I'm working on a project made in Symfony 3.1.10.

I have three entities:

MyEntity 1->n MyPivotEntity

MyPivotEntity n->1 MySuperInheritanceEntity

and I have another entity MyInheritanceEntity which inherit from MySuperInheritanceEntity with a single_table inheritance

https://www.doctrine-project.org/projects/doctrine-orm/en/2.6/reference/inheritance-mapping.html#single-table-inheritance

I made a CollectionType field of MyPivotEntity in MyEntityType form, but when I createForm from the controller I get a memory exceeded message, because the builder do a database request for each MySuperInheritanceEntity. How can I prevent this? In this case I don't need MySuperInheritanceEntity information at all, I just need MyPivotEntity fields

<?php 

/**
 * MyEntity
 *
 * @ORM\Table(name="my_entity")
 * @ORM\Entity()
 */
class MyEntity {
    /**
     * @ORM\OneToMany(targetEntity="MyPivotEntity", mappedBy="myEntity", cascade={"persist"})
     */
    private $myPivotEntity;
}

/**
 * MyPivotEntity
 *
 * @ORM\Table(name="my_pivot_entity")
 * @ORM\Entity()
 */
class MyPivotEntity {
    /**
     * @ORM\ManyToOne(targetEntity="MyEntity", inversedBy="myPivotEntity", cascade={"persist"})
     */
    private $myEntity;

    /**
     * @ORM\ManyToOne(targetEntity="MySuperInheritanceEntity", inversedBy="myPivotEntity", cascade={"persist"})
     */
    private $mySuperInheritanceEntity;
}

/**
 * MySuperInheritanceEntity
 *
 * @ORM\Table(name="my_super_inheritance_entity")
 * @ORM\Entity()
 * @ORM\InheritanceType("SINGLE_TABLE")
 */
class MySuperInheritanceEntity {
    /**
     * @ORM\OneToMany(targetEntity="MyPivotEntity", mappedBy="mySuperInheritanceEntity")
     */
    private $myPivotEntity;
}

/**
 * MyInheritanceEntity
 *
 * @ORM\Table(name="my_inheritance_entity")
 * @ORM\Entity()
 */
class MyInheritanceEntity extends MySuperInheritanceEntity {

}

class MyEntityType extends AbstractType
{
    /**
     * {@inheritdoc}
     */
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('myPivotEntity', CollectionType::class, [
                'entry_type' => MyPivotEntityType::class
            ]);
    }
}


class MyPivotEntityType extends AbstractType
{
    /**
     * {@inheritdoc}
     */
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('somField');
    }
}

class MyController extends Controller {
    /**
     * @Post("/myEntity/update")
     */
    public function postMyEntityUpdateAction(Request $request, MyEntity $myEntity) {
        $form = $this->createForm(MyEntityType::class, $myEntity);

        // here error 500 because of too mach memory
        // caused by the MyPivotEntityType which runs a request for each entry,
        // trying to retrieve all the information about MySuperInheritanceEntity and MyInheritanceEntity
        // even if I don't need it at all
        // because of the @ORM\InheritanceType("SINGLE_TABLE")
        // deleting the inheritance solves the problem, but I need it

        $form->handleRequest($request);

        if ($form->isValid()) {
            $this->getEm()->flush();
            return ['success' => true];
        }

        $errors = (string) $form->getErrors(true, false);
        throw new HttpException(400, $errors);
    }
}

The documentation says:

There is a general performance consideration with Single Table Inheritance: If the target-entity of a many-to-one or one-to-one association is an STI entity, it is preferable for performance reasons that it be a leaf entity in the inheritance hierarchy, (ie. have no subclasses).

Also keep in mind:

use plural names for OneToMany association propertiy names:

class MyEntity {
        /**
         * @ORM\OneToMany(targetEntity="MyPivotEntity", mappedBy="myEntity", cascade={"persist"})
         */
        private $myPivotEntities;
}

Update

As an alternative way, you may totaly forget about the inheritance and have separate (previously child) entities with all properties and associations replicated.