stylesheet.rb

class Stylesheet
  include MongoMapper::Document

  class Processor
    def self.errors
      []
    end

    attr_reader :contents

    def initialize(contents)
      @contents = contents
    end
  end

  class PlainProcessor < Processor
    def process
      contents
    end
  end

  class SassProcessor < Processor
    def self.errors
      [Sass::SyntaxError]
    end

    def self.options
      @options ||= {}
    end

    def process
      Sass::Engine.new(contents, self.class.options).render
    end
  end

  class ScssProcessor < SassProcessor
    def self.options
      @options ||= {:syntax => :scss}
    end
  end

  class LessProcessor < Processor
    def self.errors
      [
        Less::MixedUnitsError,
        Less::PathError,
        Less::VariableNameError,
        Less::MixinNameError,
        Less::SyntaxError,
        Less::ImportError,
        Less::CompileError
      ]
    end

    def process
      Less.parse(contents)
    end
  end

  Processors = {
    'plain' => PlainProcessor,
    'sass'  => SassProcessor,
    'scss'  => ScssProcessor,
    'less'  => LessProcessor,
  }

  def self.processors
    Processors
  end

  key :processor, String, :default => 'plain'

  validates_inclusion_of :processor, :within => Stylesheet.processors.keys
  validate :no_processor_parse_error

  def processed_contents
    processor_class.new(contents).process
  end

  private
    def processor_class
      self.class.processors[processor]
    end

    def no_processor_parse_error
      return if contents.blank?
      begin
        processed_contents
      rescue *processor_class.errors => e
        errors.add(:base, "#{processor.capitalize} parse error: #{e.message}")
      end
    end
end