How Can I Implement Conditional CSS Loading with the Rails 3.1 Asset Pipeline?

Using the Rails 3.1 Asset Pipeline for Conditional CSS Loading

When building web applications with Rails, it can be necessary to selectively render CSS files based on specific conditions. This can be achieved using the Rails 3.1 asset pipeline.

By default, the *= require_tree command in application.css includes all CSS files in the assets/stylesheets directory. However, this can lead to unwanted results if you want to render files conditionally.

One workaround is to manually specify each CSS file individually in application.css and utilize a partial to conditionally include the remaining files. However, this method can become cumbersome and inflexible.

A more efficient solution is to utilize multiple manifest files to segregate CSS files. By organizing them into relevant directories and creating separate manifest files for each directory, you can easily include or exclude files based on specific conditions.

Here's an example of how to structure the asset directory:

app/assets/stylesheets
+-- all
    +-- your_base_stylesheet.css
+-- print
    +-- blueprint
        +-- print.css
    +-- your_print_stylesheet.css
+-- ie
    +-- blueprint
        + ie.css
    +-- your_ie_hacks.css
+-- application-all.css
+-- application-print.css
+-- application-ie.css

And the corresponding manifest files:

/**
 * application-all.css
 *
 *= require_self
 *= require_tree ./all
 */

/**
 * application-print.css
 *
 *= require_self
 *= require_tree ./print
 */

/**
 * application-ie.css
 *
 *= require_self
 *= require_tree ./ie
 */

Finally, update the application layout file:

<%= stylesheet_link_tag "application-all", :media => "all" %>
<%= stylesheet_link_tag "application-print", :media => "print" %>

<!--\[if lte IE 8\]-->
    <%= stylesheet_link_tag "application-ie", :media => "all" %>
<!--\[endif\]-->

Don't forget to precompile the new manifest files in config/environments/production.rb:

config.assets.precompile += %w( application-all.css application-print.css application-ie.css )

Note that when using this structure, you need to ensure that any image references in your CSS files are either qualified with the full path, using the SASS helper image-url(), or moved to follow the same directory structure.

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