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.
• Association Mapping are explained that simplify the use-case examples.
• Association Mapping are introduced that contain entities in associations.

To master associations you should also learn about owning and inverse sides of associations

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

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

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

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<?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();
    }

    // ...
}
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Generated MySQL Schema:

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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);
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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: "_id"
referencedColumnName: "id"

As an example, consider this mapping:

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

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

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

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

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.

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:

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

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

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

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