Why does Doctrine\ORM\Configuration's "DoctrineProxies" Object contain the Universe?

Question

In my ORM code I have an Entity with a field fined like so:

//part of entity class Item:
/** @Column(name="product_id", type="integer") */
private $productId;

I then executed this code:

//3 lines ~straight out of Doctrine configuration to get EntityManager
include 'config/doctrine-config.php';
$config = Setup::createAnnotationMetadataConfiguration($paths, $isDevMode);
$em = EntityManager::create($dbParams, $config);

//my own code to retrieve an entity instance:
$instance = $em->find(Item::class, 2);
print_r($instance);

And this is the output I get (skipping few other similar properties):

Application\Entity\Item Object
(
    [id:Application\Entity\Item:private] => 2
    [description:Application\Entity\Item:private] => Product Kit
    [productId:Application\Entity\Item:private] => -1
)

Note how there are 6 (six) lines above that came out of print_r() function.

And everything was fine, Until

Next, I have changed the $productId column to ManyToOne Relationship on my Item Entity class, like so:

/** 
 * @ManyToOne(targetEntity="Product", inversedBy="id")
 * @JoinColumn(name="product_id", referencedColumnName="id")
 */
private $productId;

I ran the same code.

OUT CAME THE UNIVERSE OF 2,392,600 LINES, WHAT?

Two million, three hundred and ninety two thousand, six hundred lines lines of print_r output.

looking at the print-out I see that DoctrineProxies\__CG__\Application\Entity\Product Object contains 2,392,564 lines printed by print_r

Question:

What is exactly in this object and why is it so big as to take up nearly 300Mb of disk space when printed out?

I cannot help but wonder if such complexity is apt to cause performance issues in every-day code. For example, I am not printing out the contents of the $instance variable in my every-day code, but I surely return the humongousness from a method call. Does that mean it is a 300Mb variable that gets passed from i.e. the $em->find(Item::class, 2); call above?

(Very) Partial Listing

Application\Entity\Item Object
(
 [id:Application\Entity\Item:private] => 2
 [description:Application\Entity\Item:private] => Product Kit
 [ProductId:Application\Entity\Item:private] => DoctrineProxies\__CG__\Application\Entity\Product Object
  (
   [__initializer__] => Closure Object
    (
     [static] => Array
      (
       [entityPersister] => Doctrine\ORM\Persisters\Entity\BasicEntityPersister Object
        (
         [class:protected] => Doctrine\ORM\Mapping\ClassMetadata Object
          (
           [name] => Application\Entity\Product
           [namespace] => Application\Entity
           [rootEntityName] => Application\Entity\Product
           [inheritanceType] => 1
           [generatorType] => 5
           [fieldMappings] => Array
            (
             [id] => Array
              (
               [fieldName] => id
               [type] => integer
               [scale] => 0
               [length] => 
               [unique] => 
               [nullable] => 
               [precision] => 0
               [columnName] => id
               [id] => 1
              )
           [fieldNames] => Array
            (
             [id] => id
             [description] => description
            )

       [columnNames] => Array
            (
             [id] => id
             [description] => description
            )

       [idGenerator] => Doctrine\ORM\Id\AssignedGenerator Object
           [reflClass] => ReflectionClass Object
            (
             [name] => Application\Entity\Product
            )

       [namingStrategy:protected] => Doctrine\ORM\Mapping\DefaultNamingStrategy Object
           [instantiator:Doctrine\ORM\Mapping\ClassMetadataInfo:private] => Doctrine\Instantiator\Instantiator Object

          )

     [conn:protected] => Doctrine\DBAL\Connection Object
          (
           [_conn:protected] => Doctrine\DBAL\Driver\PDOConnection Object
            (
            )

       [_config:protected] => Doctrine\ORM\Configuration Object
            (
             [_attributes:protected] => Array
              (
               [metadataCacheImpl] => Doctrine\Common\Cache\ArrayCache Object
                (
                 [data:Doctrine\Common\Cache\ArrayCache:private] => Array
                  (
                   [dc2_b1e855bc8c5c80316087e39e6c34bc26_[Application\Entity\Item$CLASSMETADATA][1]] => Array
                    (
                     [0] => Doctrine\ORM\Mapping\ClassMetadata Object
                      (
                       [name] => Application\Entity\Item
                       [namespace] => Application\Entity
                       [rootEntityName] => Application\Entity\Item
                       [customGeneratorDefinition] => 
                       [customRepositoryClassName] => 
                       [isMappedSuperclass] => 
                       [isEmbeddedClass] => 
                       [parentClasses] => Array

    [BAZILLION LINES redacted for brevity]

Answer 1

You can't dump a proxy object without XDebug or similar tools (which limit the dumped object size).

The problem is really, really simple:

Proxy -> references EntityManager -> references UnitOfWork -> contains Proxy

This obviously leads to a recursive data-structure dump, which in turn leads to a mess any time you try to dump it without sensible limits.