i can haz railz 3

vendor/plugins/rails_upgrade/lib/gemfile_generator.rb

@@ -0,0 +1,95 @@
+module Rails
+  module Upgrading
+    class GemfileGenerator
+      def generate_new_gemfile
+        if has_environment?
+          generate_gemfile
+        else
+          raise FileNotFoundError, "Can't find environment.rb [config/environment.rb]!"
+        end
+      end
+      
+      def has_environment?
+        File.exists?("config/environment.rb")
+      end
+      
+      def environment_code
+        File.open("config/environment.rb").read
+      end
+      
+      def generate_gemfile
+        environment_file = environment_code
+        
+        # Get each line that starts with config.gem
+        gem_lines = environment_file.split("\n").select {|l| l =~ /^\s*config\.gem/}
+        
+        # Toss those lines to a generator class; the lines are evaluated in the 
+        # context of that instance.
+        config = GemfileGenerator.new
+        config.instance_eval(gem_lines.join("\n"))
+        
+        config.output
+      end
+    end
+    
+    class GemfileGenerator
+      # Creates a target for the config.gem calls
+      def config
+        self
+      end
+      
+      def initialize
+        @gems = []
+      end
+      
+      # Receive a call to add a gem to the list
+      def gem(name, options={})
+        data = {}
+        
+        # Add new keys from old keys
+        data[:require] = options[:lib] if options[:lib]
+        data[:source] = options[:source] if options[:source]
+        
+        version = options[:version]
+        @gems << [name, version, data]
+      end
+      
+      # Generate the Gemfile output
+      def output
+        # Generic preamble, taken from Yehuda Katz's blog
+        preamble = <<STR
+# Edit this Gemfile to bundle your application's dependencies.
+# This preamble is the current preamble for Rails 3 apps; edit as needed.
+source 'http://rubygems.org'
+
+gem 'rails', '3.0.0.beta3'
+
+STR
+        preamble + generate_upgraded_code
+      end
+      
+      # Get Gemfile call for all the gems
+      def generate_upgraded_code   
+        code = @gems.map do |name, version, data|
+          version_string = (version ? "'#{version}'" : nil)
+          source = data.delete(:source)
+          
+          data_string = nil
+          unless data.empty?
+            data_string = data.to_a.map {|k, v| ":#{k} => '#{v}'"}.join(", ")
+          end
+          
+          # If we have a source, generate a call to +source+ then output the
+          # gem call.  Otherwise, just generate the gem requirement.
+          if source
+            str = ["'#{name}'", version_string, data_string].compact.join(", ")
+            "source '#{source}'\ngem #{str}"
+          else
+            str = ["'#{name}'", version_string, data_string].compact.join(", ")
+            "gem #{str}"
+          end
+        end.join("\n")
+      end
+    end
+  end
+end