association-mapping.rst

Association Mapping

This chapter explains mapping associations between objects.

Instead of working with foreign keys in your code, you will always work with references to objects instead and Doctrine will convert those references to foreign keys internally.

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

One tip for working with relations is to read the relation from left to right, where the left word refers to the current Entity. For example:

• OneToMany - One instance of the current Entity has Many instances (references) to the refered Entity.
• ManyToOne - Many instances of the current Entity refer to One instance of the refered Entity.
• OneToOne - One instance of the current Entity refers to One instance of the refered Entity.

See below for all the possible relations.

An association is considered to be unidirectional if only one side of the association has a property referring to the other side.

To gain a full understanding of associations you should also read about owning and inverse sides of associations <unitofwork-associations>

Many-To-One, Unidirectional

A many-to-one association is the most common association between objects. Example: Many Users have One Address:

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

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

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

<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>

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:

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-One, Unidirectional

Here is an example of a one-to-one association with a Product entity that references one Shipment entity.

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

    /**
     * One Product has One Shipment.
     * @OneToOne(targetEntity="Shipment")
     * @JoinColumn(name="shipment_id", referencedColumnName="id")
     */
    private $shipment;

    // ...
}

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

<doctrine-mapping>
    <entity class="Product">
        <one-to-one field="shipment" target-entity="Shipment">
            <join-column name="shipment_id" referenced-column-name="id" />
        </one-to-one>
    </entity>
</doctrine-mapping>

Product:
  type: entity
  oneToOne:
    shipment:
      targetEntity: Shipment
      joinColumn:
        name: shipment_id
        referencedColumnName: id

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

Generated MySQL Schema:

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

Here we see the mappedBy and inversedBy annotations for the first time. They are used to tell Doctrine which property on the other side refers to the object.

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

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

    // ...
}

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

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

    // ...
}

<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>

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:

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);

We had a choice of sides on which to place the inversedBy attribute. Because it is on the Cart, that is the owning side of the relation, and thus holds the foreign key.

One-To-One, Self-referencing

You can define a self-referencing one-to-one relationships like below.

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

    /**
     * One Student has One 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:

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, Bidirectional

A one-to-many association has to be bidirectional, unless you are using a join table. This is because the "many" side in a one-to-many association holds the foreign key, making it the owning side. Doctrine needs the "many" side defined in order to understand the association.

This bidirectional mapping requires the mappedBy attribute on the "one" side and the inversedBy attribute on the "many" side.

This means there is no difference between a bidirectional one-to-many and a bidirectional many-to-one.

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

/** @Entity */
class Product
{
    // ...
    /**
     * One product has many features. This is the inverse side.
     * @OneToMany(targetEntity="Feature", mappedBy="product")
     */
    private $features;
    // ...

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

/** @Entity */
class Feature
{
    // ...
    /**
     * Many features have one product. This is the owning side.
     * @ManyToOne(targetEntity="Product", inversedBy="features")
     * @JoinColumn(name="product_id", referencedColumnName="id")
     */
    private $product;
    // ...
}

<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>

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:

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, 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.

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

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

    /**
     * Many User have Many Phonenumbers.
     * @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
{
    // ...
}

<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>

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:

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);

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.

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

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

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

<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>

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:

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:

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

    /**
     * Many Users have Many Groups.
     * @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
{
    // ...
}

<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>

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

Generated MySQL Schema:

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.

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

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

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

    // ...
}

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

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

    // ...
}

<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>

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.

Owning and Inverse Side on a ManyToMany Association

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 specifying the tags directly. This is why you should pick the Article as owning side, as it makes the code more understandable:

<?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:

<?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.

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

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

    /**
     * Many Users have many Users.
     * @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:

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);

Mapping Defaults

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:

<?php
/** @OneToOne(targetEntity="Shipment") */
private $shipment;

<doctrine-mapping>
    <entity class="Product">
        <one-to-one field="shipment" target-entity="Shipment" />
    </entity>
</doctrine-mapping>

Product:
  type: entity
  oneToOne:
    shipment:
      targetEntity: Shipment

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

<?php
/**
 * One Product has One Shipment.
 * @OneToOne(targetEntity="Shipment")
 * @JoinColumn(name="shipment_id", referencedColumnName="id")
 */
private $shipment;

<doctrine-mapping>
    <entity class="Product">
        <one-to-one field="shipment" target-entity="Shipment">
            <join-column name="shipment_id" referenced-column-name="id" />
        </one-to-one>
    </entity>
</doctrine-mapping>

Product:
  type: entity
  oneToOne:
    shipment:
      targetEntity: Shipment
      joinColumn:
        name: shipment_id
        referencedColumnName: id

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

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

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

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

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

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

<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>

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

Unfortunately, PHP arrays, while being great for many things, are missing features that make them suitable for lazy loading in the context of an ORM. This is why in all the examples of many-valued associations in this manual we will make use of a Collection interface and its default implementation ArrayCollection that are both defined in the Doctrine\Common\Collections namespace. A collection implements the PHP interfaces ArrayAccess, Traversable and Countable.

Note

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 model and elsewhere does not introduce a coupling to the ORM.

Initializing Collections

You should always initialize the collections of your @OneToMany and @ManyToMany associations in the constructor of your entities:

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

/** @Entity */
class User
{
    /**
     * Many Users have Many Groups.
     * @var Collection
     * @ManyToMany(targetEntity="Group")
     */
    private $groups;

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

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

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

<?php
$group = new Group();
$user = new User();
$user->getGroups()->add($group);