Association Mapping
===================

This chapter introduces association mappings which are used to explain
references between objects and are mapped to a relational database using
foreign keys.

Instead of working with the foreign keys directly you will always work with
references to objects:

- A reference to a single object is represented by a foreign key.
- A collection of objects is represented by many foreign keys pointing to the object holding the collection

This chapter is split into three different sections.

- A list of all the possible association mapping use-cases is given.
- :ref:`association_mapping_defaults` are explained that simplify the use-case examples.
- :ref:`collections` are introduced that contain entities in associations.

To master associations you should also learn about :doc:`owning and inverse sides of associations <unitofwork-associations>`

One-To-One, Unidirectional
--------------------------

A unidirectional one-to-one association is very common. Here is an
example of a ``Product`` that has one ``Shipping`` object
associated to it. The ``Shipping`` side does not reference back to
the ``Product`` so it is unidirectional.

.. configuration-block::

    .. code-block:: php

        <?php
        /** @Entity **/
        class Product
        {
            // ...

            /**
             * @OneToOne(targetEntity="Shipping")
             * @JoinColumn(name="shipping_id", referencedColumnName="id")
             **/
            private $shipping;

            // ...
        }

        /** @Entity **/
        class Shipping
        {
            // ...
        }

    .. code-block:: xml

        <doctrine-mapping>
            <entity class="Product">
                <one-to-one field="shipping" target-entity="Shipping">
                    <join-column name="shipping_id" referenced-column-name="id" />
                </one-to-one>
            </entity>
        </doctrine-mapping>

    .. code-block:: yaml

        Product:
          type: entity
          oneToOne:
            shipping:
              targetEntity: Shipping
              joinColumn:
                name: shipping_id
                referencedColumnName: id

Note that the @JoinColumn is not really necessary in this example,
as the defaults would be the same.

Generated MySQL Schema:

.. code-block:: sql

    CREATE TABLE Product (
        id INT AUTO_INCREMENT NOT NULL,
        shipping_id INT DEFAULT NULL,
        UNIQUE INDEX UNIQ_6FBC94267FE4B2B (shipping_id),
        PRIMARY KEY(id)
    ) ENGINE = InnoDB;
    CREATE TABLE Shipping (
        id INT AUTO_INCREMENT NOT NULL,
        PRIMARY KEY(id)
    ) ENGINE = InnoDB;
    ALTER TABLE Product ADD FOREIGN KEY (shipping_id) REFERENCES Shipping(id);

One-To-One, Bidirectional
-------------------------

Here is a one-to-one relationship between a ``Customer`` and a
``Cart``. The ``Cart`` has a reference back to the ``Customer`` so
it is bidirectional.

.. configuration-block::

    .. code-block:: php

        <?php
        /** @Entity **/
        class Customer
        {
            // ...

            /**
             * @OneToOne(targetEntity="Cart", mappedBy="customer")
             **/
            private $cart;

            // ...
        }

        /** @Entity **/
        class Cart
        {
            // ...

            /**
             * @OneToOne(targetEntity="Customer", inversedBy="cart")
             * @JoinColumn(name="customer_id", referencedColumnName="id")
             **/
            private $customer;

            // ...
        }

    .. code-block:: xml

        <doctrine-mapping>
            <entity name="Customer">
                <one-to-one field="cart" target-entity="Cart" mapped-by="customer" />
            </entity>
            <entity name="Cart">
                <one-to-one field="customer" target-entity="Customer" inversed-by="cart">
                    <join-column name="customer_id" referenced-column-name="id" />
                </one-to-one>
            </entity>
        </doctrine-mapping>

    .. code-block:: yaml

        Customer:
          oneToOne:
            cart:
              targetEntity: Cart
              mappedBy: customer
        Cart:
          oneToOne:
            customer:
              targetEntity: Customer
              inversedBy: cart
              joinColumn:
                name: customer_id
                referencedColumnName: id

Note that the @JoinColumn is not really necessary in this example,
as the defaults would be the same.

Generated MySQL Schema:

.. code-block:: sql

    CREATE TABLE Cart (
        id INT AUTO_INCREMENT NOT NULL,
        customer_id INT DEFAULT NULL,
        PRIMARY KEY(id)
    ) ENGINE = InnoDB;
    CREATE TABLE Customer (
        id INT AUTO_INCREMENT NOT NULL,
        PRIMARY KEY(id)
    ) ENGINE = InnoDB;
    ALTER TABLE Cart ADD FOREIGN KEY (customer_id) REFERENCES Customer(id);

See how the foreign key is defined on the owning side of the
relation, the table ``Cart``.

One-To-One, Self-referencing
----------------------------

You can easily have self referencing one-to-one relationships like
below.

.. code-block:: php

    <?php
    /** @Entity **/
    class Student
    {
        // ...

        /**
         * @OneToOne(targetEntity="Student")
         * @JoinColumn(name="mentor_id", referencedColumnName="id")
         **/
        private $mentor;

        // ...
    }

Note that the @JoinColumn is not really necessary in this example,
as the defaults would be the same.

With the generated MySQL Schema:

.. code-block:: sql

    CREATE TABLE Student (
        id INT AUTO_INCREMENT NOT NULL,
        mentor_id INT DEFAULT NULL,
        PRIMARY KEY(id)
    ) ENGINE = InnoDB;
    ALTER TABLE Student ADD FOREIGN KEY (mentor_id) REFERENCES Student(id);

One-To-Many, Unidirectional with Join Table
-------------------------------------------

A unidirectional one-to-many association can be mapped through a
join table. From Doctrine's point of view, it is simply mapped as a
unidirectional many-to-many whereby a unique constraint on one of
the join columns enforces the one-to-many cardinality.

.. note::

    One-To-Many uni-directional relations with join-table only
    work using the @ManyToMany annotation and a unique-constraint.


The following example sets up such a unidirectional one-to-many association:

.. configuration-block::

    .. code-block:: php

        <?php
        /** @Entity **/
        class User
        {
            // ...

            /**
             * @ManyToMany(targetEntity="Phonenumber")
             * @JoinTable(name="users_phonenumbers",
             *      joinColumns={@JoinColumn(name="user_id", referencedColumnName="id")},
             *      inverseJoinColumns={@JoinColumn(name="phonenumber_id", referencedColumnName="id", unique=true)}
             *      )
             **/
            private $phonenumbers;

            public function __construct()
            {
                $this->phonenumbers = new \Doctrine\Common\Collections\ArrayCollection();
            }

            // ...
        }

        /** @Entity **/
        class Phonenumber
        {
            // ...
        }

    .. code-block:: xml

        <doctrine-mapping>
            <entity name="User">
                <many-to-many field="phonenumbers" target-entity="Phonenumber">
                    <join-table name="users_phonenumbers">
                        <join-columns>
                            <join-column name="user_id" referenced-column-name="id" />
                        </join-columns>
                        <inverse-join-columns>
                            <join-column name="phonenumber_id" referenced-column-name="id" unique="true" />
                        </inverse-join-columns>
                    </join-table>
                </many-to-many>
            </entity>
        </doctrine-mapping>

    .. code-block:: yaml

        User:
          type: entity
          manyToMany:
            phonenumbers:
              targetEntity: Phonenumber
              joinTable:
                name: users_phonenumbers
                joinColumns:
                  user_id:
                    referencedColumnName: id
                inverseJoinColumns:
                  phonenumber_id:
                    referencedColumnName: id
                    unique: true


Generates the following MySQL Schema:

.. code-block:: sql

    CREATE TABLE User (
        id INT AUTO_INCREMENT NOT NULL,
        PRIMARY KEY(id)
    ) ENGINE = InnoDB;

    CREATE TABLE users_phonenumbers (
        user_id INT NOT NULL,
        phonenumber_id INT NOT NULL,
        UNIQUE INDEX users_phonenumbers_phonenumber_id_uniq (phonenumber_id),
        PRIMARY KEY(user_id, phonenumber_id)
    ) ENGINE = InnoDB;

    CREATE TABLE Phonenumber (
        id INT AUTO_INCREMENT NOT NULL,
        PRIMARY KEY(id)
    ) ENGINE = InnoDB;

    ALTER TABLE users_phonenumbers ADD FOREIGN KEY (user_id) REFERENCES User(id);
    ALTER TABLE users_phonenumbers ADD FOREIGN KEY (phonenumber_id) REFERENCES Phonenumber(id);


Many-To-One, Unidirectional
---------------------------

You can easily implement a many-to-one unidirectional association
with the following:

.. configuration-block::

    .. code-block:: php

        <?php
        /** @Entity **/
        class User
        {
            // ...

            /**
             * @ManyToOne(targetEntity="Address")
             * @JoinColumn(name="address_id", referencedColumnName="id")
             **/
            private $address;
        }

        /** @Entity **/
        class Address
        {
            // ...
        }

    .. code-block:: xml

        <doctrine-mapping>
            <entity name="User">
                <many-to-one field="address" target-entity="Address">
                    <join-column name="address_id" referenced-column-name="id" />
                </many-to-one>
            </entity>
        </doctrine-mapping>

    .. code-block:: yaml

        User:
          type: entity
          manyToOne:
            address:
              targetEntity: Address
              joinColumn:
                name: address_id
                referencedColumnName: id


.. note::

    The above ``@JoinColumn`` is optional as it would default
    to ``address_id`` and ``id`` anyways. You can omit it and let it
    use the defaults.


Generated MySQL Schema:

.. code-block:: sql

    CREATE TABLE User (
        id INT AUTO_INCREMENT NOT NULL,
        address_id INT DEFAULT NULL,
        PRIMARY KEY(id)
    ) ENGINE = InnoDB;

    CREATE TABLE Address (
        id INT AUTO_INCREMENT NOT NULL,
        PRIMARY KEY(id)
    ) ENGINE = InnoDB;

    ALTER TABLE User ADD FOREIGN KEY (address_id) REFERENCES Address(id);

One-To-Many, Bidirectional
--------------------------

Bidirectional one-to-many associations are very common. The
following code shows an example with a Product and a Feature
class:

.. configuration-block::

    .. code-block:: php

        <?php
        /** @Entity **/
        class Product
        {
            // ...
            /**
             * @OneToMany(targetEntity="Feature", mappedBy="product")
             **/
            private $features;
            // ...

            public function __construct() {
                $this->features = new \Doctrine\Common\Collections\ArrayCollection();
            }
        }

        /** @Entity **/
        class Feature
        {
            // ...
            /**
             * @ManyToOne(targetEntity="Product", inversedBy="features")
             * @JoinColumn(name="product_id", referencedColumnName="id")
             **/
            private $product;
            // ...
        }

    .. code-block:: xml

        <doctrine-mapping>
            <entity name="Product">
                <one-to-many field="features" target-entity="Feature" mapped-by="product" />
            </entity>
            <entity name="Feature">
                <many-to-one field="product" target-entity="Product" inversed-by="features">
                    <join-column name="product_id" referenced-column-name="id" />
                </many-to-one>
            </entity>
        </doctrine-mapping>

    .. code-block:: yaml

        Product:
          type: entity
          oneToMany:
            features:
              targetEntity: Feature
              mappedBy: product
        Feature:
          type: entity
          manyToOne:
            product:
              targetEntity: Product
              inversedBy: features
              joinColumn:
                name: product_id
                referencedColumnName: id


Note that the @JoinColumn is not really necessary in this example,
as the defaults would be the same.

Generated MySQL Schema:

.. code-block:: sql

    CREATE TABLE Product (
        id INT AUTO_INCREMENT NOT NULL,
        PRIMARY KEY(id)
    ) ENGINE = InnoDB;
    CREATE TABLE Feature (
        id INT AUTO_INCREMENT NOT NULL,
        product_id INT DEFAULT NULL,
        PRIMARY KEY(id)
    ) ENGINE = InnoDB;
    ALTER TABLE Feature ADD FOREIGN KEY (product_id) REFERENCES Product(id);

One-To-Many, Self-referencing
-----------------------------

You can also setup a one-to-many association that is
self-referencing. In this example we setup a hierarchy of
``Category`` objects by creating a self referencing relationship.
This effectively models a hierarchy of categories and from the
database perspective is known as an adjacency list approach.

.. configuration-block::

    .. code-block:: php

        <?php
        /** @Entity **/
        class Category
        {
            // ...
            /**
             * @OneToMany(targetEntity="Category", mappedBy="parent")
             **/
            private $children;

            /**
             * @ManyToOne(targetEntity="Category", inversedBy="children")
             * @JoinColumn(name="parent_id", referencedColumnName="id")
             **/
            private $parent;
            // ...

            public function __construct() {
                $this->children = new \Doctrine\Common\Collections\ArrayCollection();
            }
        }

    .. code-block:: xml

        <doctrine-mapping>
            <entity name="Category">
                <one-to-many field="children" target-entity="Category" mapped-by="parent" />
                <many-to-one field="parent" target-entity="Category" inversed-by="children" />
            </entity>
        </doctrine-mapping>

    .. code-block:: yaml

        Category:
          type: entity
          oneToMany:
            children:
              targetEntity: Category
              mappedBy: parent
          manyToOne:
            parent:
              targetEntity: Category
              inversedBy: children

Note that the @JoinColumn is not really necessary in this example,
as the defaults would be the same.

Generated MySQL Schema:

.. code-block:: sql

    CREATE TABLE Category (
        id INT AUTO_INCREMENT NOT NULL,
        parent_id INT DEFAULT NULL,
        PRIMARY KEY(id)
    ) ENGINE = InnoDB;
    ALTER TABLE Category ADD FOREIGN KEY (parent_id) REFERENCES Category(id);

Many-To-Many, Unidirectional
----------------------------

Real many-to-many associations are less common. The following
example shows a unidirectional association between User and Group
entities:

.. configuration-block::

    .. code-block:: php

        <?php
        /** @Entity **/
        class User
        {
            // ...

            /**
             * @ManyToMany(targetEntity="Group")
             * @JoinTable(name="users_groups",
             *      joinColumns={@JoinColumn(name="user_id", referencedColumnName="id")},
             *      inverseJoinColumns={@JoinColumn(name="group_id", referencedColumnName="id")}
             *      )
             **/
            private $groups;

            // ...

            public function __construct() {
                $this->groups = new \Doctrine\Common\Collections\ArrayCollection();
            }
        }

        /** @Entity **/
        class Group
        {
            // ...
        }

    .. code-block:: xml

        <doctrine-mapping>
            <entity name="User">
                <many-to-many field="groups" target-entity="Group">
                    <join-table name="users_groups">
                        <join-columns>
                            <join-column name="user_id" referenced-column-name="id" />
                        </join-columns>
                        <inverse-join-columns>
                            <join-column name="group_id" referenced-column-name="id" />
                        </inverse-join-columns>
                    </join-table>
                </many-to-many>
            </entity>
        </doctrine-mapping>

    .. code-block:: yaml

        User:
          type: entity
          manyToMany:
            groups:
              targetEntity: Group
              joinTable:
                name: users_groups
                joinColumns:
                  user_id:
                    referencedColumnName: id
                inverseJoinColumns:
                  group_id:
                    referencedColumnName: id

Generated MySQL Schema:

.. code-block:: sql

    CREATE TABLE User (
        id INT AUTO_INCREMENT NOT NULL,
        PRIMARY KEY(id)
    ) ENGINE = InnoDB;
    CREATE TABLE users_groups (
        user_id INT NOT NULL,
        group_id INT NOT NULL,
        PRIMARY KEY(user_id, group_id)
    ) ENGINE = InnoDB;
    CREATE TABLE Group (
        id INT AUTO_INCREMENT NOT NULL,
        PRIMARY KEY(id)
    ) ENGINE = InnoDB;
    ALTER TABLE users_groups ADD FOREIGN KEY (user_id) REFERENCES User(id);
    ALTER TABLE users_groups ADD FOREIGN KEY (group_id) REFERENCES Group(id);

.. note::

    Why are many-to-many associations less common? Because
    frequently you want to associate additional attributes with an
    association, in which case you introduce an association class.
    Consequently, the direct many-to-many association disappears and is
    replaced by one-to-many/many-to-one associations between the 3
    participating classes.

Many-To-Many, Bidirectional
---------------------------

Here is a similar many-to-many relationship as above except this
one is bidirectional.

.. configuration-block::

    .. code-block:: php

        <?php
        /** @Entity **/
        class User
        {
            // ...

            /**
             * @ManyToMany(targetEntity="Group", inversedBy="users")
             * @JoinTable(name="users_groups")
             **/
            private $groups;

            public function __construct() {
                $this->groups = new \Doctrine\Common\Collections\ArrayCollection();
            }

            // ...
        }

        /** @Entity **/
        class Group
        {
            // ...
            /**
             * @ManyToMany(targetEntity="User", mappedBy="groups")
             **/
            private $users;

            public function __construct() {
                $this->users = new \Doctrine\Common\Collections\ArrayCollection();
            }

            // ...
        }

    .. code-block:: xml

        <doctrine-mapping>
            <entity name="User">
                <many-to-many field="groups" inversed-by="users" target-entity="Group">
                    <join-table name="users_groups">
                        <join-columns>
                            <join-column name="user_id" referenced-column-name="id" />
                        </join-columns>
                        <inverse-join-columns>
                            <join-column name="group_id" referenced-column-name="id" />
                        </inverse-join-columns>
                    </join-table>
                </many-to-many>
            </entity>

            <entity name="Group">
                <many-to-many field="users" mapped-by="groups" target-entity="User"/>
            </entity>
        </doctrine-mapping>

    .. code-block:: yaml

        User:
          type: entity
          manyToMany:
            groups:
              targetEntity: Group
              inversedBy: users
              joinTable:
                name: users_groups
                joinColumns:
                  user_id:
                    referencedColumnName: id
                inverseJoinColumns:
                  group_id:
                    referencedColumnName: id

        Group:
          type: entity
          manyToMany:
            users:
              targetEntity: User
              mappedBy: groups

The MySQL schema is exactly the same as for the Many-To-Many
uni-directional case above.

Picking Owning and Inverse Side
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

For Many-To-Many associations you can chose which entity is the
owning and which the inverse side. There is a very simple semantic
rule to decide which side is more suitable to be the owning side
from a developers perspective. You only have to ask yourself, which
entity is responsible for the connection management and pick that
as the owning side.

Take an example of two entities ``Article`` and ``Tag``. Whenever
you want to connect an Article to a Tag and vice-versa, it is
mostly the Article that is responsible for this relation. Whenever
you add a new article, you want to connect it with existing or new
tags. Your create Article form will probably support this notion
and allow to specify the tags directly. This is why you should pick
the Article as owning side, as it makes the code more
understandable:

.. code-block:: php

    <?php
    class Article
    {
        private $tags;

        public function addTag(Tag $tag)
        {
            $tag->addArticle($this); // synchronously updating inverse side
            $this->tags[] = $tag;
        }
    }

    class Tag
    {
        private $articles;

        public function addArticle(Article $article)
        {
            $this->articles[] = $article;
        }
    }

This allows to group the tag adding on the ``Article`` side of the
association:

.. code-block:: php

    <?php
    $article = new Article();
    $article->addTag($tagA);
    $article->addTag($tagB);

Many-To-Many, Self-referencing
------------------------------

You can even have a self-referencing many-to-many association. A
common scenario is where a ``User`` has friends and the target
entity of that relationship is a ``User`` so it is self
referencing. In this example it is bidirectional so ``User`` has a
field named ``$friendsWithMe`` and ``$myFriends``.

.. code-block:: php

    <?php
    /** @Entity **/
    class User
    {
        // ...

        /**
         * @ManyToMany(targetEntity="User", mappedBy="myFriends")
         **/
        private $friendsWithMe;

        /**
         * @ManyToMany(targetEntity="User", inversedBy="friendsWithMe")
         * @JoinTable(name="friends",
         *      joinColumns={@JoinColumn(name="user_id", referencedColumnName="id")},
         *      inverseJoinColumns={@JoinColumn(name="friend_user_id", referencedColumnName="id")}
         *      )
         **/
        private $myFriends;

        public function __construct() {
            $this->friendsWithMe = new \Doctrine\Common\Collections\ArrayCollection();
            $this->myFriends = new \Doctrine\Common\Collections\ArrayCollection();
        }

        // ...
    }

Generated MySQL Schema:

.. code-block:: sql

    CREATE TABLE User (
        id INT AUTO_INCREMENT NOT NULL,
        PRIMARY KEY(id)
    ) ENGINE = InnoDB;
    CREATE TABLE friends (
        user_id INT NOT NULL,
        friend_user_id INT NOT NULL,
        PRIMARY KEY(user_id, friend_user_id)
    ) ENGINE = InnoDB;
    ALTER TABLE friends ADD FOREIGN KEY (user_id) REFERENCES User(id);
    ALTER TABLE friends ADD FOREIGN KEY (friend_user_id) REFERENCES User(id);

.. _association_mapping_defaults:

Mapping Defaults
----------------

Before we introduce all the association mappings in detail, you
should note that the @JoinColumn and @JoinTable definitions are
usually optional and have sensible default values. The defaults for
a join column in a one-to-one/many-to-one association is as
follows:

::

    name: "<fieldname>_id"
    referencedColumnName: "id"

As an example, consider this mapping:

.. configuration-block::

    .. code-block:: php

        <?php
        /** @OneToOne(targetEntity="Shipping") **/
        private $shipping;

    .. code-block:: xml

        <doctrine-mapping>
            <entity class="Product">
                <one-to-one field="shipping" target-entity="Shipping" />
            </entity>
        </doctrine-mapping>

    .. code-block:: yaml

        Product:
          type: entity
          oneToOne:
            shipping:
              targetEntity: Shipping

This is essentially the same as the following, more verbose,
mapping:

.. configuration-block::

    .. code-block:: php

        <?php
        /**
         * @OneToOne(targetEntity="Shipping")
         * @JoinColumn(name="shipping_id", referencedColumnName="id")
         **/
        private $shipping;

    .. code-block:: xml

        <doctrine-mapping>
            <entity class="Product">
                <one-to-one field="shipping" target-entity="Shipping">
                    <join-column name="shipping_id" referenced-column-name="id" />
                </one-to-one>
            </entity>
        </doctrine-mapping>

    .. code-block:: yaml

        Product:
          type: entity
          oneToOne:
            shipping:
              targetEntity: Shipping
              joinColumn:
                name: shipping_id
                referencedColumnName: id

The @JoinTable definition used for many-to-many mappings has
similar defaults. As an example, consider this mapping:

.. configuration-block::

    .. code-block:: php

        <?php
        class User
        {
            //...
            /** @ManyToMany(targetEntity="Group") **/
            private $groups;
            //...
        }

    .. code-block:: xml

        <doctrine-mapping>
            <entity class="User">
                <many-to-many field="groups" target-entity="Group" />
            </entity>
        </doctrine-mapping>

    .. code-block:: yaml

        User:
          type: entity
          manyToMany:
            groups:
              targetEntity: Group

This is essentially the same as the following, more verbose,
mapping:

.. configuration-block::

    .. code-block:: php

        <?php
        class User
        {
            //...
            /**
             * @ManyToMany(targetEntity="Group")
             * @JoinTable(name="User_Group",
             *      joinColumns={@JoinColumn(name="User_id", referencedColumnName="id")},
             *      inverseJoinColumns={@JoinColumn(name="Group_id", referencedColumnName="id")}
             *      )
             **/
            private $groups;
            //...
        }

    .. code-block:: xml

        <doctrine-mapping>
            <entity class="User">
                <many-to-many field="groups" target-entity="Group">
                    <join-table name="User_Group">
                        <join-columns>
                            <join-column id="User_id" referenced-column-name="id" />
                        </join-columns>
                        <inverse-join-columns>
                            <join-column id="Group_id" referenced-column-name="id" />
                        </inverse-join-columns>
                    </join-table>
                </many-to-many>
            </entity>
        </doctrine-mapping>

    .. code-block:: yaml

        User:
          type: entity
          manyToMany:
            groups:
              targetEntity: Group
              joinTable:
                name: User_Group
                joinColumns:
                  User_id:
                    referencedColumnName: id
                inverseJoinColumns:
                  Group_id:
                    referencedColumnName: id

In that case, the name of the join table defaults to a combination
of the simple, unqualified class names of the participating
classes, separated by an underscore character. The names of the
join columns default to the simple, unqualified class name of the
targeted class followed by "\_id". The referencedColumnName always
defaults to "id", just as in one-to-one or many-to-one mappings.

If you accept these defaults, you can reduce the mapping code to a
minimum.

.. _collections:

Collections
-----------

In all the examples of many-valued associations in this manual we
will make use of a ``Collection`` interface and a corresponding
default implementation ``ArrayCollection`` that are defined in the
``Doctrine\Common\Collections`` namespace. Why do we need that?
Doesn't that couple my domain model to Doctrine? Unfortunately, PHP
arrays, while being great for many things, do not make up for good
collections of business objects, especially not in the context of
an ORM. The reason is that plain PHP arrays can not be
transparently extended / instrumented in PHP code, which is
necessary for a lot of advanced ORM features. The classes /
interfaces that come closest to an OO collection are ArrayAccess
and ArrayObject but until instances of these types can be used in
all places where a plain array can be used (something that may
happen in PHP6) their usability is fairly limited. You "can"
type-hint on ``ArrayAccess`` instead of ``Collection``, since the
Collection interface extends ``ArrayAccess``, but this will
severely limit you in the way you can work with the collection,
because the ``ArrayAccess`` API is (intentionally) very primitive
and more importantly because you can not pass this collection to
all the useful PHP array functions, which makes it very hard to
work with.

.. warning::

    The Collection interface and ArrayCollection class,
    like everything else in the Doctrine namespace, are neither part of
    the ORM, nor the DBAL, it is a plain PHP class that has no outside
    dependencies apart from dependencies on PHP itself (and the SPL).
    Therefore using this class in your domain classes and elsewhere
    does not introduce a coupling to the persistence layer. The
    Collection class, like everything else in the Common namespace, is
    not part of the persistence layer. You could even copy that class
    over to your project if you want to remove Doctrine from your
    project and all your domain classes will work the same as before.



Initializing Collections
------------------------

You have to be careful when using entity fields that contain a
collection of related entities. Say we have a User entity that
contains a collection of groups:

.. code-block:: php

    <?php
    /** @Entity **/
    class User
    {
        /** @ManyToMany(targetEntity="Group") **/
        private $groups;

        public function getGroups()
        {
            return $this->groups;
        }
    }

With this code alone the ``$groups`` field only contains an
instance of ``Doctrine\Common\Collections\Collection`` if the user
is retrieved from Doctrine, however not after you instantiated a
fresh instance of the User. When your user entity is still new
``$groups`` will obviously be null.

This is why we recommend to initialize all collection fields to an
empty ``ArrayCollection`` in your entities constructor:

.. code-block:: php

    <?php
    use Doctrine\Common\Collections\ArrayCollection;

    /** @Entity **/
    class User
    {
        /** @ManyToMany(targetEntity="Group") **/
        private $groups;

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

        public function getGroups()
        {
            return $this->groups;
        }
    }

Now the following code will work even if the Entity hasn't
been associated with an EntityManager yet:

.. code-block:: php

    <?php
    $group = $entityManager->find('Group', $groupId);
    $user = new User();
    $user->getGroups()->add($group);