LazyUserData.php

<?php

/**
 * Other stuff.
 *
 * @ORM\Table("user_extra")
 * @ORM\Entity()
 * @ORM\HasLifecycleCallbacks()
 */
class LazyUserData
{
  
    /**
     * @var int
     * @ORM\Column(name="id", type="integer")
     * @ORM\Id
     */
    private $id
  
    /**
     * @var User
     * @ORM\OneToOne(targetEntity="AppBundle\Entity\User", inversedBy="extra")
     * @ORM\JoinColumn(name="id", referencedColumnName="id")
     * @Assert\Type(type="AppBundle\Entity\User")
     * @Assert\Valid()
     */
    private $seldomUsedAttribute;
}

User.php

<?php

/**
 * User.
 *
 * @ORM\Table("user")
 * @ORM\Entity()
 * @UniqueEntity("email")
 * @ORM\HasLifecycleCallbacks()
 */
class User
{
    /**
     * @var int
     * @ORM\Column(name="id", type="integer")
     * @ORM\Id
     * @ORM\GeneratedValue(strategy="AUTO")
     */
    private $id;
    
    /**
     * @var string
     * @ORM\Column(name="email", type="string", length=100, unique=true)
     * @Assert\NotBlank()
     */
    private $email;
    
    /**
     * @var LazyUserData
     * @ORM\OneToOne(targetEntity="AppBundle\Entity\LazyUserData", mappedBy="user")
     * @Assert\Valid()
     */
    private $extra;
}