Optional @import in sass / sass-rails

application.scss

// ...

// -----------------------------------------------------------------------------
// Import JS generated CSS
// -----------------------------------------------------------------------------
@import "OPTIONAL:../javascripts/bundle/jscss/main";
@import "OPTIONAL:../javascripts/bundle/jscss/admin";

sass_optional_import.rb

# config/initializers/sass_optional_import.rb

module OptionalImport
  PREFIX = 'OPTIONAL:'.freeze

  def find_relative(name, base, options)
    if prefix?(name)
      result = super(strip_prefix(name), base, options)

      if result.nil?
        Rails.logger.warn "Could not find optional import #{strip_prefix(name)}"
        return Sass::Engine.new(dummy_sass(name), options)
      end

      return result
    else
      super
    end
  end

  def find(name, options)
    if prefix?(name)
      result = super(strip_prefix(name), base, options)

      if result.nil?
        Rails.logger.warn "Could not find optional import #{strip_prefix(name)}"
        return Sass::Engine.new(dummy_sass(name), options)
      end

      return result
    else
      super
    end
  end

  def mtime(name, options)
    if prefix?(name)
      super(strip_prefix(name), options)
    else
      super
    end
  end

  def key(name, options)
    if prefix?(name)
      super(strip_prefix(name), options)
    else
      super
    end
  end

  def to_s
    "#{self.class}(#{root})"
  end

  def eql?(other)
    other.class == self.class && other.root == root
  end

  protected

  def prefix?(name)
    name.start_with?(PREFIX)
  end

  def strip_prefix(name)
    name.start_with?(PREFIX) ? name[PREFIX.length..-1] : name
  end

  def dummy_sass(name)
    ".dummy-class-#{name.parameterize} { color: #F00F00 }"
  end
end

module Sass
  module Rails
    class SassImporter
      include OptionalImport
    end
  end
end

# Based heavily on:
# https://github.com/chriseppstein/sass-css-importer/blob/master/lib/sass/css_importer/importer.rb