Doctrine 2 - Eager loading doesn't appear to work with ManyToMany

Question

I have three entities: SequenceRun has one MaterialTypeString, and has many User. I.e. There is a OneToMany relationship between SequenceRun and MaterialTypeString, and a ManyToMany relationship between SequenceRun and User.

SequenceRun.php:

/**
 * @ORM\ManyToOne(targetEntity="MaterialTypeStrings", inversedBy="sequenceRuns")
 * @ORM\JoinColumn(name="material_type_strings_id", referencedColumnName="id")
 */
private $materialTypeString;

 /**
  * Many Groups have Many Users.
  * @ORM\ManyToMany(targetEntity="FOSUser", mappedBy="sequenceRuns")
  */
 private $users;

Now without eager loading my index method:

/**
 * @Route("/sequence_run/index", name="sequence_run_index")
 */
public function indexAction() {
    // Grab all experiments from database and hand them to template.
    $repository = $this->getDoctrine()->getRepository('AppBundle:SequenceRun');
    $sequence_runs = $repository->findAll();
    return $this->render('sequence_run/index.html.twig',['sequence_runs' => $sequence_runs]);
}

Produces 4 queries for a single SequenceRun object:

SELECT t0.username AS username_1, t0.username_canonical AS username_canonical_2, t0.email AS email_3, t0.email_canonical AS email_canonical_4, t0.enabled AS enabled_5, t0.salt AS salt_6, t0.last_login AS last_login_7, t0.confirmation_token AS confirmation_token_8, t0.password_requested_at AS password_requested_at_9, t0.roles AS roles_10, t0.id AS id_11, t0.dn AS dn_12, t0.cn AS cn_13, t0.department AS department_14, t0.department_dn AS department_dn_15, t0.from_bio_control AS from_bio_control_16, t0.password AS password_17 FROM fos_user t0 WHERE t0.id = ? LIMIT 1
Parameters: [0 => 96]

SELECT t0.id AS id_1, t0.start_date AS start_date_2, t0.end_dat AS end_dat_3, t0.kit AS kit_4, t0.run_coverage_target AS run_coverage_target_5, t0.read_length AS read_length_6, t0.created_at AS created_at_7, t0.updated_at AS updated_at_8, t0.material_type_strings_id AS material_type_strings_id_9 FROM sequence_run t0
Parameters: []

SELECT t0.username AS username_1, t0.username_canonical AS username_canonical_2, t0.email AS email_3, t0.email_canonical AS email_canonical_4, t0.enabled AS enabled_5, t0.salt AS salt_6, t0.last_login AS last_login_7, t0.confirmation_token AS confirmation_token_8, t0.password_requested_at AS password_requested_at_9, t0.roles AS roles_10, t0.id AS id_11, t0.dn AS dn_12, t0.cn AS cn_13, t0.department AS department_14, t0.department_dn AS department_dn_15, t0.from_bio_control AS from_bio_control_16, t0.password AS password_17 FROM fos_user t0 INNER JOIN users_sequence_runs ON t0.id = users_sequence_runs.fosuser_id WHERE users_sequence_runs.sequence_run_id = ?
Parameters: [0 => 2]

SELECT t0.id AS id_1, t0.type AS type_2 FROM material_type_strings t0 WHERE t0.id = ?
Parameters: [0 => 5]

If I add fetch = "EAGER" to MaterialTypeString, it drops the last query (as expected). But if I add also add it to User I still get 3 queries.

Does eager loading work for ManyToMany relationships, or will I have to use DQL and write the query manually? (If so what would that look like?)

Answer 1

What are you trying to achieve? If you want to have a single query against your dB in which you fetch all the needed entities, you do need to write a custom query manually, it's not a matter of eager or lazy loading.

You could write something like this

$sequence_runs=$repository->createQueryBuilder('sr')->addSelect(['mts', 'fu'])
->leftJoin('sr.materialTypeString', 'mts')
->leftJoin('sr.users', 'fu')
->getQuery()->getResult();

You use eager loading when you want doctrine to do all the needed queries no matter if you use your associations in your code or not, while you use lazy loading to have your associations fully hydrated only if you access them in your code, but the number of queries will be the same in case you access all your associations in your code.

The point is: are you showing a paginated list of entities? Then, try to write your own query with DQL or a query builder, fetch loading all the necessary entities in reason of your views. Are you showing a single entity? Then fetch it from the repository lazy loading your associations and let doctrine do the job!