doctrine2/manual/en/working-with-objects.rst

In this chapter we will help you understand the ``EntityManager``
and the ``UnitOfWork``. A Unit of Work is similar to an
object-level transaction. A new Unit of Work is implicitly started
when an EntityManager is initially created or after
``EntityManager#flush()`` has been invoked. A Unit of Work is
committed (and a new one started) by invoking
``EntityManager#flush()``.

A Unit of Work can be manually closed by calling
EntityManager#close(). Any changes to objects within this Unit of
Work that have not yet been persisted are lost.

    **NOTE**

    It is very important to understand that only
    ``EntityManager#flush()`` ever causes write operations against the
    database to be executed. Any other methods such as
    ``EntityManager#persist($entity)`` or
    ``EntityManager#remove($entity)`` only notify the UnitOfWork to
    perform these operations during flush.

    Not calling ``EntityManager#flush()`` will lead to all changes
    during that request being lost.


Entities and the Identity Map
-----------------------------

Entities are objects with identity. Their identity has a conceptual
meaning inside your domain. In a CMS application each article has a
unique id. You can uniquely identify each article by that id.

Take the following example, where you find an article with the
headline "Hello World" with the ID 1234:

::

    <?php
    $article = $entityManager->find('CMS\Article', 1234);
    $article->setHeadline('Hello World dude!');
    
    $article2 = $entityManager->find('CMS\Article', 1234);
    echo $article2->getHeadline();

In this case the Article is accessed from the entity manager twice,
but modified in between. Doctrine 2 realizes this and will only
ever give you access to one instance of the Article with ID 1234,
no matter how often do you retrieve it from the EntityManager and
even no matter what kind of Query method you are using (find,
Repository Finder or DQL). This is called "Identity Map" pattern,
which means Doctrine keeps a map of each entity and ids that have
been retrieved per PHP request and keeps returning you the same
instances.

In the previous example the echo prints "Hello World dude!" to the
screen. You can even verify that ``$article`` and ``$article2`` are
indeed pointing to the same instance by running the following
code:

::

    <?php
    if ($article === $article2) {
        echo "Yes we are the same!";
    }

Sometimes you want to clear the identity map of an EntityManager to
start over. We use this regularly in our unit-tests to enforce
loading objects from the database again instead of serving them
from the identity map. You can call ``EntityManager#clear()`` to
achieve this result.

Entity Object Graph Traversal
-----------------------------

Although Doctrine allows for a complete separation of your domain
model (Entity classes) there will never be a situation where
objects are "missing" when traversing associations. You can walk
all the associations inside your entity models as deep as you
want.

Take the following example of a single ``Article`` entity fetched
from newly opened EntityManager.

::

    <?php
    /** @Entity */
    class Article
    {
        /** @Id @Column(type="integer") @GeneratedValue */
        private $id;
    
        /** @Column(type="string") */
        private $headline;
    
        /** @ManyToOne(targetEntity="User") */
        private $author;
    
        /** @OneToMany(targetEntity="Comment", mappedBy="article") */
        private $comments;
    
        public function __construct {
            $this->comments = new ArrayCollection();
        }
    
        public function getAuthor() { return $this->author; }
        public function getComments() { return $this->comments; }
    }
    
    $article = $em->find('Article', 1);

This code only retrieves the ``User`` instance with id 1 executing
a single SELECT statement against the user table in the database.
You can still access the associated properties author and comments
and the associated objects they contain.

This works by utilizing the lazy loading pattern. Instead of
passing you back a real Author instance and a collection of
comments Doctrine will create proxy instances for you. Only if you
access these proxies for the first time they will go through the
EntityManager and load their state from the database.

This lazy-loading process happens behind the scenes, hidden from
your code. See the following code:

::

    <?php
    $article = $em->find('Article', 1);
    
    // accessing a method of the user instance triggers the lazy-load
    echo "Author: " . $article->getAuthor()->getName() . "\n";
    
    // Lazy Loading Proxies pass instanceof tests:
    if ($article->getAuthor() instanceof User) {
        // a User Proxy is a generated "UserProxy" class
    }
    
    // accessing the comments as an iterator triggers the lazy-load
    // retrieving ALL the comments of this article from the database
    // using a single SELECT statement
    foreach ($article->getComments() AS $comment) {
        echo $comment->getText() . "\n\n";
    }
    
    // Article::$comments passes instanceof tests for the Collection interface
    // But it will NOT pass for the ArrayCollection interface
    if ($article->getComments() instanceof \Doctrine\Common\Collections\Collection) {
        echo "This will always be true!";
    }

A slice of the generated proxy classes code looks like the
following piece of code. A real proxy class override ALL public
methods along the lines of the ``getName()`` method shown below:

::

    <?php
    class UserProxy extends User implements Proxy
    {
        private function _load()
        {
            // lazy loading code
        }
    
        public function getName()
        {
            $this->_load();
            return parent::getName();
        }
        // .. other public methods of User
    }

    **Warning**

    Traversing the object graph for parts that are lazy-loaded will
    easily trigger lots of SQL queries and will perform badly if used
    to heavily. Make sure to use DQL to fetch-join all the parts of the
    object-graph that you need as efficiently as possible.


Persisting entities
-------------------

An entity can be made persistent by passing it to the
``EntityManager#persist($entity)`` method. By applying the persist
operation on some entity, that entity becomes MANAGED, which means
that its persistence is from now on managed by an EntityManager. As
a result the persistent state of such an entity will subsequently
be properly synchronized with the database when
``EntityManager#flush()`` is invoked.

    **CAUTION** Invoking the ``persist`` method on an entity does NOT
    cause an immediate SQL INSERT to be issued on the database.
    Doctrine applies a strategy called "transactional write-behind",
    which means that it will delay most SQL commands until
    ``EntityManager#flush()`` is invoked which will then issue all
    necessary SQL statements to synchronize your objects with the
    database in the most efficient way and a single, short transaction,
    taking care of maintaining referential integrity.


Example:

::

    <?php
    $user = new User;
    $user->setName('Mr.Right');
    $em->persist($user);
    $em->flush();

    **CAUTION** Generated entity identifiers / primary keys are
    guaranteed to be available after the next successful flush
    operation that involves the entity in question. You can not rely on
    a generated identifier to be available directly after invoking
    ``persist``. The inverse is also true. You can not rely on a
    generated identifier being not available after a failed flush
    operation.


The semantics of the persist operation, applied on an entity X, are
as follows:


-  If X is a new entity, it becomes managed. The entity X will be
   entered into the database as a result of the flush operation.
-  If X is a preexisting managed entity, it is ignored by the
   persist operation. However, the persist operation is cascaded to
   entities referenced by X, if the relationships from X to these
   other entities are mapped with cascade=PERSIST or cascade=ALL (see
   "Transitive Persistence").
-  If X is a removed entity, it becomes managed.
-  If X is a detached entity, an exception will be thrown on
   flush.

Removing entities
-----------------

An entity can be removed from persistent storage by passing it to
the ``EntityManager#remove($entity)`` method. By applying the
``remove`` operation on some entity, that entity becomes REMOVED,
which means that its persistent state will be deleted once
``EntityManager#flush()`` is invoked.

    **CAUTION** Just like ``persist``, invoking ``remove`` on an entity
    does NOT cause an immediate SQL DELETE to be issued on the
    database. The entity will be deleted on the next invocation of
    ``EntityManager#flush()`` that involves that entity.


Example:

::

    <?php
    $em->remove($user);
    $em->flush();

The semantics of the remove operation, applied to an entity X are
as follows:


-  If X is a new entity, it is ignored by the remove operation.
   However, the remove operation is cascaded to entities referenced by
   X, if the relationship from X to these other entities is mapped
   with cascade=REMOVE or cascade=ALL (see "Transitive Persistence").
-  If X is a managed entity, the remove operation causes it to
   become removed. The remove operation is cascaded to entities
   referenced by X, if the relationships from X to these other
   entities is mapped with cascade=REMOVE or cascade=ALL (see
   "Transitive Persistence").
-  If X is a detached entity, an InvalidArgumentException will be
   thrown.
-  If X is a removed entity, it is ignored by the remove operation.
-  A removed entity X will be removed from the database as a result
   of the flush operation.

After an entity has been removed its in-memory state is the same as
before the removal, except for generated identifiers.

Removing an entity will also automatically delete any existing
records in many-to-many join tables that link this entity. The
action taken depends on the value of the ``@joinColumn`` mapping
attribute "onDelete". Either Doctrine issues a dedicated ``DELETE``
statement for records of each join table or it depends on the
foreign key semantics of onDelete="CASCADE".

Deleting an object with all its associated objects can be achieved
in multiple ways with very different performance impacts.


1. If an association is marked as ``CASCADE=REMOVE`` Doctrine 2
   will fetch this association. If its a Single association it will
   pass this entity to
   ´EntityManager#remove()``. If the association is a collection, Doctrine will loop over all    its elements and pass them to``EntityManager#remove()\`.
   In both cases the cascade remove semantics are applied recursively.
   For large object graphs this removal strategy can be very costly.
2. Using a DQL ``DELETE`` statement allows you to delete multiple
   entities of a type with a single command and without hydrating
   these entities. This can be very efficient to delete large object
   graphs from the database.
3. Using foreign key semantics ``onDelete="CASCADE"`` can force the
   database to remove all associated objects internally. This strategy
   is a bit tricky to get right but can be very powerful and fast. You
   should be aware however that using strategy 1 (``CASCADE=REMOVE``)
   completely by-passes any foreign key ``onDelete=CASCADE`` option,
   because Doctrine will fetch and remove all associated entities
   explicitly nevertheless.

Detaching entities
------------------

An entity is detached from an EntityManager and thus no longer
managed by invoking the ``EntityManager#detach($entity)`` method on
it or by cascading the detach operation to it. Changes made to the
detached entity, if any (including removal of the entity), will not
be synchronized to the database after the entity has been
detached.

Doctrine will not hold on to any references to a detached entity.

Example:

::

    <?php
    $em->detach($entity);

The semantics of the detach operation, applied to an entity X are
as follows:


-  If X is a managed entity, the detach operation causes it to
   become detached. The detach operation is cascaded to entities
   referenced by X, if the relationships from X to these other
   entities is mapped with cascade=DETACH or cascade=ALL (see
   "Transitive Persistence"). Entities which previously referenced X
   will continue to reference X.
-  If X is a new or detached entity, it is ignored by the detach
   operation.
-  If X is a removed entity, the detach operation is cascaded to
   entities referenced by X, if the relationships from X to these
   other entities is mapped with cascade=DETACH or cascade=ALL (see
   "Transitive Persistence"). Entities which previously referenced X
   will continue to reference X.

There are several situations in which an entity is detached
automatically without invoking the ``detach`` method:


-  When ``EntityManager#clear()`` is invoked, all entities that are
   currently managed by the EntityManager instance become detached.
-  When serializing an entity. The entity retrieved upon subsequent
   unserialization will be detached (This is the case for all entities
   that are serialized and stored in some cache, i.e. when using the
   Query Result Cache).

The ``detach`` operation is usually not as frequently needed and
used as ``persist`` and ``remove``.

Merging entities
----------------

Merging entities refers to the merging of (usually detached)
entities into the context of an EntityManager so that they become
managed again. To merge the state of an entity into an
EntityManager use the ``EntityManager#merge($entity)`` method. The
state of the passed entity will be merged into a managed copy of
this entity and this copy will subsequently be returned.

Example:

::

    <?php
    $detachedEntity = unserialize($serializedEntity); // some detached entity
    $entity = $em->merge($detachedEntity);
    // $entity now refers to the fully managed copy returned by the merge operation.
    // The EntityManager $em now manages the persistence of $entity as usual.

    **CAUTION** When you want to serialize/unserialize entities you
    have to make all entity properties protected, never private. The
    reason for this is, if you serialize a class that was a proxy
    instance before, the private variables won't be serialized and a
    PHP Notice is thrown.


The semantics of the merge operation, applied to an entity X, are
as follows:


-  If X is a detached entity, the state of X is copied onto a
   pre-existing managed entity instance X' of the same identity.
-  If X is a new entity instance, a new managed copy X' will be
   created and the state of X is copied onto this managed instance.
-  If X is a removed entity instance, an InvalidArgumentException
   will be thrown.
-  If X is a managed entity, it is ignored by the merge operation,
   however, the merge operation is cascaded to entities referenced by
   relationships from X if these relationships have been mapped with
   the cascade element value MERGE or ALL (see "Transitive
   Persistence").
-  For all entities Y referenced by relationships from X having the
   cascade element value MERGE or ALL, Y is merged recursively as Y'.
   For all such Y referenced by X, X' is set to reference Y'. (Note
   that if X is managed then X is the same object as X'.)
-  If X is an entity merged to X', with a reference to another
   entity Y, where cascade=MERGE or cascade=ALL is not specified, then
   navigation of the same association from X' yields a reference to a
   managed object Y' with the same persistent identity as Y.

The ``merge`` operation will throw an ``OptimisticLockException``
if the entity being merged uses optimistic locking through a
version field and the versions of the entity being merged and the
managed copy don't match. This usually means that the entity has
been modified while being detached.

The ``merge`` operation is usually not as frequently needed and
used as ``persist`` and ``remove``. The most common scenario for
the ``merge`` operation is to reattach entities to an EntityManager
that come from some cache (and are therefore detached) and you want
to modify and persist such an entity.

    **NOTE** If you load some detached entities from a cache and you do
    not need to persist or delete them or otherwise make use of them
    without the need for persistence services there is no need to use
    ``merge``. I.e. you can simply pass detached objects from a cache
    directly to the view.


Synchronization with the Database
---------------------------------

The state of persistent entities is synchronized with the database
on flush of an ``EntityManager`` which commits the underlying
``UnitOfWork``. The synchronization involves writing any updates to
persistent entities and their relationships to the database.
Thereby bidirectional relationships are persisted based on the
references held by the owning side of the relationship as explained
in the Association Mapping chapter.

When ``EntityManager#flush()`` is called, Doctrine inspects all
managed, new and removed entities and will perform the following
operations.

Synchronizing New and Managed Entities
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The flush operation applies to a managed entity with the following
semantics:


-  The entity itself is synchronized to the database using a SQL
   UPDATE statement, only if at least one persistent field has
   changed.
-  No SQL updates are executed if the entity did not change.

The flush operation applies to a new entity with the following
semantics:


-  The entity itself is synchronized to the database using a SQL
   INSERT statement.

For all (initialized) relationships of the new or managed entity
the following semantics apply to each associated entity X:


-  If X is new and persist operations are configured to cascade on
   the relationship, X will be persisted.
-  If X is new and no persist operations are configured to cascade
   on the relationship, an exception will be thrown as this indicates
   a programming error.
-  If X is removed and persist operations are configured to cascade
   on the relationship, an exception will be thrown as this indicates
   a programming error (X would be re-persisted by the cascade).
-  If X is detached and persist operations are configured to
   cascade on the relationship, an exception will be thrown (This is
   semantically the same as passing X to persist()).

Synchronizing Removed Entities
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The flush operation applies to a removed entity by deleting its
persistent state from the database. No cascade options are relevant
for removed entities on flush, the cascade remove option is already
executed during ``EntityManager#remove($entity)``.

The size of a Unit of Work
~~~~~~~~~~~~~~~~~~~~~~~~~~

The size of a Unit of Work mainly refers to the number of managed
entities at a particular point in time.

The cost of flushing
~~~~~~~~~~~~~~~~~~~~

How costly a flush operation is, mainly depends on two factors:


-  The size of the EntityManager's current UnitOfWork.
-  The configured change tracking policies

You can get the size of a UnitOfWork as follows:

::

    <?php
    $uowSize = $em->getUnitOfWork()->size();

The size represents the number of managed entities in the Unit of
Work. This size affects the performance of flush() operations due
to change tracking (see "Change Tracking Policies") and, of course,
memory consumption, so you may want to check it from time to time
during development.

    **CAUTION** Do not invoke ``flush`` after every change to an entity
    or every single invocation of persist/remove/merge/... This is an
    anti-pattern and unnecessarily reduces the performance of your
    application. Instead, form units of work that operate on your
    objects and call ``flush`` when you are done. While serving a
    single HTTP request there should be usually no need for invoking
    ``flush`` more than 0-2 times.


Direct access to a Unit of Work
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

You can get direct access to the Unit of Work by calling
``EntityManager#getUnitOfWork()``. This will return the UnitOfWork
instance the EntityManager is currently using.

::

    <?php
    $uow = $em->getUnitOfWork();

    **NOTE** Directly manipulating a UnitOfWork is not recommended.
    When working directly with the UnitOfWork API, respect methods
    marked as INTERNAL by not using them and carefully read the API
    documentation.


Entity State
~~~~~~~~~~~~

As outlined in the architecture overview an entity can be in one of
four possible states: NEW, MANAGED, REMOVED, DETACHED. If you
explicitly need to find out what the current state of an entity is
in the context of a certain ``EntityManager`` you can ask the
underlying ``UnitOfWork``:

::

    <?php
    switch ($em->getUnitOfWork()->getEntityState($entity)) {
        case UnitOfWork::MANAGED:
            ...
        case UnitOfWork::REMOVED:
            ...
        case UnitOfWork::DETACHED:
            ...
        case UnitOfWork::NEW:
            ...
    }

An entity is in MANAGED state if it is associated with an
``EntityManager`` and it is not REMOVED.

An entity is in REMOVED state after it has been passed to
``EntityManager#remove()`` until the next flush operation of the
same EntityManager. A REMOVED entity is still associated with an
``EntityManager`` until the next flush operation.

An entity is in DETACHED state if it has persistent state and
identity but is currently not associated with an
``EntityManager``.

An entity is in NEW state if has no persistent state and identity
and is not associated with an ``EntityManager`` (for example those
just created via the "new" operator).

Querying
--------

Doctrine 2 provides the following ways, in increasing level of
power and flexibility, to query for persistent objects. You should
always start with the simplest one that suits your needs.

By Primary Key
~~~~~~~~~~~~~~

The most basic way to query for a persistent object is by its
identifier / primary key using the
``EntityManager#find($entityName, $id)`` method. Here is an
example:

::

    <?php
    // $em instanceof EntityManager
    $user = $em->find('MyProject\Domain\User', $id);

The return value is either the found entity instance or null if no
instance could be found with the given identifier.

Essentially, ``EntityManager#find()`` is just a shortcut for the
following:

::

    <?php
    // $em instanceof EntityManager
    $user = $em->getRepository('MyProject\Domain\User')->find($id);

``EntityManager#getRepository($entityName)`` returns a repository
object which provides many ways to retrieve entities of the
specified type. By default, the repository instance is of type
``Doctrine\ORM\EntityRepository``. You can also use custom
repository classes as shown later.

By Simple Conditions
~~~~~~~~~~~~~~~~~~~~

To query for one or more entities based on several conditions that
form a logical conjunction, use the ``findBy`` and ``findOneBy``
methods on a repository as follows:

::

    <?php
    // $em instanceof EntityManager
    
    // All users that are 20 years old
    $users = $em->getRepository('MyProject\Domain\User')->findBy(array('age' => 20));
    
    // All users that are 20 years old and have a surname of 'Miller'
    $users = $em->getRepository('MyProject\Domain\User')->findBy(array('age' => 20, 'surname' => 'Miller'));
    
    // A single user by its nickname
    $user = $em->getRepository('MyProject\Domain\User')->findOneBy(array('nickname' => 'romanb'));

An EntityRepository also provides a mechanism for more concise
calls through its use of ``__call``. Thus, the following two
examples are equivalent:

::

    <?php
    // A single user by its nickname
    $user = $em->getRepository('MyProject\Domain\User')->findOneBy(array('nickname' => 'romanb'));
    
    // A single user by its nickname (__call magic)
    $user = $em->getRepository('MyProject\Domain\User')->findOneByNickname('romanb');

By Eager Loading
~~~~~~~~~~~~~~~~

Whenever you query for an entity that has persistent associations
and these associations are mapped as EAGER, they will automatically
be loaded together with the entity being queried and is thus
immediately available to your application.

By Lazy Loading
~~~~~~~~~~~~~~~

Whenever you have a managed entity instance at hand, you can
traverse and use any associations of that entity that are
configured LAZY as if they were in-memory already. Doctrine will
automatically load the associated objects on demand through the
concept of lazy-loading.

By DQL
~~~~~~

The most powerful and flexible method to query for persistent
objects is the Doctrine Query Language, an object query language.
DQL enables you to query for persistent objects in the language of
objects. DQL understands classes, fields, inheritance and
associations. DQL is syntactically very similar to the familiar SQL
but *it is not SQL*.

A DQL query is represented by an instance of the
``Doctrine\ORM\Query`` class. You create a query using
``EntityManager#createQuery($dql)``. Here is a simple example:

::

    <?php
    // $em instanceof EntityManager
    
    // All users with an age between 20 and 30 (inclusive).
    $q = $em->createQuery("select u from MyDomain\Model\User u where u.age >= 20 and u.age <= 30");
    $users = $q->getResult();

Note that this query contains no knowledge about the relational
schema, only about the object model. DQL supports positional as
well as named parameters, many functions, (fetch) joins,
aggregates, subqueries and much more. Detailed information about
DQL and its syntax as well as the Doctrine class can be found in
`the dedicated chapter <http://www.doctrine-project.org/documentation/manual/2_0/en/dql-doctrine-query-language>`_.
For programmatically building up queries based on conditions that
are only known at runtime, Doctrine provides the special
``Doctrine\ORM\QueryBuilder`` class. More information on
constructing queries with a QueryBuilder can be found
`in the dedicated chapter <http://www.doctrine-project.org/documentation/manual/2_0/en/query-builder>`_.

By Native Queries
~~~~~~~~~~~~~~~~~

As an alternative to DQL or as a fallback for special SQL
statements native queries can be used. Native queries are built by
using a hand-crafted SQL query and a ResultSetMapping that
describes how the SQL result set should be transformed by Doctrine.
More information about native queries can be found in
`the dedicated chapter <http://www.doctrine-project.org/documentation/manual/2_0/en/native-sql>`_.

Custom Repositories
~~~~~~~~~~~~~~~~~~~

By default the EntityManager returns a default implementation of
``Doctrine\ORM\EntityRepository`` when you call
``EntityManager#getRepository($entityClass)``. You can overwrite
this behaviour by specifying the class name of your own Entity
Repository in the Annotation, XML or YAML metadata. In large
applications that require lots of specialized DQL queries using a
custom repository is one recommended way of grouping these queries
in a central location.

::

    <?php
    namespace MyDomain\Model;
    
    use Doctrine\ORM\EntityRepository;
    
    /**
     * @entity(repositoryClass="MyDomain\Model\UserRepository")
     */
    class User
    {
    
    }
    
    class UserRepository extends EntityRepository
    {
        public function getAllAdminUsers()
        {
            return $this->_em->createQuery('SELECT u FROM MyDomain\Model\User u WHERE u.status = "admin"')
                             ->getResult();
        }
    }

You can access your repository now by calling:

::

    <?php
    // $em instanceof EntityManager
    
    $admins = $em->getRepository('MyDomain\Model\User')->getAllAdminUsers();