<commit_before>++ Bootstrapping

Bootstrapping Doctrine is a relatively simple procedure that roughly exists of 
just 2 steps:

* Making sure Doctrine class files can be loaded on demand.
* Obtaining an EntityManager instance.

+++ Class loading

Lets start with the class loading setup. We need to set up some class loaders 
(often called "autoloader") so that Doctrine class files are loaded on demand. 
The Doctrine\Common namespace contains a very fast and minimalistic class loader 
that can be used for Doctrine and any other libraries where the coding standards 
ensure that a class's location in the directory tree is reflected by its name 
and namespace and where there is a common root namespace.

> **NOTE**
> You are not forced to use the Doctrine class loader to load Doctrine
> classes. Doctrine does not care how the classes are loaded, if you want to use a
> different class loader or your own to load Doctrine classes, just do that.
> Along the same lines, the class loader in the Doctrine\Common namespace is not
> meant to be only used for Doctrine classes, too. It is a generic class loader that can
> be used for any classes that follow some basic naming standards as described above.

The following example shows the setup of a `ClassLoader`

> **NOTE**
> This assumes you've created some kind of script to test the following code in.
> Something like a `test.php` file.

    [php]
    // test.php

    require '/path/to/lib/Doctrine/Common/ClassLoader.php';
    $classLoader = new \Doctrine\Common\ClassLoader('Doctrine', '/path/to/Doctrine2/lib);
    $classLoader->register(); // register on SPL autoload stack

For best class loading performance it is recommended that you keep your include_path short, ideally it should only contain the path to the PEAR libraries, and any other class libraries should be registered with their full base path.

+++ Obtaining an EntityManager

Once you have prepared the class loading, you acquire an EntityManager instance 
with the following minimalist configuration:

    [php]
    use Doctrine\ORM\EntityManager,
        Doctrine\ORM\Configuration;
    
    // ...
    
    $config = new Configuration;
    $cache = new \Doctrine\Common\Cache\ApcCache;
    $config->setMetadataCacheImpl($cache);
    $config->setQueryCacheImpl($cache);
    $config->setProxyDir('/path/to/myproject/lib/MyProject/Proxies');
    $config->setProxyNamespace('MyProject\Proxies');
    $connectionOptions = array(
        'driver' => 'pdo_sqlite',
        'path' => 'database.sqlite'
    );
    
    $em = EntityManager::create($connectionOptions, $config);

> **CAUTION**
> Do not use Doctrine without a metadata and query cache! Doctrine is highly
> optimized for working with caches. The main parts in Doctrine that are optimized
> for caching are the metadata mapping information with the metadata cache and the 
> DQL to SQL conversions with the query cache. These 2 caches require only an absolute
> minimum of memory yet they heavily improve the runtime performance of Doctrine.
> The recommended cache driver to use with Doctrine is [APC](http://www.php.net/apc). 
> APC provides you with an opcode-cache (which is highly recommended anyway) and 
> a very fast in-memory cache storage that you can use for the metadata and query
> caches as seen in the previous code snippet.

An EntityManager is your central access point to ORM functionality provided by Doctrine.

++ Configuration Options

The following sections describe all the configuration options available on a `Doctrine\ORM\Configuration` instance.

+++ Proxy Directory (***REQUIRED***)

    [php]
    $config->setProxyDir($dir);
    $config->getProxyDir();

Gets or sets the directory where Doctrine generates any proxy classes. For a detailed explanation on proxy classes and how they are used in Doctrine, refer to the "Proxy Objects" section further down.

+++ Proxy Namespace (***REQUIRED***)

    [php]
    $config->setProxyNamespace($namespace);
    $config->getProxyNamespace();

Gets or sets the namespace to use for generated proxy classes. For a detailed explanation on proxy classes and how they are used in Doctrine, refer to the "Proxy Objects" section further down.

+++ Metadata Driver (***REQUIRED***)

    [php]
    $config->setMetadataDriverImpl($driver);
    $config->getMetadataDriverImpl();

Gets or sets the metadata driver implementation that is used by Doctrine to acquire the object-relational metadata for your classes.

There are currently 4 available implementations:

 * `Doctrine\ORM\Mapping\Driver\AnnotationDriver`
 * `Doctrine\ORM\Mapping\Driver\XmlDriver`
 * `Doctrine\ORM\Mapping\Driver\YamlDriver`
 * `Doctrine\ORM\Mapping\Driver\DriverChain`
 
Throughout the most part of this manual the AnnotationDriver is used in the examples. For information on the usage of the XmlDriver or YamlDriver please refer to the dedicated chapters `XML Mapping` and `YAML Mapping`.

The annotation driver is configured as the default metadata driver, using Annotations as Metadata source
does not require you to set this option explicitly.

+++ Metadata Cache (***RECOMMENDED***)

    [php]
    $config->setMetadataCacheImpl($cache);
    $config->getMetadataCacheImpl();
    
Gets or sets the cache implementation to use for caching metadata information, that is, all the information you supply via annotations, xml or yaml, so that they do not need to be parsed and loaded from scratch on every single request which is a waste of resources.
The cache implementation must implement the `Doctrine\Common\Cache\Cache` interface.

Usage of a metadata cache is highly recommended.

The recommended implementations are:

 * `Doctrine\Common\Cache\ApcCache`
 * `Doctrine\Common\Cache\MemcacheCache`
 * `Doctrine\Common\Cache\XcacheCache`

+++ Query Cache (***RECOMMENDED***)

    [php]
    $config->setQueryCacheImpl($cache);
    $config->getQueryCacheImpl();
    
Gets or sets the cache implementation to use for caching DQL queries, that is, the result of a DQL parsing process that includes the final SQL as well as meta information about how to process the SQL result set of a query. Note that the query cache does not affect query results. You do not get stale data. This is a pure optimization cache without any negative side-effects (except some minimal memory usage in your cache).

Usage of a query cache is highly recommended.

The recommended implementations are:

 * `Doctrine\Common\Cache\ApcCache`
 * `Doctrine\Common\Cache\MemcacheCache`
 * `Doctrine\Common\Cache\XcacheCache`

+++ SQL Logger (***Optional***)

     [php]
     $config->setSqlLogger($logger);
     $config->getSqlLogger();

Gets or sets the logger to use for logging all SQL statements executed by Doctrine. The logger class must implement the `Doctrine\DBAL\Logging\SqlLogger` interface. A simple default implementation that logs to the standard output using `echo` and `var_dump` can be found at `Doctrine\DBAL\Logging\EchoSqlLogger`.

+++ Auto-generating Proxy Classes (***OPTIONAL***)

    [php]
    $config->setAutoGenerateProxyClasses($bool);
    $config->getAutoGenerateProxyClasses();

Gets or sets whether proxy classes should be generated automatically at runtime by Doctrine. If set to `FALSE`, proxy classes must be generated manually through the doctrine command line task `generate-proxies`. The strongly recommended value for a production environment is `FALSE`.

++ Connection Options

The `$connectionOptions` passed as the first argument to `EntityManager::create()` has to be either an array
or an instance of `Doctrine\DBAL\Connection`. If an array is passed it is directly passed along to the
DBAL Factory `Doctrine\DBAL\DriverManager::getConnection()`. The DBAL configuration is explained
in the [DBAL section](./../dbal).

++ Change Tracking Policies

Change tracking is the process of determining what has changed in managed 
entities since the last time they were synchronized with the database.

Doctrine provides 3 different change tracking policies, each having its 
particular advantages and disadvantages. The change tracking policy can 
be defined on a per-class basis (or more precisely, per-hierarchy).

+++ Deferred Implicit

The deferred implicit policy is the default change tracking policy and the most
 convenient one. With this policy, Doctrine detects the changes by a 
 property-by-property comparison at commit time and also detects changes 
 to entities or new entities that are referenced by other managed entities 
 ("persistence by reachability"). Although the most convenient policy, it can 
 have negative effects on performance if you are dealing with large units of 
 work (see "Understanding the Unit of Work"). Since Doctrine can't know what 
 has changed, it needs to check all managed entities for changes every time you 
 invoke EntityManager#flush(), making this operation rather costly.

+++ Deferred Explicit

The deferred explicit policy is similar to the deferred implicit policy in that 
it detects changes through a property-by-property comparison at commit time. The 
difference is that only entities are considered that have been explicitly marked 
for change detection through a call to EntityManager#persist(entity) or through 
a save cascade. All other entities are skipped. This policy therefore gives 
improved performance for larger units of work while sacrificing the behavior 
of "automatic dirty checking". 

Therefore, flush() operations are potentially cheaper with this policy. The 
negative aspect this has is that if you have a rather large application and 
you pass your objects through several layers for processing purposes and 
business tasks you may need to track yourself which entities have changed 
on the way so you can pass them to EntityManager#persist().

This policy can be configured as follows:

    /**
     * @Entity
     * @ChangeTrackingPolicy("DEFERRED_EXPLICIT")
     */
    class User
    {
        // ...
    }

+++ Notify

This policy is based on the assumption that the entities notify interested 
listeners of changes to their properties. For that purpose, a class that 
wants to use this policy needs to implement the NotifyPropertyChanged? 
interface from the Doctrine\Common namespace. As a guideline, such an 
implementation should look as follows:

    [php]
    use Doctrine\Common\NotifyPropertyChanged,
        Doctrine\Common\PropertyChangedListener;
    
    /**
     * @Entity
     * @ChangeTrackingPolicy("NOTIFY")
     */
    class MyEntity implements NotifyPropertyChanged
    {
        // ...

        private $_listeners = array();

        public function addPropertyChangedListener(PropertyChangedListener $listener)
        {
            $this->_listeners[] = $listener;
        }

        protected function _onPropertyChanged($propName, $oldValue, $newValue)
        {
            if ($this->_listeners) {
                foreach ($this->_listeners as $listener) {
                    $listener->propertyChanged($this, $propName, $oldValue, $newValue);
                }
            }
        }
    }

Then, in each property setter of this class or derived classes, you need to 
invoke `_onPropertyChanged` as follows to notify listeners:

    [php]
    <?php
    // ...

    class MyEntity implements NotifyPropertyChanged
    {
        // ...

        public function setData($data)
        {
            if ($data != $this->data) {
                $this->_onPropertyChanged('data', $this->data, $data);
                $this->data = $data;
            }
        }
    }

The check whether the new value is different from the old one is not mandatory 
but recommended. That way you also have full control over when you consider a 
property changed.

The negative point of this policy is obvious: You need implement an interface 
and write some plumbing code. But also note that we tried hard to keep this 
notification functionality abstract. Strictly speaking, it has nothing to do 
with the persistence layer and the Doctrine ORM or DBAL. You may find that 
property notification events come in handy in many other scenarios as well. 
As mentioned earlier, the `Doctrine\Common` namespace is not that evil and 
consists solely of very small classes and interfaces that have almost no 
external dependencies (none to the DBAL and none to the ORM) and that you 
can easily take with you should you want to swap out the persistence layer. 
This change tracking policy does not introduce a dependency on the Doctrine 
DBAL/ORM or the persistence layer.

The positive point and main advantage of this policy is its effectiveness. It 
has the best performance characteristics of the 3 policies with larger units of 
work and a flush() operation is very cheap when nothing has changed.

++ Partial Objects

A partial object is an object whose state is not fully initialized after being 
reconstituted from the database and that is disconnected from the rest of its data. The following section will describe why partial objects are problematic and what the approach of Doctrine2 to this problem is.

> **NOTE**
> The partial object problem in general does not apply to methods or
> queries where you do not retrieve the query result as objects. Examples are:
> `Query#getArrayResult()`, `Query#getScalarResult()`, `Query#getSingleScalarResult()`,
> etc.

+++ What is the problem?

In short, partial objects are problematic because they are usually objects with 
broken invariants. As such, code that uses these partial objects tends to be 
very fragile and either needs to "know" which fields or methods can be safely 
accessed or add checks around every field access or method invocation. The same 
holds true for the internals, i.e. the method implementations, of such objects. 
You usually simply assume the state you need in the method is available, after 
all you properly constructed this object before you pushed it into the database, 
right? These blind assumptions can quickly lead to null reference errors when 
working with such partial objects.

It gets worse with the scenario of an optional association (0..1 to 1). When 
the associated field is NULL, you dont know whether this object does not have 
an associated object or whether it was simply not loaded when the owning object 
was loaded from the database. 

These are reasons why many ORMs do not allow partial objects at all and instead 
you always have to load an object with all its fields (associations being proxied). 
One secure way to allow partial objects is if the programming language/platform 
allows the ORM tool to hook deeply into the object and instrument it in such a 
way that individual fields (not only associations) can be loaded lazily on first 
access. This is possible in Java, for example, through bytecode instrumentation. 
In PHP though this is not possible, so there is no way to have "secure" partial 
objects in an ORM with transparent persistence.

Doctrine, by default, does not allow partial objects. That means, any query that only selects partial object data and wants to retrieve the result as objects 
(i.e. `Query#getResult()`) will raise an exception telling you that 
partial objects are dangerous. If you want to force a query to return you partial objects, possibly as a performance tweak, you can use the `Query#HINT_FORCE_PARTIAL_LOAD` query hint as follows:

    [php]
    $q = $em->createQuery("select u.id, u.name from MyApp\Domain\User u");
    $q->setHint(Query::HINT_FORCE_PARTIAL_LOAD, true);

+++ When should I force partial objects?

Mainly for optimization purposes, especially since the stateless nature of PHP 
applications means that any fields or objects that are loaded unnecessarily in 
a request are useless (though often minimal) overhead. Be careful of premature optimization. Only force partial objects if it proves to provide an improvement to a performance problem.

++ Proxy Objects

A proxy object is an object that is put in place or used instead of the "real" object. A proxy object can add behavior to the object being proxied without that object being aware of it. In Doctrine 2, proxy objects are used to realize several features but mainly for transparent lazy-loading.

Proxy objects with their lazy-loading facilities help to keep the subset of objects that are already in memory connected to the rest of the objects. This is an essential property as without it there would always be fragile partial objects at the outer edges of your object graph.

Doctrine 2 implements a variant of the proxy pattern where it generates classes that extend your entity classes and adds lazy-loading capabilities to them. Doctrine can then give you an instance of such a proxy class whenever you request an object of the class being proxied. This happens in two situations:

**Reference Proxies**

The method `EntityManager#getReference($entityName, $identifier)` lets you obtain a reference to an entity for which the identifier is known, without loading that entity from the database. This is useful, for example, as a performance enhancement, when you want to establish an association to an entity for which you have the identifier. You could simply do this:

    [php]
    // $em instanceof EntityManager, $cart instanceof MyProject\Model\Cart
    // $itemId comes from somewhere, probably a request parameter
    $item = $em->getReference('MyProject\Model\Item', $itemId);
    $cart->addItem($item);

Here, we added an Item to a Cart without loading the Item from the database. If you invoke any method on the Item instance, it would fully initialize its state transparently from the database. Here $item is actually an instance of the proxy class that was generated for the Item class but your code does not need to care. In fact it **should not care**. Proxy objects should be transparent to your code.

**Association proxies**

The second most important situation where Doctrine uses proxy objects is when querying for objects. Whenever you query for an object that has a single-valued association to another object that is configured LAZY, without joining that association in the same query, Doctrine puts proxy objects in place where normally the associated object would be.
Just like other proxies it will transparently initialize itself on first access.

> **NOTE**
> Joining an association in a DQL or native query essentially means eager loading of that
> association in that query. This will override the 'fetch' option specified in
> the mapping for that association, but only for that query.

+++ Generating Proxy classes

Proxy classes can either be generated manually through the Doctrine Console or automatically by Doctrine. The configuration option that controls this behavior is:

    [php]
    $config->setAutoGenerateProxyClasses($bool);
    $config->getAutoGenerateProxyClasses();
    
The default value is `TRUE` for convenient development. However, this setting is not optimal for performance and therefore not recommended for a production environment.
To eliminate the overhead of proxy class generation during runtime, set this configuration option to `FALSE`. When you do this in a development environment, note that you may get class/file not found errors if certain proxy classes are not available or failing lazy-loads if new methods were added to the entity class that are not yet in the proxy class. In such a case, simply use the Doctrine Console to (re)generate the proxy classes like so:

    doctrine orm:generate-proxies

++ Multiple Metadata Sources

When using different components using Doctrine 2 you may end up with them using two different metadata drivers,
for example XML and YAML. You can use the DriverChain Metadata implementations to aggregate these drivers
based on namespaces:

    [php]
    $chain = new DriverChain();
    $chain->addDriver($xmlDriver, 'Doctrine\Tests\Models\Company');
    $chain->addDriver($yamlDriver, 'Doctrine\Tests\ORM\Mapping');

Based on the namespace of the entity the loading of entities is delegated to the appropriate driver. The chain
semantics come from the fact that the driver loops through all namespaces and matches the entity class name
against the namespace using a `strpos() === 0` call. This means you need to order the drivers correctly if
sub-namespaces use different metadata driver implementations.<commit_msg>DDC-510 - Updated docs to reflect that Annotations Metadata Driver is not a default anymore, but required to specifiy<commit_after>++ Bootstrapping

Bootstrapping Doctrine is a relatively simple procedure that roughly exists of 
just 2 steps:

* Making sure Doctrine class files can be loaded on demand.
* Obtaining an EntityManager instance.

+++ Class loading

Lets start with the class loading setup. We need to set up some class loaders 
(often called "autoloader") so that Doctrine class files are loaded on demand. 
The Doctrine\Common namespace contains a very fast and minimalistic class loader 
that can be used for Doctrine and any other libraries where the coding standards 
ensure that a class's location in the directory tree is reflected by its name 
and namespace and where there is a common root namespace.

> **NOTE**
> You are not forced to use the Doctrine class loader to load Doctrine
> classes. Doctrine does not care how the classes are loaded, if you want to use a
> different class loader or your own to load Doctrine classes, just do that.
> Along the same lines, the class loader in the Doctrine\Common namespace is not
> meant to be only used for Doctrine classes, too. It is a generic class loader that can
> be used for any classes that follow some basic naming standards as described above.

The following example shows the setup of a `ClassLoader`

> **NOTE**
> This assumes you've created some kind of script to test the following code in.
> Something like a `test.php` file.

    [php]
    // test.php

    require '/path/to/lib/Doctrine/Common/ClassLoader.php';
    $classLoader = new \Doctrine\Common\ClassLoader('Doctrine', '/path/to/Doctrine2/lib);
    $classLoader->register(); // register on SPL autoload stack

For best class loading performance it is recommended that you keep your include_path short, ideally it should only contain the path to the PEAR libraries, and any other class libraries should be registered with their full base path.

+++ Obtaining an EntityManager

Once you have prepared the class loading, you acquire an EntityManager instance 
with the following minimalist configuration:

    [php]
    use Doctrine\ORM\EntityManager,
        Doctrine\ORM\Configuration;
    
    // ...
    
    $config = new Configuration;
    $cache = new \Doctrine\Common\Cache\ApcCache;
    $config->setMetadataCacheImpl($cache);
    $driverImpl = $config->newDefaultAnnotationDriver('/path/to/lib/MyProject/Entities');
    $config->setMetadataDriverImpl($driverImpl);
    $config->setQueryCacheImpl($cache);
    $config->setProxyDir('/path/to/myproject/lib/MyProject/Proxies');
    $config->setProxyNamespace('MyProject\Proxies');
    $connectionOptions = array(
        'driver' => 'pdo_sqlite',
        'path' => 'database.sqlite'
    );
    
    $em = EntityManager::create($connectionOptions, $config);

> **CAUTION**
> Do not use Doctrine without a metadata and query cache! Doctrine is highly
> optimized for working with caches. The main parts in Doctrine that are optimized
> for caching are the metadata mapping information with the metadata cache and the 
> DQL to SQL conversions with the query cache. These 2 caches require only an absolute
> minimum of memory yet they heavily improve the runtime performance of Doctrine.
> The recommended cache driver to use with Doctrine is [APC](http://www.php.net/apc). 
> APC provides you with an opcode-cache (which is highly recommended anyway) and 
> a very fast in-memory cache storage that you can use for the metadata and query
> caches as seen in the previous code snippet.

An EntityManager is your central access point to ORM functionality provided by Doctrine.

++ Configuration Options

The following sections describe all the configuration options available on a `Doctrine\ORM\Configuration` instance.

+++ Proxy Directory (***REQUIRED***)

    [php]
    $config->setProxyDir($dir);
    $config->getProxyDir();

Gets or sets the directory where Doctrine generates any proxy classes. For a detailed explanation on proxy classes and how they are used in Doctrine, refer to the "Proxy Objects" section further down.

+++ Proxy Namespace (***REQUIRED***)

    [php]
    $config->setProxyNamespace($namespace);
    $config->getProxyNamespace();

Gets or sets the namespace to use for generated proxy classes. For a detailed explanation on proxy classes and how they are used in Doctrine, refer to the "Proxy Objects" section further down.

+++ Metadata Driver (***REQUIRED***)

    [php]
    $config->setMetadataDriverImpl($driver);
    $config->getMetadataDriverImpl();

Gets or sets the metadata driver implementation that is used by Doctrine to acquire the object-relational metadata for your classes.

There are currently 4 available implementations:

 * `Doctrine\ORM\Mapping\Driver\AnnotationDriver`
 * `Doctrine\ORM\Mapping\Driver\XmlDriver`
 * `Doctrine\ORM\Mapping\Driver\YamlDriver`
 * `Doctrine\ORM\Mapping\Driver\DriverChain`
 
Throughout the most part of this manual the AnnotationDriver is used in the examples. For information on the usage of the XmlDriver or YamlDriver please refer to the dedicated chapters `XML Mapping` and `YAML Mapping`.

The annotation driver can be configured with a factory method on the `Doctrine\ORM\Configuration`:

    [php]
    $driverImpl = $config->newDefaultAnnotationDriver('/path/to/lib/MyProject/Entities');
    $config->setMetadataDriverImpl($driverImpl);

The path information to the entities is requied for the annotation driver, because otherwise
mass-operations on all entities through the console could not work correctly.

+++ Metadata Cache (***RECOMMENDED***)

    [php]
    $config->setMetadataCacheImpl($cache);
    $config->getMetadataCacheImpl();
    
Gets or sets the cache implementation to use for caching metadata information, that is, all the information you supply via annotations, xml or yaml, so that they do not need to be parsed and loaded from scratch on every single request which is a waste of resources.
The cache implementation must implement the `Doctrine\Common\Cache\Cache` interface.

Usage of a metadata cache is highly recommended.

The recommended implementations are:

 * `Doctrine\Common\Cache\ApcCache`
 * `Doctrine\Common\Cache\MemcacheCache`
 * `Doctrine\Common\Cache\XcacheCache`

+++ Query Cache (***RECOMMENDED***)

    [php]
    $config->setQueryCacheImpl($cache);
    $config->getQueryCacheImpl();
    
Gets or sets the cache implementation to use for caching DQL queries, that is, the result of a DQL parsing process that includes the final SQL as well as meta information about how to process the SQL result set of a query. Note that the query cache does not affect query results. You do not get stale data. This is a pure optimization cache without any negative side-effects (except some minimal memory usage in your cache).

Usage of a query cache is highly recommended.

The recommended implementations are:

 * `Doctrine\Common\Cache\ApcCache`
 * `Doctrine\Common\Cache\MemcacheCache`
 * `Doctrine\Common\Cache\XcacheCache`

+++ SQL Logger (***Optional***)

     [php]
     $config->setSQLLogger($logger);
     $config->getSQLLogger();

Gets or sets the logger to use for logging all SQL statements executed by Doctrine. The logger class must implement the `Doctrine\DBAL\Logging\SqlLogger` interface. A simple default implementation that logs to the standard output using `echo` and `var_dump` can be found at `Doctrine\DBAL\Logging\EchoSqlLogger`.

+++ Auto-generating Proxy Classes (***OPTIONAL***)

    [php]
    $config->setAutoGenerateProxyClasses($bool);
    $config->getAutoGenerateProxyClasses();

Gets or sets whether proxy classes should be generated automatically at runtime by Doctrine. If set to `FALSE`, proxy classes must be generated manually through the doctrine command line task `generate-proxies`. The strongly recommended value for a production environment is `FALSE`.

++ Connection Options

The `$connectionOptions` passed as the first argument to `EntityManager::create()` has to be either an array
or an instance of `Doctrine\DBAL\Connection`. If an array is passed it is directly passed along to the
DBAL Factory `Doctrine\DBAL\DriverManager::getConnection()`. The DBAL configuration is explained
in the [DBAL section](./../dbal).

++ Change Tracking Policies

Change tracking is the process of determining what has changed in managed 
entities since the last time they were synchronized with the database.

Doctrine provides 3 different change tracking policies, each having its 
particular advantages and disadvantages. The change tracking policy can 
be defined on a per-class basis (or more precisely, per-hierarchy).

+++ Deferred Implicit

The deferred implicit policy is the default change tracking policy and the most
 convenient one. With this policy, Doctrine detects the changes by a 
 property-by-property comparison at commit time and also detects changes 
 to entities or new entities that are referenced by other managed entities 
 ("persistence by reachability"). Although the most convenient policy, it can 
 have negative effects on performance if you are dealing with large units of 
 work (see "Understanding the Unit of Work"). Since Doctrine can't know what 
 has changed, it needs to check all managed entities for changes every time you 
 invoke EntityManager#flush(), making this operation rather costly.

+++ Deferred Explicit

The deferred explicit policy is similar to the deferred implicit policy in that 
it detects changes through a property-by-property comparison at commit time. The 
difference is that only entities are considered that have been explicitly marked 
for change detection through a call to EntityManager#persist(entity) or through 
a save cascade. All other entities are skipped. This policy therefore gives 
improved performance for larger units of work while sacrificing the behavior 
of "automatic dirty checking". 

Therefore, flush() operations are potentially cheaper with this policy. The 
negative aspect this has is that if you have a rather large application and 
you pass your objects through several layers for processing purposes and 
business tasks you may need to track yourself which entities have changed 
on the way so you can pass them to EntityManager#persist().

This policy can be configured as follows:

    /**
     * @Entity
     * @ChangeTrackingPolicy("DEFERRED_EXPLICIT")
     */
    class User
    {
        // ...
    }

+++ Notify

This policy is based on the assumption that the entities notify interested 
listeners of changes to their properties. For that purpose, a class that 
wants to use this policy needs to implement the NotifyPropertyChanged? 
interface from the Doctrine\Common namespace. As a guideline, such an 
implementation should look as follows:

    [php]
    use Doctrine\Common\NotifyPropertyChanged,
        Doctrine\Common\PropertyChangedListener;
    
    /**
     * @Entity
     * @ChangeTrackingPolicy("NOTIFY")
     */
    class MyEntity implements NotifyPropertyChanged
    {
        // ...

        private $_listeners = array();

        public function addPropertyChangedListener(PropertyChangedListener $listener)
        {
            $this->_listeners[] = $listener;
        }

        protected function _onPropertyChanged($propName, $oldValue, $newValue)
        {
            if ($this->_listeners) {
                foreach ($this->_listeners as $listener) {
                    $listener->propertyChanged($this, $propName, $oldValue, $newValue);
                }
            }
        }
    }

Then, in each property setter of this class or derived classes, you need to 
invoke `_onPropertyChanged` as follows to notify listeners:

    [php]
    <?php
    // ...

    class MyEntity implements NotifyPropertyChanged
    {
        // ...

        public function setData($data)
        {
            if ($data != $this->data) {
                $this->_onPropertyChanged('data', $this->data, $data);
                $this->data = $data;
            }
        }
    }

The check whether the new value is different from the old one is not mandatory 
but recommended. That way you also have full control over when you consider a 
property changed.

The negative point of this policy is obvious: You need implement an interface 
and write some plumbing code. But also note that we tried hard to keep this 
notification functionality abstract. Strictly speaking, it has nothing to do 
with the persistence layer and the Doctrine ORM or DBAL. You may find that 
property notification events come in handy in many other scenarios as well. 
As mentioned earlier, the `Doctrine\Common` namespace is not that evil and 
consists solely of very small classes and interfaces that have almost no 
external dependencies (none to the DBAL and none to the ORM) and that you 
can easily take with you should you want to swap out the persistence layer. 
This change tracking policy does not introduce a dependency on the Doctrine 
DBAL/ORM or the persistence layer.

The positive point and main advantage of this policy is its effectiveness. It 
has the best performance characteristics of the 3 policies with larger units of 
work and a flush() operation is very cheap when nothing has changed.

++ Partial Objects

A partial object is an object whose state is not fully initialized after being 
reconstituted from the database and that is disconnected from the rest of its data. The following section will describe why partial objects are problematic and what the approach of Doctrine2 to this problem is.

> **NOTE**
> The partial object problem in general does not apply to methods or
> queries where you do not retrieve the query result as objects. Examples are:
> `Query#getArrayResult()`, `Query#getScalarResult()`, `Query#getSingleScalarResult()`,
> etc.

+++ What is the problem?

In short, partial objects are problematic because they are usually objects with 
broken invariants. As such, code that uses these partial objects tends to be 
very fragile and either needs to "know" which fields or methods can be safely 
accessed or add checks around every field access or method invocation. The same 
holds true for the internals, i.e. the method implementations, of such objects. 
You usually simply assume the state you need in the method is available, after 
all you properly constructed this object before you pushed it into the database, 
right? These blind assumptions can quickly lead to null reference errors when 
working with such partial objects.

It gets worse with the scenario of an optional association (0..1 to 1). When 
the associated field is NULL, you dont know whether this object does not have 
an associated object or whether it was simply not loaded when the owning object 
was loaded from the database. 

These are reasons why many ORMs do not allow partial objects at all and instead 
you always have to load an object with all its fields (associations being proxied). 
One secure way to allow partial objects is if the programming language/platform 
allows the ORM tool to hook deeply into the object and instrument it in such a 
way that individual fields (not only associations) can be loaded lazily on first 
access. This is possible in Java, for example, through bytecode instrumentation. 
In PHP though this is not possible, so there is no way to have "secure" partial 
objects in an ORM with transparent persistence.

Doctrine, by default, does not allow partial objects. That means, any query that only selects partial object data and wants to retrieve the result as objects 
(i.e. `Query#getResult()`) will raise an exception telling you that 
partial objects are dangerous. If you want to force a query to return you partial objects, possibly as a performance tweak, you can use the `Query#HINT_FORCE_PARTIAL_LOAD` query hint as follows:

    [php]
    $q = $em->createQuery("select u.id, u.name from MyApp\Domain\User u");
    $q->setHint(Query::HINT_FORCE_PARTIAL_LOAD, true);

+++ When should I force partial objects?

Mainly for optimization purposes, especially since the stateless nature of PHP 
applications means that any fields or objects that are loaded unnecessarily in 
a request are useless (though often minimal) overhead. Be careful of premature optimization. Only force partial objects if it proves to provide an improvement to a performance problem.

++ Proxy Objects

A proxy object is an object that is put in place or used instead of the "real" object. A proxy object can add behavior to the object being proxied without that object being aware of it. In Doctrine 2, proxy objects are used to realize several features but mainly for transparent lazy-loading.

Proxy objects with their lazy-loading facilities help to keep the subset of objects that are already in memory connected to the rest of the objects. This is an essential property as without it there would always be fragile partial objects at the outer edges of your object graph.

Doctrine 2 implements a variant of the proxy pattern where it generates classes that extend your entity classes and adds lazy-loading capabilities to them. Doctrine can then give you an instance of such a proxy class whenever you request an object of the class being proxied. This happens in two situations:

**Reference Proxies**

The method `EntityManager#getReference($entityName, $identifier)` lets you obtain a reference to an entity for which the identifier is known, without loading that entity from the database. This is useful, for example, as a performance enhancement, when you want to establish an association to an entity for which you have the identifier. You could simply do this:

    [php]
    // $em instanceof EntityManager, $cart instanceof MyProject\Model\Cart
    // $itemId comes from somewhere, probably a request parameter
    $item = $em->getReference('MyProject\Model\Item', $itemId);
    $cart->addItem($item);

Here, we added an Item to a Cart without loading the Item from the database. If you invoke any method on the Item instance, it would fully initialize its state transparently from the database. Here $item is actually an instance of the proxy class that was generated for the Item class but your code does not need to care. In fact it **should not care**. Proxy objects should be transparent to your code.

**Association proxies**

The second most important situation where Doctrine uses proxy objects is when querying for objects. Whenever you query for an object that has a single-valued association to another object that is configured LAZY, without joining that association in the same query, Doctrine puts proxy objects in place where normally the associated object would be.
Just like other proxies it will transparently initialize itself on first access.

> **NOTE**
> Joining an association in a DQL or native query essentially means eager loading of that
> association in that query. This will override the 'fetch' option specified in
> the mapping for that association, but only for that query.

+++ Generating Proxy classes

Proxy classes can either be generated manually through the Doctrine Console or automatically by Doctrine. The configuration option that controls this behavior is:

    [php]
    $config->setAutoGenerateProxyClasses($bool);
    $config->getAutoGenerateProxyClasses();
    
The default value is `TRUE` for convenient development. However, this setting is not optimal for performance and therefore not recommended for a production environment.
To eliminate the overhead of proxy class generation during runtime, set this configuration option to `FALSE`. When you do this in a development environment, note that you may get class/file not found errors if certain proxy classes are not available or failing lazy-loads if new methods were added to the entity class that are not yet in the proxy class. In such a case, simply use the Doctrine Console to (re)generate the proxy classes like so:

    doctrine orm:generate-proxies

++ Multiple Metadata Sources

When using different components using Doctrine 2 you may end up with them using two different metadata drivers,
for example XML and YAML. You can use the DriverChain Metadata implementations to aggregate these drivers
based on namespaces:

    [php]
    $chain = new DriverChain();
    $chain->addDriver($xmlDriver, 'Doctrine\Tests\Models\Company');
    $chain->addDriver($yamlDriver, 'Doctrine\Tests\ORM\Mapping');

Based on the namespace of the entity the loading of entities is delegated to the appropriate driver. The chain
semantics come from the fact that the driver loops through all namespaces and matches the entity class name
against the namespace using a `strpos() === 0` call. This means you need to order the drivers correctly if
sub-namespaces use different metadata driver implementations.
{'fraction_non_alphanumeric': 0.0579374558217759, 'fraction_numerical': 0.0007555999707509689, 'mean_word_length': 4.124016485575122, 'pattern_counts': {'":': 0, '<': 5, '<?xml version=': 0, '>': 139, 'https://': 0, 'lorem ipsum': 0, 'www.': 2, 'xml': 4}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
{'dir': 'git-commits-cleaned', 'id': '6171966', 'n_tokens_mistral': 9995, 'n_tokens_neox': 9710, 'n_words': 5425}