# UPGRADE FROM 2.0 to 2.1 ## ID generators The `Doctrine\ODM\MongoDB\Id\AbstractIdGenerator` class has been deprecated. Custom ID generators must implement the `Doctrine\ODM\MongoDB\Id\IdGenerator` interface. ## Metadata The `Doctrine\ODM\MongoDB\Mapping\ClassMetadata` class has been marked final and will no longer be extendable in 3.0. The `boolean`, `integer`, and `int_id` mapping types have been deprecated. Use the `bool`, `int`, and `int` types, respectively. These types behave exactly the same.