We've talked about Doctrine in past chapters quite a bit. Doctrine is a set of libraries for database storage and object mapping. It comes bundled with the popular Symfony2 web framework by default and, among other features, it allows you to easily decouple your application from the persistence layer, thanks to the Data Mapper pattern.

Meanwhile, the ORM stands over a powerful database abstraction layer that enables database interaction through an SQL dialect called Doctrine Query Language (DQL), which is inspired by the famous Java Hibernate framework.

If we're going to use Doctrine ORM, the first task to complete is adding the dependencies to our project through Composer:

composer require doctrine/orm