1 files changed, 91 insertions, 0 deletions
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..d7e481905a
--- /dev/null
+++ b/activerecord/lib/active_record/migration/command_recorder.rb
@@ -0,0 +1,91 @@
+module ActiveRecord
+  class Migration
+    # ActiveRecord::Migration::CommandRecorder records commands done during
+    # a migration and knows how to reverse those commands.  The CommandRecorder
+    # knows how to invert the following commands:
+    #
+    # * add_column
+    # * add_index
+    # * add_timestamp
+    # * create_table
+    # * remove_timestamps
+    # * rename_column
+    # * rename_index
+    # * rename_table
+    class CommandRecorder
+      attr_accessor :commands, :delegate
+
+      def initialize(delegate = nil)
+        @commands = []
+        @delegate = delegate
+      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+.  For example:
+      #
+      #   recorder.record(:rename_table, [:old, :new])
+      #   recorder.inverse # => [:rename_table, [:new, :old]]
+      #
+      # This method will raise an IrreversibleMigration exception if it cannot
+      # invert the +commands+.
+      def inverse
+        @commands.reverse.map { |name, args|
+          method = :"invert_#{name}"
+          raise IrreversibleMigration unless respond_to?(method, true)
+          __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]
+      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
+
+      def invert_add_index(args)
+        table, columns, _ = *args
+        [:remove_index, [table, {:column => columns}]]
+      end
+
+      def invert_remove_timestamps(args)
+        [:add_timestamps, args]
+      end
+
+      def invert_add_timestamps(args)
+        [:remove_timestamps, args]
+      end
+    end
+  end
+end