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

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 \Doctrine\Common\Collections\ArrayCollection();
}
// ...
}
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 \Doctrine\Common\Collections\ArrayCollection
*/
private $friends;
/**
* Constructor.
*/
public function __construct()
{
$this->friends = new \Doctrine\Common\Collections\ArrayCollection();
}
/**
* #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