This chapter gives an overview of the overall architecture, terminology and constraints of Doctrine 2. It is recommended to read this chapter carefully.
As the term ORM already hints at, Doctrine 2 aims to simplify the translation between database rows and the PHP object model. The primary use case for Doctrine are therefore applications that utilize the Object-Oriented Programming Paradigm. For applications that do not primarily work with objects Doctrine 2 is not suited very well.
Doctrine 2 requires a minimum of PHP 5.4. For greatly improved performance it is also recommended that you use APC with PHP.
Doctrine 2 is divided into three main packages.
- DBAL (includes Common)
- ORM (includes DBAL+Common)
This manual mainly covers the ORM package, sometimes touching parts of the underlying DBAL and Common packages. The Doctrine code base is split in to these packages for a few reasons and they are to...
- ...make things more maintainable and decoupled
- ...allow you to use the code in Doctrine Common without the ORM or DBAL
- ...allow you to use the DBAL without the ORM
The Common package contains highly reusable components that have no
dependencies beyond the package itself (and PHP, of course). The
root namespace of the Common package is
The DBAL package contains an enhanced database abstraction layer on
top of PDO but is not strongly bound to PDO. The purpose of this
layer is to provide a single API that bridges most of the
differences between the different RDBMS vendors. The root namespace
of the DBAL package is
The ORM package contains the object-relational mapping toolkit that
provides transparent relational persistence for plain PHP objects.
The root namespace of the ORM package is
An entity is a lightweight, persistent domain object. An entity can be any regular PHP class observing the following restrictions:
- An entity class must not be final or contain final methods.
- All persistent properties/field of any entity class should always be private or protected, otherwise lazy-loading might not work as expected. In case you serialize entities (for example Session) properties should be protected (See Serialize section below).
- Any two entity classes in a class hierarchy that inherit directly or indirectly from one another must not have a mapped property with the same name. That is, if B inherits from A then B must not have a mapped field with the same name as an already mapped field that is inherited from A.
Entities support inheritance, polymorphic associations, and polymorphic queries. Both abstract and concrete classes can be entities. Entities may extend non-entity classes as well as entity classes, and non-entity classes may extend entity classes.
The constructor of an entity is only ever invoked when you construct a new instance with the new keyword. Doctrine never calls entity constructors, thus you are free to use them as you wish and even have it require arguments of any type.
An entity instance can be characterized as being NEW, MANAGED, DETACHED or REMOVED.
- A NEW entity instance has no persistent identity, and is not yet associated with an EntityManager and a UnitOfWork (i.e. those just created with the "new" operator).
- A MANAGED entity instance is an instance with a persistent identity that is associated with an EntityManager and whose persistence is thus managed.
- A DETACHED entity instance is an instance with a persistent identity that is not (or no longer) associated with an EntityManager and a UnitOfWork.
- A REMOVED entity instance is an instance with a persistent identity, associated with an EntityManager, that will be removed from the database upon transaction commit.
The persistent state of an entity is represented by instance variables. An instance variable must be directly accessed only from within the methods of the entity by the entity instance itself. Instance variables must not be accessed by clients of the entity. The state of the entity is available to clients only through the entity’s methods, i.e. accessor methods (getter/setter methods) or other business methods.
Collection-valued persistent fields and properties must be defined
in terms of the
interface. The collection implementation type may be used by the
application to initialize fields or properties before the entity is
made persistent. Once the entity becomes managed (or detached),
subsequent access must be through the interface type.
Serializing entities is generally to be avoided.
If you intend to serialize (and unserialize) entity instances that still hold references to proxy objects you may run into problems, because all proxy properties will be initialized recursively, leading to large serialized object graphs, especially for circular associations.
If you really must serialize entities, regardless if proxies are
involved or not, then consider implementing the
interface and manually checking for cyclic dependencies in your
EntityManager class is a central access point to the ORM
functionality provided by Doctrine 2. The
EntityManager API is
used to manage the persistence of your objects and to query for
EntityManager and the underlying
UnitOfWork employ a
strategy called "transactional write-behind" that delays the
execution of SQL statements in order to execute them in the most
efficient way and to execute them at the end of a transaction so
that all write locks are quickly released. You should see Doctrine
as a tool to synchronize your in-memory objects with the database
in well defined units of work. Work with your objects and modify
them as usual and when you're done call
to make your changes persistent.
EntityManager uses a
UnitOfWork, which is a
typical implementation of the
Unit of Work pattern,
to keep track of all the things that need to be done the next time
flush is invoked. You usually do not directly interact with a
UnitOfWork but with the