Doctrine 2 Inheritance Mapping with Association

Question

NOTE : if what I want is not possible, a \"not possible\" answer will be accepted

In the Doctrine 2 documentation about inheritance mapping, it says

Answer 1

I think you've misunderstood, the section of the manual you've quoted is entitled "Performance impact", they're not telling you you can't do this, only that there are performance implications if you do. This makes sense for lazy loading - for heterogeneous collections of STI entities you have to go to the database and load the entity before you know what class it will be, so lazy loading isn't possible / doesn't make sense. I'm learning Doctrine 2 myself at the moment, so I mocked up your example, the following works OK for more:

namespace Entities;

/**
 * @Entity
 * @Table(name="pets")
 * @InheritanceType("SINGLE_TABLE")
 * @DiscriminatorColumn(name="pet_type", type="string")
 * @DiscriminatorMap({"cat" = "Cat", "dog" = "Dog"})
 */
class Pet
{
    /** @Id @Column(type="integer") @generatedValue */
    private $id;

    /** @Column(type="string", length=300) */
    private $name;

    /** @ManyToOne(targetEntity="User", inversedBy="id") */
    private $owner;
}


/** @Entity */
class Dog extends Pet
{

    /** @Column(type="string", length=50) */
    private $kennels;
}

/** @Entity */
class Cat extends Pet
{
    /** @Column(type="string", length=50) */
    private $cattery;
}

/**
 * @Entity
 * @Table(name="users")
 */
class User
{

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

    /** @Column(length=255, nullable=false) */
    private $name;


    /** @OneToMany(targetEntity="Pet", mappedBy="owner") */
    private $pets;
}

... and the test script ....

if (false) {
    $u = new Entities\User;
    $u->setName("Robin");

    $p = new Entities\Cat($u, 'Socks');
    $p2 = new Entities\Dog($u, 'Rover');

    $em->persist($u);
    $em->persist($p);
    $em->persist($p2);
    $em->flush();
} else if (true) {
    $u = $em->find('Entities\User', 1);
    foreach ($u->getPets() as $p) {
        printf("User %s has a pet type %s called %s\n", $u->getName(), get_class($p), $p->getName());
    }
} else {
    echo "  [1]\n";
    $p = $em->find('Entities\Cat', 2);
    echo "  [2]\n";
    printf("Pet %s has an owner called %s\n", $p->getName(), $p->getOwner()->getName());
}

All my cats and dogs load as the correct type:

If you look at the generated SQL, you'll notice that when the OneToMany targetEntity is "pet", you get SQL like this:

SELECT t0.id AS id1, t0.name AS name2, t0.owner_id AS owner_id3, pet_type, 
t0.cattery AS cattery4, t0.kennels AS kennels5 FROM pets t0 
WHERE t0.owner_id = ? AND t0.pet_type IN ('cat', 'dog')

But when it's set to Cat, you get this:

SELECT t0.id AS id1, t0.name AS name2, t0.cattery AS cattery3, t0.owner_id 
AS owner_id4, pet_type FROM pets t0 WHERE t0.owner_id = ? AND t0.pet_type IN ('cat')

HTH.

Answer 2

I'm tired, but this seems like much ado about nothing.

You missed the important bit of that warning:

If you use a STI/CTI entity as a many-to-one or one-to-one entity

That's not the case in your example! If you had not omitted the doctrine annotations, you might have noticed.

The association User::pets is OneToMany, not [One|Many]ToOne. One user has many pets.

The inverse association is OneToOne, but it's targeting User, which has no inheritance.

Robin's answer should have been a good hint -- you can log the sql queries and see what doctrine actually does to your database!

---

The bad-for-performance scenario is something like:

abstract class Pet { ... }

class Cat extends Pet { ... } 

class Dog extends Pet { ... }

class Collar {
   /**
    * @Column(length="16")
    */

   protected $color;
   /**
    * ManyToOne(targetEntity="Pet")
    */
   protected $owner;
}

Now, if you wanted to iterate over all the blue collars, Doctrine runs into some trouble. It doesn't know what class $owner is going to be, so it can't use a Proxy. Instead, it's forced to eagerly load $owner to find out whether it's a Cat or a Dog.

This isn't a problem for OneToMany or ManyToMany relationships, because in that case, lazy loading works fine. Instead of a proxy, you get a PersistentCollection. And a PersistentCollection is always just a PersistentCollection. It doesn't care about it's own contents until you actually ask for them. So lazy loading works fine.