Working with Associations

Association Example Entities

We will use a simple comment system with Users and Comments as entities to show examples of association management. See the PHP docblocks of each association in the following example for information about its type and if it's the owning or inverse side.

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<?php
#[Entity]
class User
{
    #[Id, GeneratedValue, Column]
    private int|null $id = null;

    /**
     * Bidirectional - Many users have Many favorite comments (OWNING SIDE)
     *
     * @var Collection<int, Comment>
     */
    #[ManyToMany(targetEntity: Comment::class, inversedBy: 'userFavorites')]
    #[JoinTable(name: 'user_favorite_comments')]
    private Collection $favorites;

    /**
     * Unidirectional - Many users have marked many comments as read
     *
     * @var Collection<int, Comment>
     */
    #[ManyToMany(targetEntity: Comment::class)]
    #[JoinTable(name: 'user_read_comments')]
    private Collection $commentsRead;

    /**
     * Bidirectional - One-To-Many (INVERSE SIDE)
     *
     * @var Collection<int, Comment>
     */
    #[OneToMany(targetEntity: Comment::class, mappedBy: 'author')]
    private Collection $commentsAuthored;

    /** Unidirectional - Many-To-One */
    #[ManyToOne(targetEntity: Comment::class)]
    private Comment|null $firstComment = null;
}

#[Entity]
class Comment
{
    #[Id, GeneratedValue, Column]
    private string $id;

    /**
     * Bidirectional - Many comments are favorited by many users (INVERSE SIDE)
     *
     * @var Collection<int, User>
     */
    #[ManyToMany(targetEntity: User::class, mappedBy: 'favorites')]
    private Collection $userFavorites;

    /**
     * Bidirectional - Many Comments are authored by one user (OWNING SIDE)
     */
    #[ManyToOne(targetEntity: User::class, inversedBy: 'commentsAuthored')]
    private User|null $author = null;
}
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This two entities generate the following MySQL Schema (Foreign Key definitions omitted):

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CREATE TABLE User (
    id VARCHAR(255) NOT NULL,
    firstComment_id VARCHAR(255) DEFAULT NULL,
    PRIMARY KEY(id)
) ENGINE = InnoDB;

CREATE TABLE Comment (
    id VARCHAR(255) NOT NULL,
    author_id VARCHAR(255) DEFAULT NULL,
    PRIMARY KEY(id)
) ENGINE = InnoDB;

CREATE TABLE user_favorite_comments (
    user_id VARCHAR(255) NOT NULL,
    favorite_comment_id VARCHAR(255) NOT NULL,
    PRIMARY KEY(user_id, favorite_comment_id)
) ENGINE = InnoDB;

CREATE TABLE user_read_comments (
    user_id VARCHAR(255) NOT NULL,
    comment_id VARCHAR(255) NOT NULL,
    PRIMARY KEY(user_id, comment_id)
) ENGINE = InnoDB;
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Establishing Associations

Establishing an association between two entities is straight-forward. Here are some examples for the unidirectional relations of the User:

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<?php
class User
{
    // ...
    /** @return Collection<int, Comment> */
    public function getReadComments(): Collection {
         return $this->commentsRead;
    }

    public function setFirstComment(Comment $c): void {
        $this->firstComment = $c;
    }
}
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The interaction code would then look like in the following snippet ($em here is an instance of the EntityManager):

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<?php
$user = $em->find('User', $userId);

// unidirectional many to many
$comment = $em->find('Comment', $readCommentId);
$user->getReadComments()->add($comment);

$em->flush();

// unidirectional many to one
$myFirstComment = new Comment();
$user->setFirstComment($myFirstComment);

$em->persist($myFirstComment);
$em->flush();
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In the case of bi-directional associations you have to update the fields on both sides:

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<?php
class User
{
    // ..

    /** @return Collection<int, Comment> */
    public function getAuthoredComments(): Collection {
        return $this->commentsAuthored;
    }

    /** @return Collection<int, Comment> */
    public function getFavoriteComments(): Collection {
        return $this->favorites;
    }
}

class Comment
{
    // ...

    /** @return Collection<int, User> */
    public function getUserFavorites(): Collection {
        return $this->userFavorites;
    }

    public function setAuthor(User|null $author = null): void {
        $this->author = $author;
    }
}

// Many-to-Many
$user->getFavorites()->add($favoriteComment);
$favoriteComment->getUserFavorites()->add($user);

$em->flush();

// Many-To-One / One-To-Many Bidirectional
$newComment = new Comment();
$user->getAuthoredComments()->add($newComment);
$newComment->setAuthor($user);

$em->persist($newComment);
$em->flush();
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Notice how always both sides of the bidirectional association are updated. The previous unidirectional associations were simpler to handle.

Removing Associations

Removing an association between two entities is similarly straight-forward. There are two strategies to do so, by key and by element. Here are some examples:

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<?php
// Remove by Elements
$user->getComments()->removeElement($comment);
$comment->setAuthor(null);

$user->getFavorites()->removeElement($comment);
$comment->getUserFavorites()->removeElement($user);

// Remove by Key
$user->getComments()->remove($ithComment);
$comment->setAuthor(null);
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You need to call $em->flush() to make persist these changes in the database permanently.

Notice how both sides of the bidirectional association are always updated. Unidirectional associations are consequently simpler to handle.

Also note that if you use type-hinting in your methods, you will have to specify a nullable type, i.e. setAddress(?Address $address), otherwise setAddress(null) will fail to remove the association. Another way to deal with this is to provide a special method, like removeAddress(). This can also provide better encapsulation as it hides the internal meaning of not having an address.

When working with collections, keep in mind that a Collection is essentially an ordered map (just like a PHP array). That is why the remove operation accepts an index/key. removeElement is a separate method that has O(n) complexity using array_search, where n is the size of the map.

You can also clear the contents of a whole collection using the Collections::clear() method. You should be aware that using this method can lead to a straight and optimized database delete or update call during the flush operation that is not aware of entities that have been re-added to the collection.

Say you clear a collection of tags by calling $post->getTags()->clear(); and then call $post->getTags()->add($tag). This will not recognize the tag having already been added previously and will consequently issue two separate database calls.

Association Management Methods

It is generally a good idea to encapsulate proper association management inside the entity classes. This makes it easier to use the class correctly and can encapsulate details about how the association is maintained.

The following code shows updates to the previous User and Comment example that encapsulate much of the association management code:

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<?php
class User
{
    // ...
    public function markCommentRead(Comment $comment): void {
        // Collections implement ArrayAccess
        $this->commentsRead[] = $comment;
    }

    public function addComment(Comment $comment): void {
        if (count($this->commentsAuthored) == 0) {
            $this->setFirstComment($comment);
        }
        $this->comments[] = $comment;
        $comment->setAuthor($this);
    }

    private function setFirstComment(Comment $c): void {
        $this->firstComment = $c;
    }

    public function addFavorite(Comment $comment): void {
        $this->favorites->add($comment);
        $comment->addUserFavorite($this);
    }

    public function removeFavorite(Comment $comment): void {
        $this->favorites->removeElement($comment);
        $comment->removeUserFavorite($this);
    }
}

class Comment
{
    // ..

    public function addUserFavorite(User $user): void {
        $this->userFavorites[] = $user;
    }

    public function removeUserFavorite(User $user): void {
        $this->userFavorites->removeElement($user);
    }
}
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You will notice that addUserFavorite and removeUserFavorite do not call addFavorite and removeFavorite, thus the bidirectional association is strictly-speaking still incomplete. However if you would naively add the addFavorite in addUserFavorite, you end up with an infinite loop, so more work is needed. As you can see, proper bidirectional association management in plain OOP is a non-trivial task and encapsulating all the details inside the classes can be challenging.

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<?php
class User {
    /** @return array<int, Comment> */
    public function getReadComments(): array {
        return $this->commentsRead->toArray();
    }
}
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This will however always initialize the collection, with all the performance penalties given the size. In some scenarios of large collections it might even be a good idea to completely hide the read access behind methods on the EntityRepository.

There is no single, best way for association management. It greatly depends on the requirements of your concrete domain model as well as your preferences.

Synchronizing Bidirectional Collections

In the case of Many-To-Many associations you as the developer have the responsibility of keeping the collections on the owning and inverse side in sync when you apply changes to them. Doctrine can only guarantee a consistent state for the hydration, not for your client code.

Using the User-Comment entities from above, a very simple example can show the possible caveats you can encounter:

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<?php
$user->getFavorites()->add($favoriteComment);
// not calling $favoriteComment->getUserFavorites()->add($user);

$user->getFavorites()->contains($favoriteComment); // TRUE
$favoriteComment->getUserFavorites()->contains($user); // FALSE
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There are two approaches to handle this problem in your code:

1. Ignore updating the inverse side of bidirectional collections, BUT never read from them in requests that changed their state. In the next request Doctrine hydrates the consistent collection state again.
2. Always keep the bidirectional collections in sync through association management methods. Reads of the Collections directly after changes are consistent then.

Transitive persistence / Cascade Operations

Doctrine ORM provides a mechanism for transitive persistence through cascading of certain operations. Each association to another entity or a collection of entities can be configured to automatically cascade the following operations to the associated entities: persist, remove, detach, refresh or all.

The main use case for cascade: persist is to avoid "exposing" associated entities to your PHP application. Continuing with the User-Comment example of this chapter, this is how the creation of a new user and a new comment might look like in your controller (without cascade: persist):

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<?php
$user = new User();
$myFirstComment = new Comment();
$user->addComment($myFirstComment);

$em->persist($user);
$em->persist($myFirstComment); // required, if `cascade: persist` is not set
$em->flush();
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Note that the Comment entity is instantiated right here in the controller. To avoid this, cascade: persist allows you to "hide" the Comment entity from the controller, only accessing it through the User entity:

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<?php
// User entity
class User
{
    private int $id;

    /** @var Collection<int, Comment> */
    private Collection $comments;

    public function __construct()
    {
        $this->id = User::new();
        $this->comments = new ArrayCollection();
    }

    public function comment(string $text, DateTimeInterface $time) : void
    {
        $newComment = Comment::create($text, $time);
        $newComment->setUser($this);
        $this->comments->add($newComment);
    }

    // ...
}
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If you then set up the cascading to the User#commentsAuthored property...

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<?php
class User
{
    // ...
    /** Bidirectional - One-To-Many (INVERSE SIDE) */
    #[OneToMany(targetEntity: Comment::class, mappedBy: 'author', cascade: ['persist', 'remove'])]
    private $commentsAuthored;
    // ...
}
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...you can now create a user and an associated comment like this:

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<?php
$user = new User();
$user->comment('Lorem ipsum', new DateTime());

$em->persist($user);
$em->flush();
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Thanks to cascade: remove, you can easily delete a user and all linked comments without having to loop through them:

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$user = $em->find('User', $deleteUserId);

$em->remove($user);
$em->flush();
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Even though automatic cascading is convenient, it should be used with care. Do not blindly apply cascade=all to all associations as it will unnecessarily degrade the performance of your application. For each cascade operation that gets activated, Doctrine also applies that operation to the association, be it single or collection valued.

Orphan Removal

There is another concept of cascading that is relevant only when removing entities from collections. If an Entity of type A contains references to privately owned Entities B then if the reference from A to B is removed the entity B should also be removed, because it is not used anymore.

OrphanRemoval works with one-to-one, one-to-many and many-to-many associations.

As a better example consider an Addressbook application where you have Contacts, Addresses and StandingData:

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<?php

namespace Addressbook;

use Doctrine\Common\Collections\ArrayCollection;

#[Entity]
class Contact
{
    #[Id, Column(type: 'integer'), GeneratedValue]
    private int|null $id = null;

    #[OneToOne(targetEntity: StandingData::class, cascade: ['persist'], orphanRemoval: true)]
    private StandingData|null $standingData = null;

    /** @var Collection<int, Address> */
    #[OneToMany(targetEntity: Address::class, mappedBy: 'contact', cascade: ['persist'], orphanRemoval: true)]
    private Collection $addresses;

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

    public function newStandingData(StandingData $sd): void
    {
        $this->standingData = $sd;
    }

    public function removeAddress(int $pos): void
    {
        unset($this->addresses[$pos]);
    }
}
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Now two examples of what happens when you remove the references:

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<?php

$contact = $em->find("Addressbook\Contact", $contactId);
$contact->newStandingData(new StandingData("Firstname", "Lastname", "Street"));
$contact->removeAddress(1);

$em->flush();
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In this case you have not only changed the Contact entity itself but you have also removed the references for standing data and as well as one address reference. When flush is called not only are the references removed but both the old standing data and the one address entity are also deleted from the database.

Filtering Collections

Collections have a filtering API that allows to slice parts of data from a collection. If the collection has not been loaded from the database yet, the filtering API can work on the SQL level to make optimized access to large collections.

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<?php

use Doctrine\Common\Collections\Criteria;

$group          = $entityManager->find('Group', $groupId);
$userCollection = $group->getUsers();

$criteria = Criteria::create()
    ->where(Criteria::expr()->eq("birthday", "1982-02-17"))
    ->orderBy(array("username" => Criteria::ASC))
    ->setFirstResult(0)
    ->setMaxResults(20)
;

$birthdayUsers = $userCollection->matching($criteria);
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The Criteria has a limited matching language that works both on the SQL and on the PHP collection level. This means you can use collection matching interchangeably, independent of in-memory or sql-backed collections.

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<?php

use Doctrine\Common\Collections;

class Criteria
{
    /**
     * @return Criteria
     */
    static public function create();
    /**
     * @param Expression $where
     * @return Criteria
     */
    public function where(Expression $where);
    /**
     * @param Expression $where
     * @return Criteria
     */
    public function andWhere(Expression $where);
    /**
     * @param Expression $where
     * @return Criteria
     */
    public function orWhere(Expression $where);
    /**
     * @param array $orderings
     * @return Criteria
     */
    public function orderBy(array $orderings);
    /**
     * @param int $firstResult
     * @return Criteria
     */
    public function setFirstResult($firstResult);
    /**
     * @param int $maxResults
     * @return Criteria
     */
    public function setMaxResults($maxResults);
    public function getOrderings();
    public function getWhereExpression();
    public function getFirstResult();
    public function getMaxResults();
}
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You can build expressions through the ExpressionBuilder. It has the following methods:

• andX($arg1, $arg2, ...)
• orX($arg1, $arg2, ...)
• not($expression)
• eq($field, $value)
• gt($field, $value)
• lt($field, $value)
• lte($field, $value)
• gte($field, $value)
• neq($field, $value)
• isNull($field)
• in($field, array $values)
• notIn($field, array $values)
• contains($field, $value)
• memberOf($value, $field)
• startsWith($field, $value)
• endsWith($field, $value)