partial implementation of the command recorder

2 files changed, 50 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