doctrine entities implements JsonSerializable to avoid nesting max depth error

Acme\ApiBundle\Entity\Comment.php

<?php

namespace Acme\ApiBundle\Entity;

use Doctrine\ORM\Mapping as ORM;

/**
 * Todo
 *
 * @ORM\Table(name="acme_comments")
 * @ORM\Entity(repositoryClass="Acme\ApiBundle\Entity\CommentRepository")
 */
class Comment implements JsonSerializable
{
    /**
     * @var integer
     *
     * @ORM\Column(name="id", type="integer")
     * @ORM\Id
     * @ORM\GeneratedValue(strategy="AUTO")
     */
    private $id;

    /**
     * @var string
     *
     * @ORM\Column(name="content", type="text")
     */
    private $content;

    /**
     * @var \DateTime
     *
     * @ORM\Column(name="updated_at", type="datetime")
     */
    private $updatedAt;

    /**
     * Many Comments have One Todo.
     * @ORM\ManyToOne(targetEntity="Acme\ApiBundle\Entity\Todo", inversedBy="comments")
     * @ORM\JoinColumn(name="todo_id", referencedColumnName="id")
     */
    private $todo;

    // ... GETTER AND SETTER FUNCTIONS ...
  
    /**
     *
     */
    public function jsonSerialize()
    {              
        return [
          'id' => $this->getId(),   
          'content' => $this->getContent(),
          'updatedAt' => $this->getUpdatedAt()
        ];
    }
}

Acme\ApiBundle\Entity\Todo.php

<?php

namespace Acme\ApiBundle\Entity;

use Doctrine\ORM\Mapping as ORM;

/**
 * Todo
 *
 * @ORM\Table(name="acme_todos")
 * @ORM\Entity(repositoryClass="Acme\ApiBundle\Entity\TodoRepository")
 */
class Todo implements JsonSerializable
{
    /**
     * @var integer
     *
     * @ORM\Column(name="id", type="integer")
     * @ORM\Id
     * @ORM\GeneratedValue(strategy="AUTO")
     */
    private $id;

    /**
     * @var string
     *
     * @ORM\Column(name="title", type="string", length=255)
     */
    private $title;

    /**
     * @var string
     *
     * @ORM\Column(name="content", type="text")
     */
    private $content;

    /**
     * @var boolean
     *
     * @ORM\Column(name="is_done", type="boolean")
     */
    private $isDone;

    /**
     * @var \DateTime
     *
     * @ORM\Column(name="updated_at", type="datetime")
     */
    private $updatedAt;

    /**
     * One Todo has Many Comments.
     * @var \ArrayCollection
     *
     * @OneToMany(targetEntity="Acme\ApiBundle\Entity\Comment", mappedBy="todo")
     */
    private $comments;
  
    public function __construct() {
        $this->comments = new ArrayCollection();
    }
    
    // ... GETTER AND SETTER FUNCTIONS ...
  
    /**
     *
     */
    public function jsonSerialize()
    {
        return [
          'id' => $this->getId(),  
          'title' => $this->getTitle(), 
          'content' => $this->getContent(),
          'updatedAt' => $this->getUpdatedAt(),
          'comments' => $this->getComments()
        ];
    }
}