gem_dep_manager.rb

module SimpleGemDependecyChecker
  class << self
    def initialize
      require_libraries
      system_gems = get_installed_gems
      required_gems = get_required_gems
      verify_gems(required_gems, system_gems)
    end
    
    def require_libraries
      begin
        require 'rubygems'
        require 'geminstaller'
      rescue LoadError => e
        if e.message.match(/rubygems/)
          puts "Could not find RubyGems.  Please confirm that it is installed."
        else
          raise e
        end
      rescue Gem::LoadError => e
        if e.message.match(/geminstaller/)
          puts "Could not find geminstaller.  Use the following command to install:"
          puts "gem install geminstaller"
        else
          raise e
        end
      end

      def build_geminstaller_config_string
        config_files = []
        ['default', 'production', RAILS_ENV ].each do |env| 
          config_files << File.join(File.expand_path(RAILS_ROOT), 'config', '#{env}.gems.yml') 
        end
        "--config=#{config_files.join(',')}"
      end

      # get gems defined in geminstaller config files
      def get_required_gems
        config = build_geminstaller_config_string
        gi = GemInstaller.create_application(config)
        gi_gems = gi.create_gems_from_config
        gi_gems
      end

      # get all currently installed gems
      def get_installed_gems
        specs = Gem::GemPathSearcher.new
        gems = specs.instance_variable_get("@gemspecs")
      end

      def verify_gems(gi_gems, system_gems)
        gi_gems.each do |g|
          sys_gem = system_gems.find(g.name) 
          report_missing_gem(g) unless sys_gem
          compare_gems(g, sys_gem)
        end      
      end

      def compare_gems(gi_gem, sys_gem)
        req = Gem::Requirement.create(g.version)
        req.satisfied_by? installed_gem.version
      end
    
      def report_missing_gem(missing_gem)
        puts "Could not find #{missing_gem.name}, version #{missing_gem.version}"
        puts "Please use one of the following commands to ensure that it is installed:"
        puts "geminstaller #=> This will use geminstaller to install all required gems"
        puts "rake gems:sync #=> Wrapper for geminstaller to install gems for a given RAILS_ENV"
        puts "gem install #{missing_gem.name} -v #{missing_gem.version} #=> Will install only this gem"
        Process.exit(1)
      end
  end
end