Inline CSS or JS in Rails

application.rb

config.assets.precompile += [

  # precompile any CSS or JS file that doesn't start with _
  /(^inline[^_\/]|\/[^_])[^\/]*.(js|css)$/,
  ...

inline-js-or-css.rb

def read_file_contents(stylesheet)
  if %w(test development).include?(Rails.env.to_s)
    # if we're running the full asset pipeline, 
    # just grab the body of the final output
    stylesheet.body
  else
    # in a production-like environment, read the 
    # fingerprinted and compiled file
    File.read(File.join(Rails.root, 'public', 'assets', stylesheet.digest_path))
  end
end

def inline_file(asset_path)
  file = Rails.application.assets.find_asset(asset_path)
  file.nil? ? '' : read_file_contents(file)
end

def inline_js(asset_path)
  "<script>#{inline_file asset_path}</script>"
end

def inline_css(asset_path)
  "<style>#{inline_file asset_path}</style>"
end

# use a method like this to automatically load CSS that
# follows the current controller/action name structure
# example: assets/stylesheets/views/users/show.sass
def current_view_stylesheet
  inline_css("views/#{params[:controller]}/#{params[:action]}") +
end

@averyvery you could also leverage Rails cache when reading the file contents, replacing this:
File.read(File.join(Rails.root, 'public', 'assets', stylesheet.digest_path))
by this:
render file: File.join(Rails.root, 'public', 'assets', stylesheet.digest_path)

render here will return the actual string with the file contents, because it's being called from a view template 😃
That will avoid a lot of I/O operations and make your users happier!