3 files changed, 122 insertions, 0 deletions
diff --git a/activerecord/lib/active_record/migration.rb b/activerecord/lib/active_record/migration.rb
index b303ff4225..e7063bca11 100644
--- a/activerecord/lib/active_record/migration.rb
+++ b/activerecord/lib/active_record/migration.rb
@@ -284,6 +284,8 @@ module ActiveRecord
   # In application.rb.
   #
   class Migration
+    autoload :CommandRecorder, 'active_record/migration/command_recorder'
+
     class << self
       attr_accessor :delegate # :nodoc:
     end
diff --git a/activerecord/lib/active_record/migration/command_recorder.rb b/activerecord/lib/active_record/migration/command_recorder.rb
new file mode 100644
index 0000000000..25b8649350
--- /dev/null
+++ b/activerecord/lib/active_record/migration/command_recorder.rb
@@ -0,0 +1,48 @@
+module ActiveRecord
+  class Migration
+    # ActiveRecord::Migration::CommandRecorder records commands done during
+    # a migration and knows how to reverse those commands.
+    class CommandRecorder
+      attr_reader :commands
+
+      def initialize
+        @commands = []
+      end
+
+      # record +command+.  +command+ should be a method name and arguments.
+      # For example:
+      #
+      #   recorder.record(:method_name, [:arg1, arg2])
+      def record(*command)
+        @commands << command
+      end
+
+      # Returns a list that represents commands that are the inverse of the
+      # commands stored in +commands+.
+      def inverse
+        @commands.map { |name, args| send(:"invert_#{name}", args) }
+      end
+
+      private
+      def invert_create_table(args)
+        [:drop_table, args]
+      end
+
+      def invert_rename_table(args)
+        [:rename_table, args.reverse]
+      end
+
+      def invert_add_column(args)
+        [:remove_column, args.first(2)]
+      end
+
+      def invert_rename_index(args)
+        [:rename_index, args.reverse]
+      end
+
+      def invert_rename_column(args)
+        [:rename_column, [args.first] + args.last(2).reverse]
+      end
+    end
+  end
+end
diff --git a/activerecord/test/cases/migration/command_recorder_test.rb b/activerecord/test/cases/migration/command_recorder_test.rb
new file mode 100644
index 0000000000..c33ce7cadf
--- /dev/null
+++ b/activerecord/test/cases/migration/command_recorder_test.rb
@@ -0,0 +1,72 @@
+require "cases/helper"
+
+module ActiveRecord
+  class Migration
+    class CommandRecorderTest < ActiveRecord::TestCase
+      def setup
+        @recorder = CommandRecorder.new
+      end
+
+      def test_record
+        @recorder.record :create_table, [:system_settings]
+        assert_equal 1, @recorder.commands.length
+      end
+
+      def test_inverse
+        @recorder.record :create_table, [:system_settings]
+        assert_equal 1, @recorder.inverse.length
+
+        @recorder.record :rename_table, [:old, :new]
+        assert_equal 2, @recorder.inverse.length
+      end
+
+      def test_invert_create_table
+        @recorder.record :create_table, [:system_settings]
+        drop_table = @recorder.inverse.first
+        assert_equal [:drop_table, [:system_settings]], drop_table
+      end
+
+      def test_invert_rename_table
+        @recorder.record :rename_table, [:old, :new]
+        rename = @recorder.inverse.first
+        assert_equal [:rename_table, [:new, :old]], rename
+      end
+
+      def test_invert_add_column
+        @recorder.record :add_column, [:table, :column, :type, {}]
+        remove = @recorder.inverse.first
+        assert_equal [:remove_column, [:table, :column]], remove
+      end
+
+      def test_invert_rename_column
+        @recorder.record :rename_column, [:table, :old, :new]
+        rename = @recorder.inverse.first
+        assert_equal [:rename_column, [:table, :new, :old]], rename
+      end
+
+      def test_invert_add_index
+        @recorder.record :add_index, [:table, [:one, :two], {:options => true}]
+        remove = @recorder.inverse.first
+        assert_equal [:remove_index, [:table, {:column => [:one, :two]}]], remove
+      end
+
+      def test_invert_rename_index
+        @recorder.record :rename_index, [:old, :new]
+        rename = @recorder.inverse.first
+        assert_equal [:rename_index, [:new, :old]], rename
+      end
+
+      def test_invert_add_timestamps
+        @recorder.record :add_timestamps, [:table]
+        remove = @recorder.inverse.first
+        assert_equal [:remove_timestamps, [:table]], remove
+      end
+
+      def test_invert_remove_timestamps
+        @recorder.record :remove_timestamps, [:table]
+        add = @recorder.inverse.first
+        assert_equal [:add_timestamps, [:table]], add
+      end
+    end
+  end
+end