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.

2 files changed, 33 insertions, 1 deletions

diff --git a/activerecord/test/cases/migration/logger_test.rb b/activerecord/test/cases/migration/logger_test.rb
index ee0c20747e..97efb94b66 100644
--- a/activerecord/test/cases/migration/logger_test.rb
+++ b/activerecord/test/cases/migration/logger_test.rb
@@ -7,6 +7,7 @@ module ActiveRecord
       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 @@ module ActiveRecord
     end
   end
 end
-
diff --git a/activerecord/test/cases/migration_test.rb b/activerecord/test/cases/migration_test.rb
index 831b2ee2b4..960d28fcf5 100644
--- a/activerecord/test/cases/migration_test.rb
+++ b/activerecord/test/cases/migration_test.rb
@@ -254,7 +254,39 @@ class MigrationTest < ActiveRecord::TestCase
     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