an +erb+ extension to a CSS asset, making it something such as +application.css.erb+ then you can use the +asset_path+ helper in your CSS rules:
+If you want to use a "data URI":http://en.wikipedia.org/wiki/Data_URI_scheme -- a method of embedding the image data directly into the CSS file -- you can use the +asset_data_uri+ helper.
 
 <plain>
-.class{background-image:<%= asset_path 'image.png' %>}
+#logo { background: url(<%= asset_data_uri 'logo.png' %>) }
 </plain>
 
-This will write the path to the particular asset being referenced. In this example, it would make sense to have an image in one of the asset load paths, such as +app/assets/images/image.png+, which would be referenced here. If this image is already available in +public/assets+ as a fingerprinted file then that path will be referenced.
+This inserts a correctly-formatted data URI into the CSS source.
 
 Note that the closing tag cannot be of the style +-%>+.
 
-h5. CSS and SCSS
+h5. CSS and Sass
 
-When using the asset pipeline, paths to assets must be re-written and +sass-rails+ provides +_url+ and +_path+ helpers for the following asset classes: image, font, video, audio, javascript, stylesheet.
+When using the asset pipeline, paths to assets must be re-written and +sass-rails+ provides +-url+ and +-path+ helpers (hyphenated in Sass, underscored in Ruby) for the following asset classes: image, font, video, audio, JavaScript and stylesheet.
 
-* +image_url("rails.png")+ becomes +url(/assets/rails.png)+
-* +image_path("rails.png")+ becomes +"/assets/rails.png"+.
+* +image-url("rails.png")+ becomes +url(/assets/rails.png)+
+* +image-path("rails.png")+ becomes +"/assets/rails.png"+.
 
 The more generic form can also be used but the asset path and class must both be specified:
 
-* +asset_url("rails.png", "image")+ becomes +url(/assets/rails.png)+
-* +asset_path("rails.png", "image")+ becomes +"/assets/rails.png"+
+* +asset-url("rails.png", image)+ becomes +url(/assets/rails.png)+
+* +asset-path("rails.png", image)+ becomes +"/assets/rails.png"+
+
+h5. JavaScript/CoffeeScript and ERB
+
+If you add an +erb+ extension to a JavaScript asset, making it something such as +application.js.erb+, then you can use the +asset_path+ helper in your JavaScript code:
+
+<erb>
+$('#logo').attr({
+  src: "<%= asset_path('logo.png') %>"
+});
+</erb>
+
+This writes the path to the particular asset being referenced.
+
+Similarly, you can use the +asset_path+ helper in CoffeeScript files with +erb+ extension (eg. +application.js.coffee.erb+):
+
+<plain>
+$('#logo').attr src: "<%= asset_path('logo.png') %>"
+</plain>
 
 h4. Manifest Files and Directives
 
-Sprockets uses manifest files to determine which assets to include and serve. These manifest files contain _directives_ - instructions that tell Sprockets which files to require in order to build a single CSS or JavaScript file. With these directives, Sprockets will load the files specified, process them if necessary, concatenate them into one single file and then compress them (if +Rails.application.config.assets.compress+ is set to +true+). By serving one file rather than many, a page's load time is greatly reduced as there is not as many requests to make for each file.
+Sprockets uses manifest files to determine which assets to include and serve. These manifest files contain _directives_ -- instructions that tell Sprockets which files to require in order to build a single CSS or JavaScript file. With these directives, Sprockets loads the files specified, processes them if necessary, concatenates them into one single file and then compresses them (if +Rails.application.config.assets.compress+ is true). By serving one file rather than many, the load time of pages are greatly reduced as there are fewer requests to make.
 
 For example, in the default Rails application there's a +app/assets/javascripts/application.js+ file which contains the following lines:
 
 <plain>
+// ...
 //= require jquery
 //= require jquery_ujs
 //= require_tree .
 </plain>
 
-In JavaScript files, directives begin with +//=+. In this case, the following file is using the +require+ directive and the +require_tree+ directive. The +require+ directive tells Sprockets that we would like to require a file called +jquery.js+ that is available somewhere in the search path for Sprockets. By default, this is located inside the +vendor/assets/javascripts+ directory contained within the +jquery-rails+ gem. An identical event takes place for the +jquery_ujs+ require
+In JavaScript files, the directives begin with +//=+. In this case, the file is using the +require+ and the +require_tree+ directives. The +require+ directive is used to tell Sprockets the files that you wish to require. Here, you are requiring the files +jquery.js+ and +jquery_ujs.js+ that are available somewhere in the search path for Sprockets. You need not supply the extensions explicitly. Sprockets assumes you are requiring a +.js+ file when done from within a +.js+ file.
+
+NOTE. In Rails 3.1 the +jquery-rails+ gem provides the +jquery.js+ and +jquery_ujs.js+ files via the asset pipeline. You won't see them in the application tree.
 
-The +require_tree .+ directive tells Sprockets to include _all_ JavaScript files in this directory into the output. Only a path relative to the file can be specified.
+The +require_tree+ directive tells Sprockets to recursively include _all_ JavaScript files in this directory into the output. Only a path relative to the manifest file can be specified. There is also a +require_directory+ directive which includes all JavaScript files only in the directory specified (no nesting).
+
+Directives are processed top to bottom, but the order in which files are included by +require_tree+ is unspecified. You should not rely on any particular order among those. If you need to ensure some particular JavaScript ends up above some other, require it before in the manifest. Note that the family of +require+ directives prevents files from being included twice in the output.
 
 There's also a default +app/assets/stylesheets/application.css+ file which contains these lines:
 
@@ -189,13 +230,15 @@ There's also a default +app/assets/stylesheets/application.css+ file which conta
 */
 </plain>
 
-The directives that work in the JavaScript files will also work in stylesheets, obviously including stylesheets rather than JavaScript files. The +require_tree+ directive here works the same way as the JavaScript one, requiring all stylesheets from the current directory.
+The directives that work in the JavaScript files also work in stylesheets, obviously including stylesheets rather than JavaScript files. The +require_tree+ directive here works the same way as the JavaScript one, requiring all stylesheets from the current directory.
+
+In this example +require_self+ is used. This puts the CSS contained within the file (if any) at the precise location of the +require_self+ call. If +require_self+ is called more than once, only the last call is respected.
 
-In this example +require_self+ is used. This will put the CSS contained within the file (if any) at the top of any other CSS in this file unless +require_self+ is specified after another +require+ directive.
+NOTE. If you want to use multiple Sass files, use the "Sass +@import+ rule":http://sass-lang.com/docs/yardoc/file.SASS_REFERENCE.html#import instead of the Sprockets directives. Using Sprockets directives all Sass files exist within their own scope, making variables or mixins only available within the document they were defined in.
 
 You can have as many manifest files as you need. For example the +admin.css+ and +admin.js+ manifest could contain the JS and CSS files that are used for the admin section of an application.
 
-For some assets (like CSS) the compiled order is important. You can specify individual files and they will be compiled in the order specified:
+The same remarks about ordering made above apply. In particular, you can specify individual files and they are compiled in the order specified:
 
 <plain>
 /* ...
@@ -208,98 +251,159 @@ For some assets (like CSS) the compiled order is important. You can specify indi
 
 h4. Preprocessing
 
-The file extensions used on an asset will determine what preprocessing will be applied. When a controller or a scaffold is generated with the default Rails gemset, a CoffeeScript file and a SCSS file will be generated in place of a regular JavaScript and CSS file. The example used before was a controller called "projects", which generated an +app/assets/javascripts/projects.js.coffee+ and a +app/assets/stylesheets/projects.css.scss+ file.
+The file extensions used on an asset determine what preprocessing is applied. When a controller or a scaffold is generated with the default Rails gemset, a CoffeeScript file and a SCSS file are generated in place of a regular JavaScript and CSS file. The example used before was a controller called "projects", which generated an +app/assets/javascripts/projects.js.coffee+ and an +app/assets/stylesheets/projects.css.scss+ file.
 
-When these files are requested, they will be processed by the processors provided by the +coffee-script+ and +sass-rails+ gems and then sent back to the browser as JavaScript and CSS respectively.
+When these files are requested, they are processed by the processors provided by the +coffee-script+ and +sass-rails+ gems and then sent back to the browser as JavaScript and CSS respectively.
 
-Additional layers of pre-processing can be requested by adding other extensions, where each extension will be processed in a right-to-left manner. These should be used in the order the processing should be applied. For example, a stylesheet called +app/assets/stylesheets/projects.css.scss.erb+ would first be processed as ERB, then SCSS and finally served as CSS. The same applies to a JavaScript file - +app/assets/javascripts/projects.js.coffee.erb+ would be process as ERB, CoffeeScript and served as JavaScript.
+Additional layers of preprocessing can be requested by adding other extensions, where each extension is processed in a right-to-left manner. These should be used in the order the processing should be applied. For example, a stylesheet called +app/assets/stylesheets/projects.css.scss.erb+ is first processed as ERB, then SCSS and finally served as CSS. The same applies to a JavaScript file -- +app/assets/javascripts/projects.js.coffee.erb+ is processed as ERB, CoffeeScript, and served as JavaScript.
 
-Keep in mind that the order of these pre-processors is important. For example, if we called our JavaScript file +app/assets/javascripts/projects.js.erb.coffee+ then it would be processed with the CoffeeScript interpreter first, which wouldn't understand ERB and therefore we would run into problems.
+Keep in mind that the order of these preprocessors is important. For example, if you called your JavaScript file +app/assets/javascripts/projects.js.erb.coffee+ then it would be processed with the CoffeeScript interpreter first, which wouldn't understand ERB and therefore you would run into problems.
 
 h3. In Development
 
-In the development environment assets are compiled and cached on the first request after the server is started. Sprockets sets a +must-validate+ Cache-Control HTTP header to reduce request overhead on subsequent requests - on these the browser gets a 304 (not-modified) response.
-
-If any of the files in the manifest have changed between requests, the server will respond with a new compiled file.
-
-h4. Debugging Assets
+In development mode assets are served as separate files in the order they are specified in the manifest file.
 
-You can put +?debug_assets=true+ or +?debug_assets=1+ at the end of a URL and Sprockets will expand the lines which load the assets. For example, if we had an +app/assets/javascripts/application.js+ file containing these lines:
+This manifest +app/assets/javascripts/application.js+:
 
 <plain>
-//= require "projects"
-//= require "tickets"
+//= require core
+//= require projects
+//= require tickets
 </plain>
 
-By default, this would only render this line when used with +<%= javascript_include_tag "application" %>+ in a view or layout:
+would generate this HTML:
 
 <html>
-<script src='/assets/application.js'></script>
+<script src="/assets/core.js?body=1" type="text/javascript"></script>
+<script src="/assets/projects.js?body=1" type="text/javascript"></script>
+<script src="/assets/tickets.js?body=1" type="text/javascript"></script>
 </html>
 
-When the +debug_assets+ parameter is set, this line will be expanded out into three separate lines, separating out the combined file into their parts.
+The +body+ param is required by Sprockets.
+
+h4. Turning Debugging off
+
+You can turn off debug mode by updating +config/environments/development.rb+ to include:
+
+<ruby>
+config.assets.debug = false
+</ruby>
+
+When debug mode is off Sprockets concatenates and runs the necessary preprocessors on all files. With debug mode turned off the manifest above would generate instead:
 
 <html>
-<script src='/assets/application.js'></script>
-<script src='/assets/projects.js'></script>
-<script src='/assets/tickets.js'></script>
+<script src="/assets/application.js" type="text/javascript"></script>
 </html>
 
-This allows the individual parts of an asset to be rendered and debugged separately.
+Assets are compiled and cached on the first request after the server is started. Sprockets sets a +must-revalidate+ Cache-Control HTTP header to reduce request overhead on subsequent requests -- on these the browser gets a 304 (Not Modified) response.
+
+If any of the files in the manifest have changed between requests, the server responds with a new compiled file.
+
+Debug mode can also be enabled in the Rails helper methods:
+
+<erb>
+<%= stylesheet_link_tag "application", :debug => true %>
+<%= javascript_include_tag "application", :debug => true %>
+</erb>
+
+The +:debug+ option is redundant if debug mode is on.
+
+You could potentially also enable compression in development mode as a sanity check, and disable it on-demand as required for debugging.
 
 h3. In Production
 
-In the production environment, assets are served slightly differently.
+In the production environment Rails uses the fingerprinting scheme outlined above. By default it is assumed that assets have been precompiled and will be served as static assets by your web server.
 
-On the first request the assets are compiled and cached as described above, however the manifest names are altered to include an MD5 hash. Files names typically will look like these:
+During the precompilation phase an MD5 is generated from the contents of the compiled files, and inserted into the filenames as they are written to disc. These fingerprinted names are used by the Rails helpers in place of the manifest name.
 
-<plain>
-/assets/application-908e25f4bf641868d8683022a5b62f54.js
-/assets/application-4dd5b109ee3439da54f5bdfd78a80473.css
-</plain>
+For example this:
 
-The MD5 is generated from the contents of the compiled files, and is included in the HTTP +Content-MD5+ header.
+<erb>
+<%= javascript_include_tag "application" %>
+<%= stylesheet_link_tag "application" %>
+</erb>
 
-Sprockets also sets the +Cache-Control+ HTTP header to +max-age=31536000+. This signals all caches between your server and the client browser that this content (the file served) can be cached for 1 year. The effect of this is to reduce the number of requests for this asset from your server; the asset has a good chance of being in the local browser cache or some intermediate cache.
+generates something like this:
+
+<html>
+<script src="/assets/application-908e25f4bf641868d8683022a5b62f54.js" type="text/javascript"></script>
+<link href="/assets/application-4dd5b109ee3439da54f5bdfd78a80473.css" media="screen" rel="stylesheet" type="text/css" />
+</html>
+
+The fingerprinting behavior is controlled by the setting of +config.assets.digest+ setting in Rails (which is +true+ for production, +false+ for everything else).
 
-This behavior is controlled by the setting of +config.action_controller.perform_caching+ setting in Rails (which is +true+ for production, +false+ for everything else). This value is propagated to Sprockets during initialization for use when action_controller is not available.
+NOTE: Under normal circumstances the default option should not be changed. If there are no digests in the filenames, and far-future headers are set, remote clients will never know to refetch the files when their content changes.
 
 h4. Precompiling Assets
 
-Even though assets are served by Rack::Cache with far-future headers, in high traffic sites this may not be fast enough.
+Rails comes bundled with a rake task to compile the asset manifests and other files in the pipeline to the disk.
 
-Rails comes bundled with a rake task to compile the manifests to files on disc. These are located in the +public/assets+ directory where they will be served by your web server instead of the Rails application.
+Compiled assets are written to the location specified in +config.assets.prefix+. The default setting will use the +public/assets+ directory.
+
+You must use this task either during deployment or locally if you do not have write access to your production filesystem.
 
 The rake task is:
 
 <plain>
-rake assets:precompile
+bundle exec rake assets:precompile
 </plain>
 
-You can run this as part of a Capistrano deployment:
+For faster asset precompiles, you can partially load your application by setting
++config.assets.initialize_on_precompile+ to false in +config/application.rb+, though in that case templates
+cannot see application objects or methods. *Heroku requires this to be false.*
+
+WARNING: If you set +config.assets.initialize_on_precompile+ to false, be sure to
+test +rake assets:precompile+ locally before deploying. It may expose bugs where
+your assets reference application objects or methods, since those are still
+in scope in development mode regardless of the value of this flag.
+
+Capistrano (v2.8.0 and above) has a recipe to handle this in deployment. Add the following line to +Capfile+:
 
 <erb>
-before 'deploy:symlink' do
-  run "cd #{release_path}; RAILS_ENV=#{rails_env} rake assets:precompile"
-end
+load 'deploy/assets'
 </erb>
 
-If you are not precompiling your assets, and you are using the default cache file store (which is the file system), you will need to symlink +rails_root/tmp/cache/assets+ from the shared folder that is part of the Capistrano deployment structure in order to persist the cached file between deployments.
+This links the folder specified in +config.assets.prefix+ to +shared/assets+. If you already use this shared folder you'll need to write your own deployment task.
 
-TODO: Extend above task to allow for this and add task to set it up (See commits 8f0e0b6 and 704ee0df). Note: Capistrano folks are working on a recipe - update this when it available (see https://github.com/capistrano/capistrano/pull/35).
+It is important that this folder is shared between deployments so that remotely cached pages that reference the old compiled assets still work for the life of the cached page.
 
-The default matcher for compiling files will include +application.js+, +application.css+ and all files that do not end in +js+ or +css+:
+NOTE. If you are precompiling your assets locally, you can use +bundle install --without assets+ on the server to avoid installing the assets gems (the gems in the assets group in the Gemfile).
+
+The default matcher for compiling files includes +application.js+, +application.css+ and all files that do not end in +js+ or +css+:
 
 <ruby>
-[ /\w+\.(?!js|css).+/, /application.(css|js)$/ ]
+[ /\w<plus>\.(?!js|css).<plus>/, /application.(css|js)$/ ]
 </ruby>
 
-If you have other manifests or individual stylesheets and JavaScript files to include, you can append them to the +precompile+ array:
+If you have other manifests or individual stylesheets and JavaScript files to include, you can add them to the +precompile+ array:
 
 <erb>
-config.assets.precompile << ['admin.js', 'admin.css', 'swfObject.js']
+config.assets.precompile += ['admin.js', 'admin.css', 'swfObject.js']
 </erb>
 
+The rake task also generates a +manifest.yml+ that contains a list with all your assets and their respective fingerprints. This is used by the Rails helper methods and avoids handing the mapping requests back to Sprockets. A typical manifest file looks like:
+
+<plain>
+---
+rails.png: rails-bd9ad5a560b5a3a7be0808c5cd76a798.png
+jquery-ui.min.js: jquery-ui-7e33882a28fc84ad0e0e47e46cbf901c.min.js
+jquery.min.js: jquery-8a50feed8d29566738ad005e19fe1c2d.min.js
+application.js: application-3fdab497b8fb70d20cfc5495239dfc29.js
+application.css: application-8af74128f904600e41a6e39241464e03.css
+</plain>
+
+The default location for the manifest is the root of the location specified in +config.assets.prefix+ ('/assets' by default).
+
+This can be changed with the +config.assets.manifest+ option. A fully specified path is required:
+
+<erb>
+config.assets.manifest = '/path/to/some/other/location'
+</erb>
+
+NOTE: If there are missing precompiled files in production you will get an <tt>Sprockets::Helpers::RailsHelper::AssetPaths::AssetNotPrecompiledError</tt> exception indicating the name of the missing file(s).
+
+h5. Server Configuration
+
 Precompiled assets exist on the filesystem and are served directly by your webserver. They do not have far-future headers by default, so to get the benefit of fingerprinting you'll have to update your server configuration to add them.
 
 For Apache:
@@ -318,45 +422,102 @@ For Apache:
 </LocationMatch>
 </plain>
 
-TODO: NGINX instructions
+For nginx:
+
+<plain>
+location ~ ^/assets/ {
+  expires 1y;
+  add_header Cache-Control public;
 
-When files are precompiled Sprockets also creates "Gzip":http://en.wikipedia.org/wiki/Gzip (.gz) version of your assets. This avoids the server having to do this for any requests; it can simply read the compressed files from disc. You must configure your server to use gzip compression and serve the compressed assets that will be stored in the public/assets folder. The following configuration options can be used:
+  # Some browsers still send conditional-GET requests if there's a
+  # Last-Modified header or an ETag header even if they haven't
+  # reached the expiry date sent in the Expires header.
+  add_header Last-Modified "";
+  add_header ETag "";
+  break;
+}
+</plain>
 
-TODO: Apache instructions
+When files are precompiled, Sprockets also creates a "gzipped":http://en.wikipedia.org/wiki/Gzip (.gz) version of your assets. Web servers are typically configured to use a moderate compression ratio as a compromise, but since precompilation happens once, Sprockets uses the maximum compression ratio, thus reducing the size of the data transfer to the minimum. On the other hand, web servers can be configured to serve compressed content directly from disk, rather than deflating non-compressed files themselves.
 
+Nginx is able to do this automatically enabling +gzip_static+:
 
-h3. Customizing the Pipeline
+<plain>
+location ~ ^/(assets)/  {
+  root /path/to/public;
+  gzip_static on; # to serve pre-gzipped version
+  expires max;
+  add_header Cache-Control public;
+}
+</plain>
+
+This directive is available if the core module that provides this feature was compiled with the web server. Ubuntu packages, even +nginx-light+ have the module compiled. Otherwise, you may need to perform a manual compilation:
+
+<plain>
+./configure --with-http_gzip_static_module
+</plain>
+
+If you're compiling nginx with Phusion Passenger you'll need to pass that option when prompted.
+
+Unfortunately, a robust configuration for Apache is possible but tricky, please Google around.
+
+h4. Live Compilation
 
+In some circumstances you may wish to use live compilation. In this mode all requests for assets in the pipeline are handled by Sprockets directly.
+
+To enable this option set:
+
+<erb>
+config.assets.compile = true
+</erb>
+
+On the first request the assets are compiled and cached as outlined in development above, and the manifest names used in the helpers are altered to include the MD5 hash.
+
+Sprockets also sets the +Cache-Control+ HTTP header to +max-age=31536000+. This signals all caches between your server and the client browser that this content (the file served) can be cached for 1 year. The effect of this is to reduce the number of requests for this asset from your server; the asset has a good chance of being in the local browser cache or some intermediate cache.
+
+This mode uses more memory, performs poorer than the default and is not recommended.
+
+When deploying a production application to a system without any pre-existing JavaScript runtimes, you may want to add one to your Gemfile:
+
+<plain>
+group :production do
+  gem 'therubyracer'
+end
+</plain>
+
+h3. Customizing the Pipeline
 
 h4. CSS Compression
 
-There is currently one option for compressing CSS - YUI. This Gem extends the CSS syntax and offers minification.
+There is currently one option for compressing CSS, YUI. The "YUI CSS compressor":http://developer.yahoo.com/yui/compressor/css.html provides minification.
 
-The following line will enable YUI compression, and requires the +yui-compressor+ gem.
+The following line enables YUI compression, and requires the +yui-compressor+ gem.
 
 <erb>
 config.assets.css_compressor = :yui
 </erb>
 
-The +config.assets.compress+ must be set to +true+ to enable CSS compression
+The +config.assets.compress+ must be set to +true+ to enable CSS compression.
 
-h4. JavaScript
+h4. JavaScript Compression
 
 Possible options for JavaScript compression are +:closure+, +:uglifier+ and +:yui+. These require the use of the +closure-compiler+, +uglifier+ or +yui-compressor+ gems respectively.
 
-The default Gemfile includes "uglifier":https://github.com/lautis/uglifier. This gem wraps "UglifierJS":https://github.com/mishoo/UglifyJS (written for NodeJS) in Ruby. It compress your code by removing white spaces and other magical things like changing your +if+ and +else+ statements to ternary operators where possible.
+The default Gemfile includes "uglifier":https://github.com/lautis/uglifier. This gem wraps "UglifierJS":https://github.com/mishoo/UglifyJS (written for NodeJS) in Ruby. It compresses your code by removing white space and other magical things like changing your +if+ and +else+ statements to ternary operators where possible.
 
-The following line will invoke uglifier for JavaScript compression.
+The following line invokes +uglifier+ for JavaScript compression.
 
 <erb>
-config.assets.js_compressor  = :uglifier
+config.assets.js_compressor = :uglifier
 </erb>
 
 The +config.assets.compress+ must be set to +true+ to enable JavaScript compression
 
+NOTE: You will need a "ExecJS":https://github.com/sstephenson/execjs#readme supported runtime in order to use +uglifier+. If you are using Mac OS X or Windows you have installed a JavaScript runtime in your operating system. Check "ExecJS":https://github.com/sstephenson/execjs#readme documentation to know all supported JavaScript runtimes.
+
 h4. Using Your Own Compressor
 
-The compressor config settings for CSS and JavaScript will also take any Object. This object must have a +compress+ method that takes a string as the sole argument and it must return a string.
+The compressor config settings for CSS and JavaScript also take any Object. This object must have a +compress+ method that takes a string as the sole argument and it must return a string.
 
 <erb>
 class Transformer
@@ -366,7 +527,7 @@ class Transformer
 end
 </erb>
 
-To enable this pass a +new+ Object to the config option in +application.rb+:
+To enable this, pass a +new+ Object to the config option in +application.rb+:
 
 <erb>
 config.assets.css_compressor = Transformer.new
@@ -387,21 +548,20 @@ This is a handy option if you have any existing project (pre Rails 3.1) that alr
 
 h4. X-Sendfile Headers
 
-The X-Sendfile header is a directive to the server to ignore the response from the application, and instead serve the file specified in the headers. In production Rails (via Sprockets) does not send the asset - just the location and a zero-length response - relying on the web server to do the file serving, which is usually faster. Both Apache and nginx support this option.
+The X-Sendfile header is a directive to the server to ignore the response from the application, and instead serve the file specified in the headers. This option is off by default, but can be enabled if your server supports it. When enabled, this passes responsibility for serving the file to the web server, which is faster.
 
-The configuration is available in <tt>config/environments/production.rb</tt>.
+Apache and nginx support this option which is enabled in <tt>config/environments/production.rb</tt>.
 
 <erb>
-config.action_dispatch.x_sendfile_header = "X-Sendfile" # Use 'X-Accel-Redirect' for nginx
+# config.action_dispatch.x_sendfile_header = "X-Sendfile" # for apache
+# config.action_dispatch.x_sendfile_header = 'X-Accel-Redirect' # for nginx
 </erb>
 
-You should check that your server or hosting service actually supports this, otherwise comment it out.
-
 WARNING: If you are upgrading an existing application and intend to use this option, take care to paste this configuration option only into +production.rb+ (and not +application.rb+) and any other environment you define with production behavior.
 
 h3. How Caching Works
 
-Sprockets uses the default rails cache store to cache assets in dev and production. The only difference is file names are fingerprinted and get far-future headers in production.
+Sprockets uses the default Rails cache store to cache assets in development and production.
 
 TODO: Add more about changing the default store.
 
@@ -414,3 +574,88 @@ A good example of this is the +jquery-rails+ gem which comes with Rails as the s
 h3. Making Your Library or Gem a Pre-Processor
 
 TODO: Registering gems on "Tilt":https://github.com/rtomayko/tilt enabling Sprockets to find them.
+
+h3. Upgrading from Old Versions of Rails
+
+There are two issues when upgrading. The first is moving the files to the new locations. See the section above for guidance on the correct locations for different file types.
+
+The second is updating the various environment files with the correct default options. The following changes reflect the defaults in version 3.1.0.
+
+In +application.rb+:
+
+<erb>
+# Enable the asset pipeline
+config.assets.enabled = true
+
+# Version of your assets, change this if you want to expire all your assets
+config.assets.version = '1.0'
+
+# Change the path that assets are served from
+# config.assets.prefix = "/assets"
+</erb>
+
+In +development.rb+:
+
+<erb>
+# Do not compress assets
+config.assets.compress = false
+
+# Expands the lines which load the assets
+config.assets.debug = true
+</erb>
+
+And in +production.rb+:
+
+<erb>
+# Compress JavaScripts and CSS
+config.assets.compress = true
+
+# Choose the compressors to use
+# config.assets.js_compressor  = :uglifier
+# config.assets.css_compressor = :yui
+
+# Don't fallback to assets pipeline if a precompiled asset is missed
+config.assets.compile = false
+
+# Generate digests for assets URLs.
+config.assets.digest = true
+
+# Defaults to Rails.root.join("public/assets")
+# config.assets.manifest = YOUR_PATH
+
+# Precompile additional assets (application.js, application.css, and all non-JS/CSS are already added)
+# config.assets.precompile += %w( search.js )
+</erb>
+
+There are no changes to +test.rb+. The defaults in the test environment are: +config.assets.compile+ is true and +config.assets.compress+, +config.assets.debug+ and +config.assets.digest+ are false.
+
+The following should also be added to +Gemfile+:
+
+<plain>
+# Gems used only for assets and not required
+# in production environments by default.
+group :assets do
+  gem 'sass-rails',   "~> 3.1.0"
+  gem 'coffee-rails', "~> 3.1.0"
+  gem 'uglifier'
+end
+</plain>
+
+If you use the +assets+ group with Bundler, please make sure that your +config/application.rb+ has the following Bundler require statement.
+
+<ruby>
+if defined?(Bundler)
+  # If you precompile assets before deploying to production, use this line
+  Bundler.require *Rails.groups(:assets => %w(development test))
+  # If you want your assets lazily compiled in production, use this line
+  # Bundler.require(:default, :assets, Rails.env)
+end
+</ruby>
+
+Instead of the old Rails 3.0 one
+
+<ruby>
+# If you have a Gemfile, require the gems listed there, including any gems
+# you've limited to :test, :development, or :production.
+Bundler.require(:default, Rails.env) if defined?(Bundler)
+</ruby>