Doctrine2多对一关联不会使用JOIN查询

Question

I have a Car entity with a many-to-one relationship with an entity Owner . If I select all cars, Doctrine does one query on the Car table, and subsequently one query on the Owner table for each car . So fetching N cars becomes N+1 queries instead of a single JOIN query between the Car and Owner tables.

My entities are as follows:

/** @Entity */
class Car {

  /** @Id @Column(type="smallint") */
  private $id;

  /** @ManyToOne(targetEntity="Owner", fetch="EAGER")
      @JoinColumn(name="owner", referencedColumnName="id") */
  private $owner;

  public function getId()    { return $this->id; }
  public function getOwner() { return $this->owner; }
}

/** @Entity */
class Owner {

  /** @Id @Column(type="smallint") */
  private $id;

  /** @Column(type="string") */
  private $name;

  public function getName() { return $this->name; }
}

If I want to list the cars with their owners, I do:

$repo = $em->getRepository('Car');
$cars = $repo->findAll();

foreach($cars as $car) 
  echo 'Car no. ' . $car->getId() . 
       ' owned by ' . $car->getOwner()->getName() . '\n';

Now this all works very well, apart from the fact that Doctrine issues a query for each car.

SELECT * FROM Car;
SELECT * FROM Owner WHERE id = 1;
SELECT * FROM Owner WHERE id = 2;
SELECT * FROM Owner WHERE id = 3;
....

Of course I'd want my query log to look like this:

SELECT * FROM Car JOIN Owner ON Car.owner = Owner.id;

Whether I have fetch="EAGER" or fetch="LAZY" doesn't matter, and even if I make a custom DQL query with JOIN between the two entities, $car->getOwner() still causes Doctrine to query the database (unless I use EAGER, in which case $repo->findAll() causes all of them).

Am I just too tired here, and this is the way it is supposed to work - or is there a clever way to force Doctrine to do the JOIN query instead?

Answer 1

At least in 1.x Doctrine if you wanted to query for the related objects, you had to use DQL. For your case, the DQL query would look something like this:

//Assuming $em is EntityManager
$query = $em->createQuery('SELECT c, o FROM Car c JOIN c.owner o');
$cars = $query->execute();

Answer 2

Run first a DQL query where you select all the cars joined (DQL JOIN) with the owner. Put the owner in the select() .

// preload cars
$qb = $em->createQueryBuilder()
        ->select('car, owner')
        ->from('\Entity\Car', 'car')
        ->leftJoin('c.owner',  'owner');

    $query = $qb->getQuery();

    // the following seems not needed, but I think it depends on the conf
    $query->setFetchMode("\Entity\Car", "owner", "EAGER");

    $query->execute(); //you don't have to use this result here, Doctrine will keep it

Doctrine 2 will then perform a JOIN (normally faster as it requires less db queries depending on the number of records). Now launch your foreach , Doctrine will find the entities internally and it won't run single queries when you need the owner .

Monitor the number of queries first/after each change (eg. mysql general log)

Answer 3

Your query...

$car->getOwner() // "go and fetch this car's owner"

... is in a foreach loop so it will certainly issue the query several times.

If you're writing custom DQL to deal with this, $car->getOwner() shouldn't feature in this at all. This is a function of the Car class. The custom DQL you would write would mimick the exact SQL query you point out and get your join done efficiently.