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@@ -1,5 +1,7 @@
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 *Edge*
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+* Transactional migrations for databases which support them.  #834 [divoxx, Adam Wiggins, Tarmo Tänav]
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+
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 * Set config.active_record.timestamped_migrations = false to have migrations with numeric prefix instead of UTC timestamp. #446. [Andrew Stone, Nik Wakelin]
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 * change_column_default preserves the not-null constraint.  #617 [Tarmo Tänav]

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@@ -51,6 +51,13 @@ module ActiveRecord
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         true
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       end
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+      # Does this adapter support DDL rollbacks in transactions?  That is, would
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+      # CREATE TABLE or ALTER TABLE get rolled back by a transaction?  PostgreSQL,
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+      # SQL Server, and others support this.  MySQL and others do not.
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+      def supports_ddl_transactions?
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+        false
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+      end
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+
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       # Should primary key values be selected from their corresponding
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       # sequence before the insert statement?  If true, next_sequence_value
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       # is called before each insert to set the record's primary key.

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@@ -335,6 +335,10 @@ module ActiveRecord
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         postgresql_version >= 80200
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       end
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+      def supports_ddl_transactions?
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+        true
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+      end
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+
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       # Returns the configured supported identifier length supported by PostgreSQL,
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       # or report the default of 63 on PostgreSQL 7.x.
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       def table_alias_length

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@@ -461,14 +461,22 @@ module ActiveRecord
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         Base.logger.info "Migrating to #{migration.name} (#{migration.version})"
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         # On our way up, we skip migrating the ones we've already migrated
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-        # On our way down, we skip reverting the ones we've never migrated
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         next if up? && migrated.include?(migration.version.to_i)
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+        # On our way down, we skip reverting the ones we've never migrated
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         if down? && !migrated.include?(migration.version.to_i)
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           migration.announce 'never migrated, skipping'; migration.write
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-        else
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-          migration.migrate(@direction)
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-          record_version_state_after_migrating(migration.version)
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+          next
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+        end
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+
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+        begin
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+          ddl_transaction do
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+            migration.migrate(@direction)
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+            record_version_state_after_migrating(migration.version)
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+          end
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+        rescue => e
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+          canceled_msg = Base.connection.supports_ddl_transactions? ? "this and " : ""
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+          raise StandardError, "An error has occurred, #{canceled_msg}all later migrations canceled:\n\n#{e}", e.backtrace
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         end
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       end
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     end
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@@ -531,5 +539,14 @@ module ActiveRecord
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       def down?
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         @direction == :down
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       end
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+
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+      # Wrap the migration in a transaction only if supported by the adapter.
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+      def ddl_transaction(&block)
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+        if Base.connection.supports_ddl_transactions?
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+          Base.transaction { block.call }
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+        else
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+          block.call
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+        end
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+      end
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   end
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 end

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@@ -937,6 +937,21 @@ if ActiveRecord::Base.connection.supports_migrations?
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       assert_equal(0, ActiveRecord::Migrator.current_version)
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     end
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+    if current_adapter?(:PostgreSQLAdapter)
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+      def test_migrator_one_up_with_exception_and_rollback
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+        assert !Person.column_methods_hash.include?(:last_name)
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+
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+        e = assert_raises(StandardError) do
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+          ActiveRecord::Migrator.up(MIGRATIONS_ROOT + "/broken", 100)
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+        end
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+
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+        assert_equal "An error has occurred, this and all later migrations canceled:\n\nSomething broke", e.message
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+
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+        Person.reset_column_information
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+        assert !Person.column_methods_hash.include?(:last_name)
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+      end
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+    end
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+
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     def test_finds_migrations
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       migrations = ActiveRecord::Migrator.new(:up, MIGRATIONS_ROOT + "/valid").migrations
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       [['1', 'people_have_last_names'],