gemfile.rake

namespace :gemfile do

  desc "filters the gemfile, leaving only gems belonging to specific groups"
  task :filter => :environment do
    groups = (ENV['groups'] || "").split(',')

    if groups.empty?
      puts "! Invalid usage"
      puts
      puts "Please specify the groups you want to filter. Example:"
      puts
      puts "  rake gemfile:filter groups=development,staging"
      puts
      exit
    end

    task = FilterGemfile.new
    task.run(*groups)

    require_relative "gemfile_parser"
    code = File.read(Rails.root.join("Gemfile"))
    parser = GemfileParser.new(groups, code)
    File.open(gemfile, 'w') {|f| f.write(parser.parse) }

  end
end

gemfile_parser.rb

class GemfileParser

  def initialize(groups, code)
    @groups = groups
    @code = code
  end

  def parse
    @output = ""
    instance_eval(@code)
    @output
  end

  def source(source, options = {})
    @output += "source #{source.inspect}"
    if options.any?
      @output += ", " + options.map { |key, value| "#{key.inspect} => #{value.inspect}" }.join(", ")
    end
    @output += "\n"
  end

  def gem(name, *args)
    @output += "gem #{name.inspect}"
    if args.any?
      @output += ", " + args.map { |arg| arg.inspect }.join(", ")
    end
    @output += "\n"
  end

  def group(*groups, &blk)
    intersection = groups.map(&:to_s) & @groups.map(&:to_s)
    yield if intersection.any?
  end

end