Symfony2 ORM prePersist Not Working

prePersist() method of my entity is not being invoked.

<?php

namespace Acme\DemoBundle\Entity;

use Doctrine\ORM\Mapping as ORM;

/**
 * Users
 *
 * @ORM\Table(name="users")
 * @ORM\Entity
 * @ORM\HasLifecycleCallbacks()
 */
class Users
{
    /**
     * @var string
     *
     * @ORM\Column(name="username", type="string", length=255, nullable=false)
     */
    private $username;

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

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

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

    /**
     * @var string
     *
     * @ORM\Column(name="password", type="string", length=32, nullable=false)
     */
    private $password;

    /**
     * @var boolean
     *
     * @ORM\Column(name="active", type="boolean", nullable=false)
     */
    private $active;

    /**
     * @var \DateTime
     *
     * @ORM\Column(name="created", type="datetime", nullable=false)
     */
    private $created;

    /**
     * @var \DateTime
     *
     * @ORM\Column(name="modified", type="datetime", nullable=false)
     */
    private $modified;

    /**
     * @var integer
     *
     * @ORM\Column(name="id", type="integer")
     * @ORM\Id
     * @ORM\GeneratedValue(strategy="IDENTITY")
     */
    private $id;



    /**
     * Set username
     *
     * @param string $username
     * @return Users
     */
    public function setUsername($username)
    {
        $this->username = $username;

        return $this;
    }

    /**
     * Get username
     *
     * @return string 
     */
    public function getUsername()
    {
        return $this->username;
    }

    /**
     * Set firstname
     *
     * @param string $firstname
     * @ret