Associations between entities are represented just like in regular object-oriented PHP, with references to other objects
or collections of objects. When it comes to persistence, it is important to understand three main things:

 * The concept of owning and inverse sides in bidirectional associations as described [here](http://www.doctrine-project.org/documentation/manual/2_0/en/association-mapping#owning-side-and-inverse-side).
 * If an entity is removed from a collection, the association is removed, not the entity itself. A collection of entities always only represents the association to the containing entities, not the entity itself.
 * Collection-valued persistent fields have to be instances of the `Doctrine\Common\Collections\Collection` interface. [See here](http://www.doctrine-project.org/documentation/manual/2_0/en/architecture#entities:persistent-fields) for more details.

Changes to associations in your code are not synchronized to the database directly, but upon calling `EntityManager#flush()`.

To describe all the concepts of working with associations we introduce a specific set of example entities that show
all the different flavors of association management in Doctrine.

## 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 its the owning or inverse side.

    <?php
    /** @Entity */
    class User
    {
        /** @Id @GeneratedValue @Column(type="string") */
        private $id;

        /**
         * Bidirectional - Many users have Many favorite comments (OWNING SIDE)
         *
         * @ManyToMany(targetEntity="Comment", inversedBy="userFavorites")
         * @JoinTable(name="user_favorite_comments",
         *   joinColumns={@JoinColumn(name="user_id", referencedColumnName="id")},
         *   inverseJoinColumns={@JoinColumn(name="favorite_comment_id", referencedColumnName="id")}
         * )
         */
        private $favorites;

        /**
         * Unidirectional - Many users have marked many comments as read
         *
         * @ManyToMany(targetEntity="Comment")
         * @JoinTable(name="user_read_comments",
         *   joinColumns={@JoinColumn(name="user_id", referencedColumnName="id")},
         *   inverseJoinColumns={@JoinColumn(name="comment_id", referencedColumnName="id")}
         * )
         */
        private $commentsRead;

        /**
         * Bidirectional - One-To-Many (INVERSE SIDE)
         *
         * @OneToMany(targetEntity="Comment", mappedBy="author")
         */
        private $commentsAuthored;

        /**
         * Unidirectional - Many-To-One
         *
         * @ManyToOne(targetEntity="Comment")
         */
        private $firstComment;
    }

    /** @Entity */
    class Comment
    {
        /** @Id @GeneratedValue @Column(type="string") */
        private $id;

        /**
         * Bidirectional - Many comments are favorited by many users (INVERSE SIDE)
         *
         * @ManyToMany(targetEntity="User", mappedBy="favorites")
         */
        private $userFavorites;

        /**
         * Bidirectional - Many Comments are authored by one user (OWNING SIDE)
         *
         * @ManyToOne(targetEntity="User", inversedBy="authoredComments")
         */
         private $author;
    }

This two entities generate the following MySQL Schema (Foreign Key definitions omitted):

    [sql]
    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;

## Establishing Associations

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

    <?php
    class User
    {
        // ...
        public function getReadComments() {
             return $this->commentsRead;
        }

        public function setFirstComment(Comment $c) {
            $this->firstComment = $c;
        }
    }

The interaction code would then look like in the following snippet (`$em` here is an instance of the EntityManager):

    <?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();

In the case of bi-directional associations you have to update the fields on both sides:

    <?php
    class User
    {
        // ..

        public function getAuthoredComments() {
            return $this->commentsAuthored;
        }

        public function getFavoriteComments() {
            return $this->favorites;
        }
    }

    class Comment
    {
        // ...

        public function getUserFavorites() {
            return $this->userFavorites;
        }

        public function setAuthor(User $author = null) {
            $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();


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:

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

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

    // Remove by Key
    $user->getComments()->removeElement($ithComment);
    $comment->setAuthor(null);

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 type-hint your methods, i.e. `setAddress(Address $address)`, then PHP does only
allows null values if `null` is set as default value. Otherwise setAddress(null) will fail for removing the association.
If you insist on type-hinting a typical 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.

> **NOTE**
>
> Since Doctrine always only looks at the owning side of a bidirectional association for updates, it is not necessary
> for write operations that an inverse collection of a bidirectional one-to-many or many-to-many association is updated.
> This knowledge can often be used to improve performance by avoiding the loading of the inverse collection.

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 tag being already added before and issue
two 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:

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

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

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

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

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

    class Comment
    {
        // ..

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

        public function removeUserFavorite(User $user) {
            $this->userFavorites->removeElement($user);
        }
    }

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.

> **NOTE**
>
> If you want to make sure that your collections are perfectly encapsulated you should not return
> them from a `getCollectionName()` method directly, but call `$collection->toArray()`. This way a client programmer
> for the entity cannot circumvent the logic you implement on your entity for association management. For example:

    <?php
    class User {
        public function getReadComments() {
            return $this->commentsRead->toArray();
        }
    }

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 are responsible to keep the collections on the
owning and inverse side up 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:

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

    $user->getFavorites()->contains($favoriteComment); // TRUE
    $favoriteComment->getUserFavorites()->contains($user); // FALSE

There are to 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

Persisting, removing, detaching and merging individual entities can become pretty
cumbersome, especially when a larger object graph with collections is involved.
Therefore Doctrine 2 provides a mechanism for transitive persistence through
cascading of these operations. Each association to another entity or a collection
of entities can be configured to automatically cascade certain operations. By
default, no operations are cascaded.

The following cascade options exist:

 * persist : Cascades persist operations to the associated entities.
 * remove : Cascades remove operations to the associated entities.
 * merge : Cascades merge operations to the associated entities.
 * detach : Cascades detach operations to the associated entities.
 * all : Cascades persist, remove, merge and detach operations to associated entities.

The following example is an extension to the User-Comment example of this chapter.
Suppose in our application a user is created whenever he writes his first comment.
In this case we would use the following code:

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

    $em->persist($user);
    $em->persist($myFirstComment);
    $em->flush();

Even if you *persist* a new User that contains our new Comment this code would fail
if you removed the call to `EntityManager#persist($myFirstComment)`. Doctrine 2 does
not cascade the persist operation to all nested entities that are new as well.

More complicated is the deletion of all a users comments when he is removed from the system:

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

    foreach ($user->getAuthoredComments() AS $comment) {
        $em->remove($comment);
    }
    $em->remove($user);
    $em->flush();

Without the loop over all the authored comments Doctrine would use an UPDATE statement only
to set the foreign key to NULL and only the User would be deleted from the database
during the flush()-Operation.

To have Doctrine handle both cases automatically we can change the `User#commentsAuthored`
property to cascade both the "persist" and the "remove" operation.

    <?php
    class User
    {
        //...
        /**
         * Bidirectional - One-To-Many (INVERSE SIDE)
         *
         * @OneToMany(targetEntity="Comment", mappedBy="author", cascade={"persist", "remove"})
         */
        private $commentsAuthored;
        //...
    }

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.

### Persistence by Reachability: Cascade Persist

There are additional semantics that apply to the Cascade Persist operation.
During each flush() operation Doctrine detects if there are new entities in any
collection and three possible cases can happen:

1. New entities in a collection marked as cascade persist will be directly persisted by Doctrine.
2. New entities in a collection not marked as cascade persist will produce an Exception and rollback the flush() operation.
3. Collections without new entities are skipped.

This concept is called Persistence by Reachability: New entities that are found on
already managed entities are automatically persisted as long as the association is defined
as cascade persist.