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05.12. Doctrine 2

Note: This article was originally published at Planet PHP on 5 December 2011.
Planet PHP
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Object-relational mapping (ORM) frameworks have been around for several years now and for some people, ORM is already outdated by now. As we have seen with other technologies and concepts before, PHP is not exactly what we call an early adopter among the programming languages. Thus it took some time for ORM to grow up in the PHP context.

There have been some frameworks before Doctrine 2 that implement ORM (remember e.g. Propel) specific tasks but most of them lack the required maturity to be used in large projects. With Doctrine 2, PHP takes a huge step into the right direction A- Doctrine 2 is fast, extensible and easy to use.

This article will take you on a tour through the main concepts of Doctrine 2 in the first part and then explain how to use it in a real world application in the second part. Since at the time of writing Zend Framework 1.11.xx (ZF) is very popular, we will integrate Doctrine 2 into a ZF project.

Basic Concepts

To understand Doctrine 2, we have to take a look at some relevant terms (or in this case objects), study their behavior and practice their usage. We start with some introductory phrases on ORM systems and then go on to the concepts underlying Doctrine 2: Entity Objects, the Entity Manager, Repositories and Proxies.

Object-relational Mapping

Since the beginning of Object-Orientation, people had to manage the persistence of their application's state resp. their objects. In the context of Web Application Development, this usually involves a Database server which is being consulted using a Query Language. One example for this pattern is a PHP application that uses some kind of SQL server by sending SQL queries to it. Another one is an application using a CouchDB server by querying it via its REST API.
Due to the author's laziness, we will talk in terms of relational databases from now on. Keep in mind, that you can accomplish almost everything mentioned here with NoSQL databases, too.

ORM relates value objects that exist in an application's business logic to database records. Thus every object that should be persistent is saved in one row of a database table. The most common approach is to map classes to tables and the classes' objects to rows in the these tables.
Besides writing objects to a database, ORM systems are also intended to ease the process of finding data stored in the database. When talking in terms of ORM, finding data always means making the framework fetch one or many objects that meet a certain criteria.

Entity Objects

The objects that are being managed by an ORM system are called Entity Objects. Every entity object relates to one entry in a table. In Doctrine 2, the classes that represent entities do not have to fulfill special requirements like inheriting from a certain super class (as you might have seen in other database abstraction frameworks like Zend_Db). When creating a new entity class with Doctrine 2, all you have to do is to write down a regular PHP class with properties. Besides this, you have to provide some hints on how these attributes should be persisted. The information how entity attributes relate to columns in the DB is called Metadata. Metadata can be described in different ways: By default there are metadata drivers for descriptions in XML, YAML and PHP. The fourth and most popular driver is based on DocBlock annotations (since in PHP, annotations aren't a language feature as in Java (see Wikipedia), they are contained by the classes' and attributes' DocBlocks). We will use annotations to describe our entities metadata. To get an impression on how easy this is, take a look at the following example.

This example contains all it needs to tell Doctrine 2 about the new entity User. With this class, you can create, find, delete and modify user objects and persist their state to the underlying database. But keep in mind: as long as you don't need any persistence features, you can use your user objects just like any other objects!

The next two objects resp. object types we will describe

Truncated by Planet PHP, read more at the original (another 13928 bytes)