example.php

<?php

$hydrator = new UserHydrator(new BlogPostsHydrator(new BlogPostHydrator()));

$user = $em->find('UserEntity', 123);

var_dump($hydrator->extract($user));

PostEntity.php

<?php

class PostEntity
{
    private $id;
    private $user;
    private $title;
    private $content;
    private $date;
    
    // @todo getters/setters
}

PostHydrator.php

<?php

use Zend\Stdlib\Hydrator\HydratorInterface;

class PostHydrator implements HydratorInterface
{
    private $postsHydrator;
    
    public function __construct(HydratorInterface $postsHydrator)
    {
        $this->postsHydrator = $postsHydrator;
    }
    
    public function hydrate(array $data, $object)
    {
        // @todo won't cover this
    }

    public function extract($object)
    {
        if (! $object instanceof PostEntity) {
            throw new UnsupportedObjectException();
        }
        
        return array(
            'id' => $object->getId(),
            'title' => $object->getTitle(),
            'content' => $object->getContent(),
            'date' => $object->getDate()->format('Y-m-d H:i:s'),
        );
    }
}

PostsHydrator.php

<?php

use Zend\Stdlib\Hydrator\HydratorInterface;

class PostsHydrator implements HydratorInterface
{
    private $postHydrator;
    
    public function __construct(HydratorInterface $postsHydrator)
    {
        $this->postHydrator = $postsHydrator;
    }
    
    public function hydrate(array $data, $object)
    {
        // @todo won't cover this
    }

    public function extract($object)
    {
        if (! $object instanceof \Doctrine\Common\Collections\Collection) {
            throw new UnsupportedObjectException();
        }
        
        $data = [];
        
        foreach ($object as $key => $value) {
            $data[$key] = $this->postHydrator->extract($value);
        }
        
        return $data;
    }
}

UserEntity.php

<?php

class UserEntity
{
    private $id;
    private $name;
    private $surname;
    private $posts;
    
    public function __construct()
    {
      $this->posts = new \Doctrine\Common\Collections\ArrayCollection();
    }
    
    public function getId()
    {
      return $this->id;
    }
    
    public function getName()
    {
      return $this->name;
    }
    
    public function getSurname()
    {
      return $this->surname;
    }
    
    public function getPosts()
    {
      return $this->posts;
    }
    
    public function setName($name) { /* ... */ }
    public function setSurname($surname) { /* ... */ }
}

UserHydrator.php

<?php

use Zend\Stdlib\Hydrator\HydratorInterface;

class UserHydrator implements HydratorInterface
{
    private $postsHydrator;
    
    public function __construct(HydratorInterface $postsHydrator)
    {
        $this->postsHydrator = $postsHydrator;
    }
    
    public function hydrate(array $data, $object)
    {
        // @todo won't cover this
    }

    public function extract($object)
    {
        if (! $object instanceof UserEntity) {
            throw new UnsupportedObjectException();
        }
        return array(
            'id' => $object->getId(),
            'name' => $object->getName(),
            'surname' => $object->getSurname(),
            'posts' => $this->postsHydrator->extract($object->getPosts()),
        );
    }
}