doctrine2/cookbook/en/getting-started-xml-edition.txt

Doctrine 2 is a project that aims to handle the persistence of the domain model in a non-interfering way.
The Data Mapper pattern is at the heart of this project, aiming for a complete separation of the domain/business logic
from the persistence in a relational database management system. The benefit of Doctrine for the programmer is the
possibility can focus soley on the business and worry about persistence only as a secondary task. This doesn't mean
persistence is not important to Doctrine 2, however it is our belief that there are considerable benefits for object-oriented
programming, if persistence and entities are kept perfectly seperated.

## What are Entities?

Entities are leightweight PHP Objects that don't need to extend any abstract base class or interface.
An entity class must not be final or contain final methods. Additionally it must not implement __clone
nor __wakeup or [do so safely](http://www.doctrine-project.org/documentation/cookbook/2_0/en/implementing-wakeup-or-clone).

An entity contains persistable properties. A persistable property is an instance variable of the entity
that contains the data which is persisted and retrieved by Doctrine's data mapping capabilities.

## An Example Model: Bug Tracker

For this Getting Started Guide for Doctrine we will implement the Bug Tracker domain model from the [Zend_Db_Table](http://framework.zend.com/manual/en/zend.db.table.html)
documentation. Reading their documentation we can extract the requirements to be:

* A Bugs has a description, creation date, status, reporter and engineer
* A bug can occour on different products (platforms)
* Products have a name.
* Bug Reporter and Engineers are both Users of the System.
* A user can create new bugs.
* The assigned engineer can close a bug.
* A user can see all his reported or assigned bugs.
* Bugs can be paginated through a list-view.

> **WARNING**
>
> This tutorial is incrementally building up your Doctrine 2 knowledge and even lets you make some mistakes, to
> show some common pitfalls in mapping Entities to a database. Don't blindly copy-paste the examples here, it
> is not production ready without the additional comments and knowledge this tutorial teaches.

## A first prototype

A first simplified design for this domain model might look like the following set of classes:

    [php]
    class Bug
    {
        public $id;
        public $description;
        public $created;
        public $status;
        public $products = array();
        public $reporter;
        public $engineer;
    }
    class Product
    {
        public $id;
        public $name;
    }
    class User
    {
        public $id;
        public $name;
        public $reportedBugs = array();
        public $assignedBugs = array();
    }

> **WARNING**
>
> This is only a prototype, please don't use public properties with Doctrine 2 at all,
> the "Queries for Application Use-Cases" section shows you why. In combination with proxies
> public properties can make up for pretty nasty bugs.

Because we will focus on the mapping aspect, no effort is being made to encapsulate the business logic in this example.
All peristable properties are public in visibility. We will soon see that this is not the best solution in combination
with Doctrine 2, one restriction that actually forces you to encapsulate your properties. For persistence Doctrine 2
actually uses Reflection to access the values in all your entities properties.

Many of the fields are single scalar values, for example the 3 ID fields of the entities, their names, description,
status and change dates. Doctrine 2 can easily handle these single values as can any other ORM. From a point of our
domain model they are ready to be used right now and we will see at a later stage how they are mapped to the database.

There are also several references between objects in this domain model, whose semantics are discussed case by case at this point
to explain how Doctrine handles them. In general each OneToOne or ManyToOne Relation in the Database is replaced by an
instance of the related object in the domain model. Each OneToMany or ManyToMany Relation is replaced by a collection
of instances in the domain model.

If you think this through carefully you realize Doctrine 2 will load up the complete database in memory if you access
one object. However by default Doctrine generates Lazy Load proxies of entities or collections of all the relations
that haven't been explicitly retrieved from the database yet.

To be able to use lazyload with collections, simple PHP arrays have to be replaced by a generic collection
interface Doctrine\Common\Collections\Collection which tries to act as array as much as possible using ArrayAccess,
IteratorAggregate and Countable interfaces. The class \Doctrine\Common\Collections\ArrayCollection is the most simple
implementation of this interface.

Now that we know this, we have to clear up our domain model to cope with the assumptions about related collections:

    [php]
    use Doctrine\Common\Collections\ArrayCollection;

    class Bug
    {
        public $products = null;

        public function __construct()
        {
            $this->products = new ArrayCollection();
        }
    }

    class User
    {
        public $reportedBugs = null;
        public $assignedBugs = null;

        public function __construct()
        {
            $this->reportedBugs = new ArrayCollection();
            $this->assignedBugs = new ArrayCollection();
        }
    }

Whenever an entity is recreated from the database, an Collection implementation of the type
Doctrine\ORM\PersistantCollection is injected into your entity instead of an array. Compared
to the ArrayCollection this implementation helps the Doctrine ORM understand the changes that
have happend to the collection which are noteworthy for persistence.

> **Warning**
> Lazy load proxies always contain an instance of Doctrine's EntityManager and all its dependencies. Therefore a var_dump()
> will possibly dump a very large recursive structure which is impossible to render and read. You have to use
> `Doctrine\Common\Util\Debug::dump()` to restrict the dumping to a human readable level. Additionally you should be aware
> that dumping the EntityManager to a Browser may take several minutes, and the Debug::dump() method just ignores any
> occurences of it in Proxy instances.

Because we only work with collections for the references we must be careful to implement a bidirectional reference in
the domain model. The concept of owning or inverse side of a relation is central to this notion and should always
be kept in mind. The following assumptions are made about relations and have to be followed to be able to work with Doctrine 2.
These assumptions are not unique to Doctrine 2 but are best practices in handling database relations and Object-Relational Mapping.

* Changes to Collections are saved or updated, when the entity on the *ownin*g side of the collection is saved or updated.
* Saving an Entity at the inverse side of a relation never triggers a persist operation to changes to the collection.
* In a one-to-one relation the entity holding the foreign key of the related entity on its own database table is *always* the owning side of the relation.
* In a many-to-many relation, both sides can be the owning side of the relation. However in a bi-directional many-tomany relation only one is allowed to be.
* In a many-to-one relation the Many-side is the owning side by default, because it holds the foreign key.
* The OneToMany side of a relation is inverse by default, since the foreign key is saved on the Many side. A OneToMany relation can only be the owning side, if its implemented using a ManyToMany relation with join table and restricting the one side to allow only UNIQUE values per database constraint.

> **Important**
>
> Consistency of bi-directional references on the inverse side of a relation have to be managed in userland application code.
> Doctrine cannot magically update your collections to be consistent.

In the case of Users and Bugs we have references back and forth to the assigned and reported bugs from a user,
making this relation bi-directional. We have to change the code to ensure consistency of the bi-directional reference:

    [php]
    class Bug
    {
        protected $engineer;
        protected $reporter;

        public function setEngineer($engineer)
        {
            $engineer->assignedToBug($this);
            $this->engineer = $engineer;
        }

        public function setReporter($reporter)
        {
            $reporter->addReportedBug($this);
            $this->reporter = $reporter;
        }

        public function getEngineer()
        {
            return $this->engineer;
        }

        public function getReporter()
        {
            return $this->reporter;
        }
    }
    class User
    {
        public function addReportedBug($bug)
        {
            $this->reportedBugs[] = $bug;
        }

        public function assignedToBug($bug)
        {
            $this->assignedBugs[] = $bug;
        }
    }

I chose to name the inverse methods in past-tense, which should indicate that the actual assigning has already taken
place and the methods are only used for ensuring consistency of the references. You can see from `User::addReportedBug()`
and `User::assignedToBug()`  that using this method in userland alone would not add the Bug to the collection of the owning
side in Bug::$reporter or Bug::$engineer. Using these methods and calling Doctrine for persistence would not update
the collections representation in the database.

Only using `Bug::setEngineer()` or `Bug::setReporter()` correctly saves the relation information. We also set both collection
instance variables to protected, however with PHP 5.3's new features Doctrine is still able to use Reflection to set and get values
from protected and private properties.

The `Bug::$reporter` and `Bug::$engineer` properties are Many-To-One relations, which point to a User. In a normalized
relational model the foreign key is saved on the Bug's table, hence in our object-relation model the Bug is at the owning
side of the relation. You should always make sure that the use-cases of your domain model should drive which side
is an inverse or owning one in your Doctrine mapping. In our example, whenever a new bug is saved or an engineer is assigned
to the bug, we don't want to update the User to persist the reference, but the Bug.
This is the case with the Bug being at the owning side of the relation.

Bugs reference Products by a uni-directional ManyToMany relation in the database that points from from Bugs to Products.

    [php]
    class Bug
    {
        protected $products = null; // Set protected for encapsulation

        public function assignToProduct($product)
        {
            $this->products[] = $product;
        }

        public function getProducts()
        {
            return $this->products;
        }
    }

We are now finished with the domain model given the requirements. From the simple model with public properties only
we had to do quite some work to get to a model where we encapsulated the references between the objects to make sure
we don't break its consistent state when using Doctrine.

However up to now the assumptions Doctrine imposed on our business objects have not restricting us much in our domain
modelling capabilities. Actually we would have encapsulated access to all the properties anyways by using
object-oriented best-practices.

## Metadata Mappings for our Entities

Up to now we have only implemented our Entites as Data-Structures without actually telling Doctrine how to persist
them in the database. If perfect in-memory databases would exist, we could now finish the application  using these entities
by implementing code to fullfil all the requirements. However the world isn't perfect and we have to persist our
entities in some storage to make sure we don't loose their state. Doctrine currently serves Relational Database Management Systems.
In the future we are thinking to support NoSQL vendors like CouchDb or MongoDb, however this is still far in the future.

The next step for persistance with Doctrine is to describe the structure of our domain model entities to Doctrine
using a metadata language. The metadata language describes how entities, their properties and references should be
persisted and what constraints should be applied to them.

Metadata for entities are loaded using a `Doctrine\ORM\Mapping\Driver\Driver` implementation and Doctrine 2 already comes
with XML, YAML and Annotations Drivers. In this Getting Started Guide I will use the XML Mapping Driver. I think XML
beats YAML because of schema validation, and my favorite IDE netbeans offers me auto-completion for the XML mapping files
which is awesome to work with and you don't have to look up all the different metadata mapping commands all the time.

Since we haven't namespaced our three entities, we have to implement three mapping files called Bug.dcm.xml,
Product.dcm.xml and User.dcm.xml and put them into a distinct folder for mapping configurations.

The first discussed definition will be for the Product, since it is the most simple one:

    [xml]
    <doctrine-mapping xmlns="http://doctrine-project.org/schemas/orm/doctrine-mapping"
          xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
          xsi:schemaLocation="http://doctrine-project.org/schemas/orm/doctrine-mapping
                        http://doctrine-project.org/schemas/orm/doctrine-mapping.xsd">

          <entity name="Product" table="zf_products">
              <id name="id" type="integer" column="product_id">
                  <generator strategy="AUTO" />
              </id>

              <field name="name" column="product_name" type="string" />
          </entity>

    </doctrine-mapping>

The toplevel `entity` definition tag specifies information about the class and table-name. The
primitive type `Product::$name` is defined as `field` attributes. The Id property is defined with the `id` tag.
The id has a `generator` tag nested inside which defines that the primary key generation mechanism
automatically uses the database platforms native id generation strategy, for example AUTO INCREMENT
in the case of MySql or Sequences in the case of PostgreSql and Oracle.

We then go on specifying the definition of a Bug:

    [xml]
    <doctrine-mapping xmlns="http://doctrine-project.org/schemas/orm/doctrine-mapping"
          xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
          xsi:schemaLocation="http://doctrine-project.org/schemas/orm/doctrine-mapping
                        http://doctrine-project.org/schemas/orm/doctrine-mapping.xsd">

        <entity name="Bug" table="zf_bugs">
            <id name="id" type="integer" column="bug_id">
                <generator strategy="AUTO" />
            </id>

            <field name="description" column="bug_description" type="text" />
            <field name="created" column="bug_created" type="datetime" />
            <field name="status" column="bug_status" type="string" />

            <many-to-one target-entity="User" field="reporter" inversed-by="reportedBugs">
                <join-column name="reporter_id" referenced-column-name="account_id" />
            </many-to-one>

            <many-to-one target-entity="User" field="engineer" inversed-by="assignedBugs">
                <join-column name="engineer_id" referenced-column-name="account_id" />
            </many-to-one>

            <many-to-many target-entity="Product" field="products">
                <join-table name="zf_bugs_products">
                    <join-columns>
                        <join-column name="bug_id" referenced-column-name="bug_id" />
                    </join-columns>
                    <inverse-join-columns>
                        <join-column name="product_id" referenced-column-name="product_id" />
                    </inverse-join-columns>
                </join-table>
            </many-to-many>
        </entity>

    </doctrine-mapping>

Here again we have the entity, id and primitive type definitions.
The column names are used from the Zend_Db_Table examples and have different names than the properties
on the Bug class. Additionally for the "created" field it is specified that it is of the Type "DATETIME",
which translates the YYYY-mm-dd HH:mm:ss Database format into a PHP DateTime instance and back.

After the field definitions the two qualified references to the user entity are defined. They are created by
the `many-to-one` tag. The class name of the related entity has to be specified with the `target-entity`
attribute, which is enough information for the database mapper to access the foreign-table. The
`join-column` tags are used to specifiy how the foreign and referend columns are named, an information
Doctrine needs to construct joins between those two entities correctly. Since `reporter` and `engineer`
are on the owning side of a bi-direcitonal relation we also have to specify the `inversed-by` attribute.
They have to point to the field names on the inverse side of the relationship.

The last missing property is the `Bug::$products` collection. It holds all products where the specific
bug is occouring in. Again you have to define the `target-entity` and `field` attributes on the `many-to-many`
tag. Furthermore you have to specifiy the details of the many-to-many join-table and its foreign key columns.
The definition is rather complex, however relying on the XML auto-completion I got it working easily, although
I forget the schema details all the time.

The last missing definition is that of the User entity:

    [xml]
    <doctrine-mapping xmlns="http://doctrine-project.org/schemas/orm/doctrine-mapping"
          xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
          xsi:schemaLocation="http://doctrine-project.org/schemas/orm/doctrine-mapping
                        http://doctrine-project.org/schemas/orm/doctrine-mapping.xsd">

         <entity name="User" table="zf_accounts">
             <id name="id" type="integer" column="account_id">
                 <generator strategy="AUTO" />
             </id>

             <field name="name" column="account_name" type="string" />

             <one-to-many target-entity="Bug" field="reportedBugs" mapped-by="reporter" />
             <one-to-many target-entity="Bug" field="assignedBugs" mapped-by="engineer" />

         </entity>

    </doctrine-mapping>

Here are some new things to mention about the `one-to-many` tags. Remember that we discussed about
the inverse and owning side. Now both reportedBugs and assignedBugs are inverse relations,
which means the join details have already been defined on the owning side. Therefore we only
have to specify the property on the Bug class that holds the owning sides.

This example has a fair overview of the most basic features of the metadata definition language.

## Obtaining the EntityManager

Doctrine's public interface is the EntityManager, it provides the access point to the complete
lifecycle management of your entities and transforms entities from and back to persistence. You
have to configure and create it to use your entities with Doctrine 2. I will show the configuration
steps and then discuss them step by step:

    [php]
    // Setup Autoloader (1)
    require '/path/to/lib/Doctrine/Common/ClassLoader.php';
    $loader = new Doctrine\Common\ClassLoader("Doctrine", '/path/to/Doctrine/trunk/lib/');
    $loader->register();

    $config = new Doctrine\ORM\Configuration(); // (2)

    // Proxy Configuration (3)
    $config->setProxyDir(__DIR__.'/lib/MyProject/Proxies');
    $config->setProxyNamespace('MyProject\Proxies');
    $config->setAutoGenerateProxyClasses((APPLICATION_ENV == "development"));

    // Mapping Configuration (4)
    $driverImpl = new Doctrine\ORM\Mapping\Driver\XmlDriver(__DIR__."/config/mappings");
    $config->setMetadataDriverImpl($driverImpl);

    // Caching Configuration (5)
    if (APPLICATION_ENV == "develoment") {
        $cache = new \Doctrine\Common\Cache\ArayCache();
    } else {
        $cache = new \Doctrine\Common\Cache\ApcCache();
    }
    $config->setMetadataCacheImpl($cache);
    $config->setQueryCacheImpl($cache);

    // database configuration parameters (6)
    $conn = array(
        'driver' => 'pdo_sqlite',
        'path' => __DIR__ . '/db.sqlite',
    );

    // obtaining the entity manager (7)
    $evm = new Doctrine\Common\EventManager()
    $entityManager = \Doctrine\ORM\EntityManager::create($conn, $config, $evm);

The first block sets up the autoloading capabilities of Doctrine. I am registering the Doctrine
namespace to the given path. To add your own namespace you can instantiate another `CloassLoader`
with different namespace and path arguments. There is no requirement to use the Doctrine `ClassLoader`
for your autoloading needs, you can use whatever suits you best.

The second block contains of the instantiation of the ORM Configuration object. Besides the
configuration shown in the next blocks there are several others with are all explained
in the [Configuration section of the manual](http://www.doctrine-project.org/documentation/manual/2_0/en/configuration#configuration-options).

The Proxy Configuration is a required block for your application, you have to specifiy where
Doctrine writes the PHP code for Proxy Generation. Proxies are children of your entities generated
by Doctrine to allow for type-safe lazy loading. We will see in a later chapter how exactly this works.
Besides the path to the proxies we also specifiy which namespace they will reside under aswell as
a flag `autoGenerateProxyClasses` indicating that proxies should be re-generated on each request,
which is recommended for development. In production this should be prevented at all costs,
the proxy class generation can be quite costly.

The fourth block contains the mapping driver details. We will use XML Mapping in this example, so
we configure the `XmlDriver` instance with a path to mappings configuration folder where we put
the Bug.dcm.xml, Product.dcm.xml and User.dcm.xml.

In the 5th block the caching configuration is set. In production we use caching only on a per request-basis
using the ArrayCache. In production it is literally required to use Apc, Memcache or XCache to get the full
speed out of Doctrine. Internally Doctrine uses caching heavily for the Metadata and DQL Query Language
so make sure you use a caching mechanism.

The 6th block shows the configuration options required to connect to a database, in my case a file-based
sqlite database. All the configuration options for all the shipped drivers are given in the [DBAL Configuration
section of the manual](http://www.doctrine-project.org/documentation/manual/2_0/en/dbal).

The last block shows how the `EntityManager` is obtained from a factory method, Here we also pass
in an `EventManager` instance which is optional. However using the EventManager you can hook in to the lifecycle
of entities, which is a common use-case, so you know how to configure it already.

## Generating the Database Schema

Now that we have defined the Metadata Mappings and bootstrapped the EntityManager
we want to generate the relational database schema from it.
Doctrine has a Command-Line-Interface that allows you to access the SchemaTool, a component that generates
the required tables to work with the metadata.

For the commandline tool to work a cli-config.php file has to be present in the project root directry,
where you will execute the doctrine command. Its a fairly simple file:

    [php]
    $cliConfig = new Doctrine\Common\Cli\Configuration();
    $cliConfig->setAttribute('em', $entityManager);

You can then use your favorite console tool to call:

    [console]
    doctrine@my-desktop> cd myproject/
    doctrine@my-desktop> doctrine orm:schema-tool:create

During the development you probably need to re-create the database several times when changing the Entity
metadata. You can then either re-create the database:

    [console]
    doctrine@my-desktop> doctrine orm:schema-tool:drop
    doctrine@my-dekstop> doctrine orm:schema-tool:create

Or use the update functionality:

    [console]
    doctrine@my-desktop> doctrine orm:schema-tool:update

The updating of databases uses a Diff Algorithm for a given Database Schema, a cornerstone of the `Doctrine\DBAL`
package, which can even be used without the Doctrine ORM package. However its not available in SQLite since
it does not support ALTER TABLE.

## Writing Entities into the Database

Having created the schema we can now start and save entities in the database. For starters we need a create user use-case:

    [php]
    $newUsername = "beberlei";

    $user = new User();
    $user->name = $newUsername;

    $entityManager->persist($user);
    $entityManager->flush();

Products can also be created:

    [php]
    $newProductName = "My Product";

    $product = new Product();
    $product->name = $newProductName;

    $entityManager->persist($product);
    $entityManager->flush();

So what is happening in those two snippets? In both examples the class creation is pretty standard, the interesting
bits are the communication with the `EntityManager`. To notify the EntityManager that a new entity should be inserted
into the database you have to call `persist()`. However the EntityManager does not act on this, its merely notified.
You have to explicitly call `flush()` to have the EntityManager write those two entities to the database.

You might wonder why does this distinction between persist notification and flush exist? Doctrine 2 uses the
UnitOfWork pattern to aggregate all writes (INSERT, UDPATE, DELETE) into one single fast transaction, which
is executed when flush is called. Using this approach the write-performance is significantly faster than
in a scenario where updates are done for each entity in isolation. In more complex scenarios than the
previous two, you are free to request updates on many different entities and all flush them at once.

Doctrine's UnitOfWork detects entities that have changed after retrieval from the database automatically when
the flush operation is called, so that you only have to keep track of those entities that are new or to be removed and pass them to
`EntityManager#persist()` and `EntityManager#remove()` respectively. This comparison to find dirty
entities that need updating is using a very efficient algorithm that has almost no additional
memory overhead and can even save you computing power by only updating those database columns
that really changed.

We are now getting to the "Create a New Bug" requirement and the code for this scenario may look like this:

    [php]
    $reporter = $entityManager->find("User", $theReporterId);
    $engineer = $entityManager->find("User", $theDefaultEngineerId);

    $bug = new Bug();
    $bug->description = "Something does not work!";
    $bug->created = new DateTime("now");
    $bug->status = "NEW";

    foreach ($productIds AS $productId) {
        $product = $entityManager->find("Product", $productId);
        $bug->assignToProduct($product);
    }

    $bug->setReporter($reporter);
    $bug->setEngineer($engineer);

    $entityManager->persist($bug);
    $entityManager->flush();

    echo "Your new Bug Id: ".$bug->id."\n";

This is the first contact with the read API of the EntityManager, showing that a call to
`EntityManager#find($name, $id)` returns a single instance of an entity queried by primary key.
Besides this we see the persist + flush pattern again to save the Bug into the database.

See how simple relating Bug, Reporter, Engineer and Products is done by using the discussed methods
in the "A first prototype" section. The UnitOfWork will detect this relations when flush
is called and relate them in the database appropriately.

## Queries for Application Use-Cases

### List of Bugs

Using the previous examples we can fill up the database quite a bit, however we now need to discuss how to query the underlying
mapper for the required view representations. When opening the application, bugs can be paginated through a list-view, which is the first
read-only use-case:

    [php]
    $dql = "SELECT b, e, r FROM Bug b JOIN b.engineer e JOIN b.reporter r ORDER BY b.created DESC";

    $query = $entityManager->createQuery($dql);
    $query->setMaxResults(30);
    $bugs = $query->getResult();

    foreach($bugs AS $bug) {
        echo $bug->description." - ".$bug->created->format('d.m.Y')."\n";
        echo "    Reported by: ".$bug->getReporter()->name."\n";
        echo "    Assigned to: ".$bug->getEngineer()->name."\n";
        foreach($bug->getProducts() AS $product) {
            echo "    Platform: ".$product->name."\n";
        }
        echo "\n";
    }

The DQL Query in this example fetches the 30 most recent bugs with their respective engineer and reporter
in one single SQL statement. The console output of this script is then:

    Something does not work! - 02.04.2010
        Reported by: beberlei
        Assigned to: beberlei
        Platform: My Product

> **NOTE**
>
> **Dql is not Sql**
>
> You may wonder why we start writing SQL at the beginning of this use-case. Don't we use an ORM to get rid
> of all the endless hand-writing of SQL? Doctrine introduces DQL which is best described
> as **object-query-language** and is a dialect of [OQL](http://en.wikipedia.org/wiki/Object_Query_Language) and
> similar to [HQL](http://www.hibernate.org) or [JPQL](http://en.wikipedia.org/wiki/Java_Persistence_Query_Language).
> It does not know the concept of columns and tables, but only those
> of Entity-Class and property. Using the Metadata we defined before it allows for very short distinctive
> and powerful queries.
>
> An important reason why DQL is favourable to the Query API of most ORMs is its similarity to SQL. The DQL language
> allows query constructs that most ORMs don't, GROUP BY even with HAVING, Subselects, Fetch-Joins of nested
> classes, mixed results with entities and scalar data such as COUNT() results and much more. Using
> DQL you should seldom come to the point where you want to throw your ORM into the dumpster, because it
> doesn't support some the more powerful SQL concepts.
>
> Besides handwriting DQL you can however also use the `QueryBuilder` retrieved by calling `$entityManager->createQueryBuilder()`
> which is a Query Object around the DQL language.
>
> As a last resort you can however also use Native SQL and a description of the result set to retrieve
> entities from the database. DQL boils down to a Native SQL statement and a `ResultSetMapping` instance itself.
> Using Native SQL you could even use stored procedures for data retrieval, or make use of advanced non-portable
> database queries like PostgreSql's recursive queries.

### Array Hydration of the Bug List

In the previous use-case we retrieved the result as their respective object instances.
We are not limitied to retrieving objects only from Doctrine however. For a simple list view
like the previous one we only need read access to our entities and can switch the hydration
from objects to simple PHP arrays instead. This can obviously yiel considerable performance benefits for read-only requests.

Implementing the same list view as before using array hydration we can rewrite our code:

    [php]
    $dql = "SELECT b, e, r, p FROM Bug b JOIN b.engineer e ".
           "JOIN b.reporter r JOIN b.products p ORDER BY b.created DESC";
    $query = $em->createQuery($dql);
    $bugs = $query->getArrayResult();

    foreach ($bugs AS $bug) {
        echo $bug['description'] . " - " . $bug['created']->format('d.m.Y')."\n";
        echo "    Reported by: ".$bug['reporter']['name']."\n";
        echo "    Assigned to: ".$bug['engineer']['name']."\n";
        foreach($bug['products'] AS $product) {
            echo "    Platform: ".$product['name']."\n";
        }
        echo "\n";
    }

There is one significant difference in the DQL query however, we have
to add an additional fetch-join for the products connected to a bug. The resulting
SQL query for this single select statement is pretty large, however still
more efficient to retrieve compared to hydrating objects.

### Find by Primary Key

The next Use-Case is displaying a Bug by primary key. This could be done using DQL as in the previous example with a where clause,
however there is a convenience method on the Entity Manager that handles loading by primary key, which we have already
seen in the write scenarios:

    [php]
    $bug = $entityManager->find("Bug", (int)$theBugId);

However we will soon see another problem with our entities using this approach. Try displaying the engineer's name:

    [php]
    echo "Bug: ".$bug->description."\n";
    echo "Engineer: ".$bug->getEngineer()->name."\n";

It will be null! What is happening? It worked in the previous example, so it can't be a problem with the persistance
code of Doctrine. What is it then? You walked in the public property trap.

Since we only retrieved the bug by primary key both the engineer and reporter are not immediately loaded
from the database but are replaced by LazyLoading proxies. Sample
code of this proxy generated code can be found in the specified Proxy Directory, it looks like:

    [php]
    namespace MyProject\Proxies;

    /**
     * THIS CLASS WAS GENERATED BY THE DOCTRINE ORM. DO NOT EDIT THIS FILE.
     */
    class UserProxy extends \User implements \Doctrine\ORM\Proxy\Proxy
    {
        // .. lazy load code here

        public function addReportedBug($bug)
        {
            $this->_load();
            return parent::addReportedBug($bug);
        }

        public function assignedToBug($bug)
        {
            $this->_load();
            return parent::assignedToBug($bug);
        }
    }

See how upon each method call the proxy is lazily loaded from the database? Using public properties however
we never call a method and Doctrine has no way to hook into the PHP Engine to detect a direct access to a public property
and trigger the lazy load. We need to rewrite our entities, make all the properties private or protected and add getters
and setters to get a working example:

    [php]
    echo "Bug: ".$bug->getDescription()."\n";
    echo "Engineer: ".$bug->getEngineer()->getName()."\n";

    /**
    Bug: Something does not work!
    Engineer: beberlei
    */

Being required to use private or protected properties Doctrine 2 actually enforces you to encapsulate
your objects according to object-oriented best-practices.

## Dashboard of the User

For the next use-case we want to retrieve the dashboard view, a list of all open bugs the user reported or
was assigned to. This will be achieved using DQL again, this time with some WHERE clauses and usage of bound parameters:

    [php]
    $dql = "SELECT b, e, r FROM Bug b JOIN b.engineer e JOIN b.reporter r ".
           "WHERE b.status = 'OPEN' AND e.id = ?1 OR r.id = ?1 ORDER BY b.created DESC";

    $myBugs = $entityManager->createQuery($dql)
                            ->setParameter(1, $theUserId)
                            ->setMaxResults(15)
                            ->getResult();

    foreach ($myBugs AS $bug) {
        echo $bug->getDescription()."\n";
    }

That is it for the read-scenarios of this example, we will continue with the last missing bit, engineers
being able to close a bug.

## Number of Bugs

Until now we only retrieved entities or their array representation. Doctrine also supports the retrieval
of non-entities through DQL. These values are called "scalar result values" and may even be aggregate
values using COUNT, SUM, MIN, MAX or AVG functions.

We will need this knowledge to retrieve the number of open bugs grouped by product:

    [php]
    $dql = "SELECT p.id, p.name, count(b.id) AS openBugs FROM Bug b ".
           "JOIN b.products p WHERE b.status = 'OPEN' GROUP BY p.id";
    $productBugs = $em->createQuery($dql)->getScalarResult();

    foreach($productBugs as $productBug) {
        echo $productBug['name']." has " . $productBug['openBugs'] . " open bugs!\n";
    }

## Updating Entities

There is a single use-case missing from the requirements, Engineers should be able to close a bug. This looks like:

    [php]
    $bug = $entityManager->find("Bug", (int)$theBugId);
    $bug->close();

    $entityManager->flush();

When retrieving the Bug from the database it is inserted into the IdentityMap inside the UnitOfWork of Doctrine.
This means your Bug with exactly this id can only exist once during the whole request no matter how often you
call `EntityManager#find()`. It even detects entities that are hydrated using DQL and are already present in
the Identity Map.

When flush is called the EntityManager loops over all the entities in the identity map and performs a comparison
between the values originally retrieved from the database and those values the entity currently has. If at
least one of these properties is different the entity is scheduled for an UPDATE against the database.
Only the changed columns are updated, which offers a pretty good performance improvement compared to updating
all the properties.

This tutorial is over here, I hope you had fun. Additional content will be added to this tutorial
incrementally, topics will include:

    * Entity Repositories
    * More on Association Mappings
    * Lifecycle Events triggered in the UnitOfWork
    * Ordering of Collections

Additional details on all the topics discussed here can be found in the respective manual chapters.