131 lines
5.3 KiB
ReStructuredText
131 lines
5.3 KiB
ReStructuredText
Architecture
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============
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This chapter gives an overview of the overall architecture,
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terminology and constraints of Doctrine 2. It is recommended to
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read this chapter carefully.
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Entities
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--------
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An entity is a lightweight, persistent domain object. An entity can
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be any regular PHP class observing the following restrictions:
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- An entity class must not be final or contain final methods.
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- All persistent properties/field of any entity class should
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always be private or protected, otherwise lazy-loading might not
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work as expected.
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- An entity class must not implement ``__clone`` or
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:doc:`do so safely <../cookbook/implementing-wakeup-or-clone>`.
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- An entity class must not implement ``__wakeup`` or
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:doc:`do so safely <../cookbook/implementing-wakeup-or-clone>`.
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Also consider implementing
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`Serializable <http://de3.php.net/manual/en/class.serializable.php>`_
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instead.
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- Any two entity classes in a class hierarchy that inherit
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directly or indirectly from one another must not have a mapped
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property with the same name. That is, if B inherits from A then B
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must not have a mapped field with the same name as an already
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mapped field that is inherited from A.
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Entities support inheritance, polymorphic associations, and
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polymorphic queries. Both abstract and concrete classes can be
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entities. Entities may extend non-entity classes as well as entity
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classes, and non-entity classes may extend entity classes.
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**TIP** The constructor of an entity is only ever invoked when
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*you* construct a new instance with the *new* keyword. Doctrine
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never calls entity constructors, thus you are free to use them as
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you wish and even have it require arguments of any type.
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Entity states
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~~~~~~~~~~~~~
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An entity instance can be characterized as being NEW, MANAGED,
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DETACHED or REMOVED.
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- A NEW entity instance has no persistent identity, and is not yet
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associated with an EntityManager and a UnitOfWork (i.e. those just
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created with the "new" operator).
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- A MANAGED entity instance is an instance with a persistent
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identity that is associated with an EntityManager and whose
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persistence is thus managed.
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- A DETACHED entity instance is an instance with a persistent
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identity that is not (or no longer) associated with an
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EntityManager and a UnitOfWork.
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- A REMOVED entity instance is an instance with a persistent
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identity, associated with an EntityManager, that will be removed
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from the database upon transaction commit.
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.. _architecture_persistent_fields:
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Persistent fields
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~~~~~~~~~~~~~~~~~
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The persistent state of an entity is represented by instance
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variables. An instance variable must be directly accessed only from
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within the methods of the entity by the entity instance itself.
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Instance variables must not be accessed by clients of the entity.
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The state of the entity is available to clients only through the
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entity’s methods, i.e. accessor methods (getter/setter methods) or
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other business methods.
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Collection-valued persistent fields and properties must be defined
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in terms of the ``Doctrine\Common\Collections\Collection``
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interface. The collection implementation type may be used by the
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application to initialize fields or properties before the entity is
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made persistent. Once the entity becomes managed (or detached),
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subsequent access must be through the interface type.
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Serializing entities
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~~~~~~~~~~~~~~~~~~~~
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Serializing entities can be problematic and is not really
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recommended, at least not as long as an entity instance still holds
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references to proxy objects or is still managed by an
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EntityManager. If you intend to serialize (and unserialize) entity
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instances that still hold references to proxy objects you may run
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into problems with private properties because of technical
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limitations. Proxy objects implement ``__sleep`` and it is not
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possible for ``__sleep`` to return names of private properties in
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parent classes. On the other hand it is not a solution for proxy
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objects to implement ``Serializable`` because Serializable does not
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work well with any potential cyclic object references (at least we
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did not find a way yet, if you did, please contact us).
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The EntityManager
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-----------------
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The ``EntityManager`` class is a central access point to the ORM
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functionality provided by Doctrine 2. The ``EntityManager`` API is
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used to manage the persistence of your objects and to query for
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persistent objects.
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Transactional write-behind
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~~~~~~~~~~~~~~~~~~~~~~~~~~
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An ``EntityManager`` and the underlying ``UnitOfWork`` employ a
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strategy called "transactional write-behind" that delays the
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execution of SQL statements in order to execute them in the most
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efficient way and to execute them at the end of a transaction so
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that all write locks are quickly released. You should see Doctrine
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as a tool to synchronize your in-memory objects with the database
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in well defined units of work. Work with your objects and modify
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them as usual and when you're done call ``EntityManager#flush()``
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to make your changes persistent.
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The Unit of Work
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~~~~~~~~~~~~~~~~
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Internally an ``EntityManager`` uses a ``UnitOfWork``, which is a
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typical implementation of the
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`Unit of Work pattern <http://martinfowler.com/eaaCatalog/unitOfWork.html>`_,
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to keep track of all the things that need to be done the next time
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``flush`` is invoked. You usually do not directly interact with a
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``UnitOfWork`` but with the ``EntityManager`` instead.
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