5. 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.
単一のオブジェクトへの参照は、外部キーによって表されます。A collection of objects is represented by many foreign keys pointing to the object holding the collection
オブジェクトのコレクションは、コレクションを保持しているオブジェクトを指す多くの外部キーによって表されます
This chapter is split into three different sections.
A list of all the possible association mapping use-cases is given.
すべての可能なアソシエーション マッピングの使用例のリストが示されています。Mapping Defaults are explained that simplify the use-case examples.
ユースケースの例を簡素化するマッピングのデフォルトについて説明します。Collections are introduced that contain entities in associations.
関連付けにエンティティを含むコレクションが導入されました。
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 referred Entity.
OneToMany - 現在のエンティティの 1 つのインスタンスには、参照されたエンティティへの多数のインスタンス (参照) があります。ManyToOne - Many instances of the current Entity refer to One instance of the referred Entity.
ManyToOne - 現在のエンティティの多数のインスタンスが、参照されたエンティティの 1 つのインスタンスを参照します。OneToOne - One instance of the current Entity refers to One instance of the referred Entity.
OneToOne - 現在のエンティティの 1 つのインスタンスは、参照されたエンティティの 1 つのインスタンスを参照します。
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
5.1. Many-To-One, Unidirectional¶
A many-to-one association is the most common association between objects. Example: Many Users have One Address:
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.
Likewise, inside the #[ManyToOne] attribute you can omit the
targetEntity argument and it will default to Address.
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);
5.2. One-To-One, Unidirectional¶
Here is an example of a one-to-one association with a Product entity that
references one Shipment entity.
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);
5.3. 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 attributes for the first time.
They are used to tell Doctrine which property on the other side refers to the
object.
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,
UNIQUE INDEX UNIQ_BA388B79395C3F3 (customer_id),
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.
5.4. 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 Mentor. */
#[OneToOne(targetEntity: Student::class)]
#[JoinColumn(name: 'mentor_id', referencedColumnName: 'id')]
private Student|null $mentor = null;
// ...
}
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);
5.5. 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.
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);
5.6. 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:
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);
5.7. 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.
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);
5.8. Many-To-Many, Unidirectional¶
Real many-to-many associations are less common. The following example shows a unidirectional association between User and Group entities:
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.
5.9. Many-To-Many, Bidirectional¶
Here is a similar many-to-many relationship as above except this one is bidirectional.
The MySQL schema is exactly the same as for the Many-To-Many uni-directional case above.
5.9.1. 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 Collection $tags;
public function addTag(Tag $tag): void
{
$tag->addArticle($this); // synchronously updating inverse side
$this->tags[] = $tag;
}
}
class Tag
{
private Collection $articles;
public function addArticle(Article $article): void
{
$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);
5.10. 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.
* @var Collection<int, User>
*/
#[ManyToMany(targetEntity: User::class, mappedBy: 'myFriends')]
private Collection $friendsWithMe;
/**
* Many Users have many Users.
* @var Collection<int, User>
*/
#[JoinTable(name: 'friends')]
#[JoinColumn(name: 'user_id', referencedColumnName: 'id')]
#[InverseJoinColumn(name: 'friend_user_id', referencedColumnName: 'id')]
#[ManyToMany(targetEntity: 'User', inversedBy: 'friendsWithMe')]
private Collection $myFriends;
public function __construct() {
$this->friendsWithMe = new ArrayCollection();
$this->myFriends = new 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);
5.11. 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:
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.
Additionally, when using typed properties with Doctrine 2.9 or newer
you can skip targetEntity in ManyToOne and OneToOne
associations as they will be set based on type. Also nullable
attribute on JoinColumn will be inherited from PHP type. So that:
Is essentially the same as following:
If you accept these defaults, you can reduce the mapping code to a minimum.
5.12. 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.
5.13. 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. */
#[ManyToMany(targetEntity: Group::class)]
private Collection $groups;
public function __construct()
{
$this->groups = new ArrayCollection();
}
public function getGroups(): Collection
{
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);
