command recorder will record commands sent to a delegate object

activerecord/lib/active_record/migration/command_recorder.rb

@@ -3,10 +3,11 @@ module ActiveRecord
     # ActiveRecord::Migration::CommandRecorder records commands done during
     # a migration and knows how to reverse those commands.
     class CommandRecorder
-      attr_reader :commands
+      attr_reader :commands, :delegate
 
-      def initialize
+      def initialize(delegate = nil)
         @commands = []
+        @delegate = delegate
       end
 
       # record +command+.  +command+ should be a method name and arguments.
@@ -29,10 +30,19 @@ module ActiveRecord
         @commands.reverse.map { |name, args|
           method = :"invert_#{name}"
           raise IrreversibleMigration unless respond_to?(method, true)
-          send(method, args)
+          __send__(method, args)
         }
       end
 
+      def respond_to?(*args) # :nodoc:
+        super || delegate.respond_to?(*args)
+      end
+
+      def send(method, *args) # :nodoc:
+        return super unless respond_to?(method)
+        record(method, args)
+      end
+
       private
       def invert_create_table(args)
         [:drop_table, args]

activerecord/test/cases/migration/command_recorder_test.rb

@@ -7,6 +7,28 @@ module ActiveRecord
         @recorder = CommandRecorder.new
       end
 
+      def test_respond_to_delegates
+        recorder = CommandRecorder.new(Class.new {
+          def america; end
+        }.new)
+        assert recorder.respond_to?(:america)
+      end
+
+      def test_send_calls_super
+        assert_raises(NoMethodError) do
+          @recorder.send(:create_table, :horses)
+        end
+      end
+
+      def test_send_delegates_to_record
+        recorder = CommandRecorder.new(Class.new {
+          def create_table(name); end
+        }.new)
+        assert recorder.respond_to?(:create_table), 'respond_to? create_table'
+        recorder.send(:create_table, :horses)
+        assert_equal [[:create_table, [:horses]]], recorder.commands
+      end
+
       def test_unknown_commands_raise_exception
         @recorder.record :execute, ['some sql']
         assert_raises(ActiveRecord::IrreversibleMigration) do