yii2中使用Active Record模式的方法

return [
  'class' => 'yii\db\Connection',
  'dsn' => 'mysql:host=localhost;dbname=yii2basic',
  'username' => 'root',
  'password' => '',
  'charset' => 'utf8',
];

class m150225_022640_modify_book_table extends Migration
{
  public function up()
  {
    $this->addColumn("book", "book_desc", yii\db\mssql\Schema::TYPE_TEXT);
  }
  public function down()
  {
    echo "m150225_022640_modify_book_table cannot be reverted.\n";
    return false;
  }
}