transactions can be turned off per Migration.

Closes #9483.

There are SQL Queries that can't run inside a transaction. Since
the Migrator used to wrap all Migrations inside a transaction there
was no way to run these queries within a migration.

This patch adds `self.disable_ddl_transaction!` to the migration to
turn transactions off when necessary.

activerecord/CHANGELOG.md

@@ -1,5 +1,20 @@
 ## Rails 4.0.0 (unreleased) ##
 
+*   Make it possible to execute migrations without a transaction even
+    if the database adapter supports DDL transactions.
+    Fixes #9483.
+
+    Example:
+
+        class ChangeEnum < ActiveRecord::Migration
+          self.disable_ddl_transaction!
+          def up
+            execute "ALTER TYPE model_size ADD VALUE 'new_value'"
+          end
+        end
+
+    *Yves Senn*
+
 *   Assigning "0.0" to a nullable numeric column does not make it dirty.
     Fix #9034.
 

activerecord/lib/active_record/migration.rb

@@ -330,6 +330,23 @@ def initialize
   #
   # For a list of commands that are reversible, please see
   # <tt>ActiveRecord::Migration::CommandRecorder</tt>.
+  #
+  # == Transactional Migrations
+  #
+  # If the database adapter supports DDL transactions, all migrations will
+  # automatically be wrapped in a transaction. There are queries that you
+  # can't execute inside a transaction though, and for these situations
+  # you can turn the automatic transactions off.
+  #
+  #   class ChangeEnum < ActiveRecord::Migration
+  #     self.disable_ddl_transaction!
+  #     def up
+  #       execute "ALTER TYPE model_size ADD VALUE 'new_value'"
+  #     end
+  #   end
+  #
+  # Remember that you can still open your own transactions, even if you
+  # are in a Migration with <tt>self.disable_ddl_transaction!</tt>.
   class Migration
     autoload :CommandRecorder, 'active_record/migration/command_recorder'
 
@@ -351,6 +368,7 @@ def call(env)
 
     class << self
       attr_accessor :delegate # :nodoc:
+      attr_accessor :disable_ddl_transaction # :nodoc:
     end
 
     def self.check_pending!
@@ -365,8 +383,16 @@ def self.migrate(direction)
       new.migrate direction
     end
 
-    cattr_accessor :verbose
+    # Disable DDL transactions for this migration.
+    def self.disable_ddl_transaction!
+      @disable_ddl_transaction = true
+    end
+
+    def disable_ddl_transaction # :nodoc:
+      self.class.disable_ddl_transaction
+    end
 
+    cattr_accessor :verbose
     attr_accessor :name, :version
 
     def initialize(name = self.class.name, version = nil)
@@ -375,8 +401,8 @@ def initialize(name = self.class.name, version = nil)
       @connection = nil
     end
 
+    self.verbose = true
     # instantiate the delegate object after initialize is defined
-    self.verbose  = true
     self.delegate = new
 
     # Reverses the migration commands for the given block and
@@ -663,7 +689,7 @@ def basename
       File.basename(filename)
     end
 
-    delegate :migrate, :announce, :write, :to => :migration
+    delegate :migrate, :announce, :write, :disable_ddl_transaction, to: :migration
 
     private
 
@@ -856,12 +882,12 @@ def migrate
         Base.logger.info "Migrating to #{migration.name} (#{migration.version})" if Base.logger
 
         begin
-          ddl_transaction do
+          ddl_transaction(migration) do
             migration.migrate(@direction)
             record_version_state_after_migrating(migration.version)
           end
         rescue => e
-          canceled_msg = Base.connection.supports_ddl_transactions? ? "this and " : ""
+          canceled_msg = use_transaction?(migration) ? "this and " : ""
           raise StandardError, "An error has occurred, #{canceled_msg}all later migrations canceled:\n\n#{e}", e.backtrace
         end
       end
@@ -935,12 +961,16 @@ def down?
     end
 
     # Wrap the migration in a transaction only if supported by the adapter.
-    def ddl_transaction
-      if Base.connection.supports_ddl_transactions?
+    def ddl_transaction(migration)
+      if use_transaction?(migration)
         Base.transaction { yield }
       else
         yield
       end
     end
+
+    def use_transaction?(migration)
+      !migration.disable_ddl_transaction && Base.connection.supports_ddl_transactions?
+    end
   end
 end

activerecord/test/cases/migration/logger_test.rb

@@ -7,6 +7,7 @@ class LoggerTest < ActiveRecord::TestCase
       self.use_transactional_fixtures = false
 
       Migration = Struct.new(:name, :version) do
+        def disable_ddl_transaction; false end
         def migrate direction
           # do nothing
         end
@@ -34,4 +35,3 @@ def test_migration_should_be_run_without_logger
     end
   end
 end
-

activerecord/test/cases/migration_test.rb

@@ -254,7 +254,39 @@ def migrate(x)
     assert_equal "An error has occurred, this and all later migrations canceled:\n\nSomething broke", e.message
 
     Person.reset_column_information
+    assert_not Person.column_methods_hash.include?(:last_name),
+     "On error, the Migrator should revert schema changes but it did not."
+  end
+
+  def test_migration_without_transaction
+    unless ActiveRecord::Base.connection.supports_ddl_transactions?
+      skip "not supported on #{ActiveRecord::Base.connection.class}"
+    end
+
     assert_not Person.column_methods_hash.include?(:last_name)
+
+    migration = Class.new(ActiveRecord::Migration) {
+      self.disable_ddl_transaction!
+
+      def version; 101 end
+      def migrate(x)
+        add_column "people", "last_name", :string
+        raise 'Something broke'
+      end
+    }.new
+
+    migrator = ActiveRecord::Migrator.new(:up, [migration], 101)
+    e = assert_raise(StandardError) { migrator.migrate }
+    assert_equal "An error has occurred, all later migrations canceled:\n\nSomething broke", e.message
+
+    Person.reset_column_information
+    assert Person.column_methods_hash.include?(:last_name),
+     "without ddl transactions, the Migrator should not rollback on error but it did."
+  ensure
+    Person.reset_column_information
+    if Person.column_methods_hash.include?(:last_name)
+      Person.connection.remove_column('people', 'last_name')
+    end
   end
 
   def test_schema_migrations_table_name

guides/source/migrations.md

@@ -61,6 +61,10 @@ migrations are wrapped in a transaction. If the database does not support this
 then when a migration fails the parts of it that succeeded will not be rolled
 back. You will have to rollback the changes that were made by hand.
 
+NOTE: There are certain queries that can't run inside a transaction. If your
+adapter supports DDL transactions you can use `disable_ddl_transaction!` to
+disable them for a single migration.
+
 If you wish for a migration to do something that Active Record doesn't know how
 to reverse, you can use `reversible`:
 
@@ -180,7 +184,7 @@ end
 ```
 
 If the migration name is of the form "CreateXXX" and is
-followed by a list of column names and types then a migration creating the table 
+followed by a list of column names and types then a migration creating the table
 XXX with the columns listed will be generated. For example:
 
 ```bash