php - Doctrine's Many-To-Many Self-Referencing and reciprocity

Question

By default, self-referencing ManyToMany relationships under Doctrine involve an owning side and an inverse side, as explained in the documentation.

Is there a way to implement a reciprocal association whithout difference between both sides?

Following the example in the docs:

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

    /**
     * @ManyToMany(targetEntity="User")
     **/
    private $friends;

    public function __construct() {
        $this->friends = new DoctrineCommonCollectionsArrayCollection();
    }

    // ...
}

So, adding entity1 to entity2s friends implies that entity2 will be in entity1s friends.

Answer 1

There are a number of ways to solve this problem, all depending on what the requirements for the "friends" relation are.

Unidirectional

A simple approach would be to use a unidirectional ManyToMany association, and treat it as if it where a bidirectional one (keeping both sides in sync):

/**
 * @Entity
 */
class User
{
    /**
     * @Id
     * @Column(type="integer")
     */
    private $id;

    /**
     * @ManyToMany(targetEntity="User")
     * @JoinTable(name="friends",
     *     joinColumns={@JoinColumn(name="user_a_id", referencedColumnName="id")},
     *     inverseJoinColumns={@JoinColumn(name="user_b_id", referencedColumnName="id")}
     * )
     * @var DoctrineCommonCollectionsArrayCollection
     */
    private $friends;

    /**
     * Constructor.
     */
    public function __construct()
    {
        $this->friends = new DoctrineCommonCollectionsArrayCollection();
    }

    /**
     * @return array
     */
    public function getFriends()
    {
        return $this->friends->toArray();
    }

    /**
     * @param  User $user
     * @return void
     */
    public function addFriend(User $user)
    {
        if (!$this->friends->contains($user)) {
            $this->friends->add($user);
            $user->addFriend($this);
        }
    }

    /**
     * @param  User $user
     * @return void
     */
    public function removeFriend(User $user)
    {
        if ($this->friends->contains($user)) {
            $this->friends->removeElement($user);
            $user->removeFriend($this);
        }
    }

    // ...

}

When you call $userA->addFriend($userB), $userB will be added to the friends-collection in $userA, and $userA will be added to the friends-collection in $userB.

It will also result in 2 records added to the "friends" table (1,2 and 2,1). While this can be seen as duplicate data, it will simplify your code a lot. For example when you need to find all friends of $userA, you can simply do:

SELECT u FROM User u JOIN u.friends f WHERE f.id = :userId

No need to check 2 different properties as you would with a bidirectional association.

Bidirectional

When using a bidirectional association the User entity will have 2 properties, $myFriends and $friendsWithMe for example. You can keep them in sync the same way as described above.

The main difference is that on a database level you'll only have one record representing the relationship (either 1,2 or 2,1). This makes "find all friends" queries a bit more complex because you'll have to check both properties.

You could of course still use 2 records in the database by making sure addFriend() will update both $myFriends and $friendsWithMe (and keep the other side in sync). This will add some complexity in your entities, but queries become a little less complex.

OneToMany / ManyToOne

If you need a system where a user can add a friend, but that friend has to confirm that they are indeed friends, you'll need to store that confirmation in the join-table. You then no longer have a ManyToMany association, but something like User <- OneToMany -> Friendship <- ManyToOne -> User.

You can read my blog-posts on this subject:

• Doctrine 2: How to handle join tables with extra columns
• More on one-to-many/many-to-one associations in Doctrine 2