Working with Indexed Associations
=================================

Doctrine ORM collections are modelled after PHPs native arrays. PHP arrays are an ordered hashmap, but in
the first version of Doctrine keys retrieved from the database were always numerical unless ``INDEX BY``
was used. You can index your collections by a value in the related entity.
This is a first step towards full ordered hashmap support through the Doctrine ORM.
The feature works like an implicit ``INDEX BY`` for the selected association but has several
downsides also:

-  You have to manage both the key and field if you want to change the index by field value.
-  On each request the keys are regenerated from the field value, and not from the previous collection key.
-  Values of the Index-By keys are never considered during persistence. They only exist for accessing purposes.
-  Fields that are used for the index by feature **HAVE** to be unique in the database. The behavior for multiple entities
   with the same index-by field value is undefined.

As an example we will design a simple stock exchange list view. The domain consists of the entity ``Stock``
and ``Market`` where each Stock has a symbol and is traded on a single market. Instead of having a numerical
list of stocks traded on a market they will be indexed by their symbol, which is unique across all markets.

Mapping Indexed Associations
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

You can map indexed associations by adding:

    * ``indexBy`` attribute to any ``@OneToMany`` or ``@ManyToMany`` annotation.
    * ``index-by`` attribute to any ``<one-to-many />`` or ``<many-to-many />`` xml element.
    * ``indexBy:`` key-value pair to any association defined in ``manyToMany:`` or ``oneToMany:`` YAML mapping files.

The code and mappings for the Market entity looks like this:

.. configuration-block::
    .. code-block:: php

        <?php
        namespace Doctrine\Tests\Models\StockExchange;

        use Doctrine\Common\Collections\ArrayCollection;

        /**
         * @Entity
         * @Table(name="exchange_markets")
         */
        class Market
        {
            /**
             * @Id @Column(type="integer") @GeneratedValue
             * @var int
             */
            private $id;

            /**
             * @Column(type="string")
             * @var string
             */
            private $name;

            /**
             * @OneToMany(targetEntity="Stock", mappedBy="market", indexBy="symbol")
             * @var Stock[]
             */
            private $stocks;

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

            public function getId()
            {
                return $this->id;
            }

            public function getName()
            {
                return $this->name;
            }

            public function addStock(Stock $stock)
            {
                $this->stocks[$stock->getSymbol()] = $stock;
            }

            public function getStock($symbol)
            {
                if (!isset($this->stocks[$symbol])) {
                    throw new \InvalidArgumentException("Symbol is not traded on this market.");
                }

                return $this->stocks[$symbol];
            }

            public function getStocks()
            {
                return $this->stocks->toArray();
            }
        }

    .. code-block:: xml

        <?xml version="1.0" encoding="UTF-8"?>
        <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
                                  https://www.doctrine-project.org/schemas/orm/doctrine-mapping.xsd">

            <entity name="Doctrine\Tests\Models\StockExchange\Market">
                <id name="id" type="integer">
                    <generator strategy="AUTO" />
                </id>

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

                <one-to-many target-entity="Stock" mapped-by="market" field="stocks" index-by="symbol" />
            </entity>
        </doctrine-mapping>

    .. code-block:: yaml

        Doctrine\Tests\Models\StockExchange\Market:
          type: entity
          id:
            id:
              type: integer
              generator:
                strategy: AUTO
          fields:
            name:
              type:string
          oneToMany:
            stocks:
              targetEntity: Stock
              mappedBy: market
              indexBy: symbol

Inside the ``addStock()`` method you can see how we directly set the key of the association to the symbol,
so that we can work with the indexed association directly after invoking ``addStock()``. Inside ``getStock($symbol)``
we pick a stock traded on the particular market by symbol. If this stock doesn't exist an exception is thrown.

The ``Stock`` entity doesn't contain any special instructions that are new, but for completeness
here are the code and mappings for it:

.. configuration-block::
    .. code-block:: php

        <?php
        namespace Doctrine\Tests\Models\StockExchange;

        /**
         * @Entity
         * @Table(name="exchange_stocks")
         */
        class Stock
        {
            /**
             * @Id @GeneratedValue @Column(type="integer")
             * @var int
             */
            private $id;

            /**
             * @Column(type="string", unique=true)
             */
            private $symbol;

            /**
             * @ManyToOne(targetEntity="Market", inversedBy="stocks")
             * @var Market
             */
            private $market;

            public function __construct($symbol, Market $market)
            {
                $this->symbol = $symbol;
                $this->market = $market;
                $market->addStock($this);
            }

            public function getSymbol()
            {
                return $this->symbol;
            }
        }

    .. code-block:: xml

        <?xml version="1.0" encoding="UTF-8"?>
        <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
                                  https://www.doctrine-project.org/schemas/orm/doctrine-mapping.xsd">

            <entity name="Doctrine\Tests\Models\StockExchange\Stock">
                <id name="id" type="integer">
                    <generator strategy="AUTO" />
                </id>

                <field name="symbol" type="string" unique="true" />
                <many-to-one target-entity="Market" field="market" inversed-by="stocks" />
            </entity>
        </doctrine-mapping>

    .. code-block:: yaml

        Doctrine\Tests\Models\StockExchange\Stock:
          type: entity
          id:
            id:
              type: integer
              generator:
                strategy: AUTO
          fields:
            symbol:
              type: string
          manyToOne:
            market:
              targetEntity: Market
              inversedBy: stocks

Querying indexed associations
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Now that we defined the stocks collection to be indexed by symbol, we can take a look at some code
that makes use of the indexing.

First we will populate our database with two example stocks traded on a single market:

.. code-block:: php

    <?php
    // $em is the EntityManager

    $market = new Market("Some Exchange");
    $stock1 = new Stock("AAPL", $market);
    $stock2 = new Stock("GOOG", $market);

    $em->persist($market);
    $em->persist($stock1);
    $em->persist($stock2);
    $em->flush();

This code is not particular interesting since the indexing feature is not yet used. In a new request we could
now query for the market:

.. code-block:: php

    <?php
    // $em is the EntityManager
    $marketId = 1;
    $symbol = "AAPL";
    
    $market = $em->find("Doctrine\Tests\Models\StockExchange\Market", $marketId);

    // Access the stocks by symbol now:
    $stock = $market->getStock($symbol);

    echo $stock->getSymbol(); // will print "AAPL"

The implementation of ``Market::addStock()``, in combination with ``indexBy``, allows us to access the collection
consistently by the Stock symbol. It does not matter if Stock is managed by Doctrine or not.

The same applies to DQL queries: The ``indexBy`` configuration acts as implicit "INDEX BY" to a join association.

.. code-block:: php

    <?php
    // $em is the EntityManager
    $marketId = 1;
    $symbol = "AAPL";

    $dql = "SELECT m, s FROM Doctrine\Tests\Models\StockExchange\Market m JOIN m.stocks s WHERE m.id = ?1";
    $market = $em->createQuery($dql)
                 ->setParameter(1, $marketId)
                 ->getSingleResult();

    // Access the stocks by symbol now:
    $stock = $market->getStock($symbol);

    echo $stock->getSymbol(); // will print "AAPL"

If you want to use ``INDEX BY`` explicitly on an indexed association you are free to do so. Additionally,
indexed associations also work with the ``Collection::slice()`` functionality, even if the association's fetch mode is
LAZY or EXTRA_LAZY.

Outlook into the Future
~~~~~~~~~~~~~~~~~~~~~~~

For the inverse side of a many-to-many associations there will be a way to persist the keys and the order
as a third and fourth parameter into the join table. This feature is discussed in `#2817 <https://github.com/doctrine/orm/issues/2817>`_
This feature cannot be implemented for one-to-many associations, because they are never the owning side.
