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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
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.
- PHP
- XML
- YAML
Note that the @JoinColumn is not really necessary in this example, as the defaults would be the same.
Generated MySQL Schema:
1 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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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.
- PHP
1 <?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; // ... } 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 - XML
1 <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> 2 3 4 5 6 7 8 9 10 - YAML
Note that the @JoinColumn is not really necessary in this example, as the defaults would be the same.
Generated MySQL Schema:
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.
Note that the @JoinColumn is not really necessary in this example, as the defaults would be the same.
With the generated MySQL Schema:
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.
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:
- PHP
1 <?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 { // ... } 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 - XML
1 <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> 2 3 4 5 6 7 8 9 10 11 12 13 14 - YAML
Generates the following MySQL Schema:
1 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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Many-To-One, Unidirectional
You can easily implement a many-to-one unidirectional association with the following:
- PHP
- XML
- YAML
The above |
Generated MySQL Schema:
1 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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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:
- PHP
1 <?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; // ... } 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 - XML
1 <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> 2 3 4 5 6 7 8 9 10 - YAML
Note that the @JoinColumn is not really necessary in this example, as the defaults would be the same.
Generated MySQL Schema:
1 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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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
1 <?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(); } } 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 - XML
- YAML
Note that the @JoinColumn is not really necessary in this example, as the defaults would be the same.
Generated MySQL Schema:
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
1 <?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 { // ... } 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 - XML
1 <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> 2 3 4 5 6 7 8 9 10 11 12 13 14 - YAML
Generated MySQL Schema:
1 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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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
1 <?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(); } // ... } 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 - XML
1 <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> 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 - YAML
1 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 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
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:
1 <?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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This allows to group the tag adding on the Article
side of the
association:
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
.
1 <?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:
1 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:
- PHP
- XML
- YAML
This is essentially the same as the following, more verbose, mapping:
- PHP
- XML
- YAML
The @JoinTable definition used for many-to-many mappings has similar defaults. As an example, consider this mapping:
- PHP
- XML
- YAML
This is essentially the same as the following, more verbose, mapping:
- PHP
1 <?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; //... } 2 3 4 5 6 7 8 9 10 11 12 13 14 - XML
1 <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> 2 3 4 5 6 7 8 9 10 11 12 13 14 - YAML
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.
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:
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:
1 <?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: