Challenge.php

<?php

use DateTime;
use Doctrine\ORM\Mapping as ORM;
use Symfony\Component\Validator\Constraints as Assert;
use Symfony\Bridge\Doctrine\Validator\Constraints as DoctrineAssert;

/**
 * @ORM\Entity
 * @ORM\Table(name="challenges")
 * @DoctrineAssert\UniqueEntity(fields="slug", message="That slug is already in use.")
 */
class Challenge
{
    /**
     * @ORM\Id
     * @ORM\Column(type="integer")
     * @ORM\GeneratedValue(strategy="AUTO")
     */
    protected $id;
    
    /**
     * @Assert\NotBlank()
     * @ORM\Column(type="string")
     */
    protected $name;
    
    /**
     * @ORM\OneToOne(targetEntity="Owner", mappedBy="challenge")
     */
    protected $owner;
    
    /**
     * @ORM\OneToOne(targetEntity="Rival", mappedBy="challenge")
     */
    protected $rival;
    
    public function getId()
    {
        return $this->id;
    }

    public function getName()
    {
        return $this->name;
    }

    public function setName($name)
    {
        $this->name = $name;
        
        return $this;
    }

    public function setOwner(Owner $owner)
    {
        $this->owner = $owner;
        $owner->setChallenge($this);
        
        return $this;
    }

    public function getOwner()
    {
        return $this->owner;
    }

    public function setRival(Rival $rival)
    {
        $this->rival = $rival;
        $rival->setChallenge($this);
        
        return $this;
    }

    public function getRival()
    {
        return $this->rival;
    }
}

Owner.php

<?php

use Doctrine\ORM\Mapping as ORM;
use Symfony\Component\Validator\Constraints as Assert;

/**
 * @ORM\Entity
 * @ORM\Table(name="owners")
 */
class Owner extends Participant
{
    /**
     * @ORM\OneToOne(targetEntity="Challenge", mappedBy="owner")
     */
    protected $challenge;

    public function setChallenge(Challenge $challenge)
    {
        $this->challenge = $challenge;
    }

    public function getChallenge()
    {
        return $this->challenge;
    }
}

Participant.php

<?php

use DateTime;
use Doctrine\ORM\Mapping as ORM;
use Symfony\Component\Validator\Constraints as Assert;

/**
 * @ORM\Entity
 * @ORM\HasLifecycleCallbacks
 * @ORM\InheritanceType("JOINED")
 * @ORM\DiscriminatorColumn(name="type", type="string")
 * @ORM\DiscriminatorMap({
 *     "owner" = "Owner",
 *     "rival" = "Rival"
 * })
 * @ORM\Table(name="participants",
 *     uniqueConstraints={
 *         @ORM\UniqueConstraint(name="uix_participants_userId_challengeId", columns={
 *             "userId",
 *             "challengeId"
 *         })
 *     }
 * )
 */
class Participant
{
    /**
     * @ORM\Id
     * @ORM\Column(type="integer")
     * @ORM\GeneratedValue(strategy="AUTO")
     */
    protected $id;
    
    /**
     * @ORM\Column(type="integer")
     */
    protected $challengeId;
    
    /**
     * @ORM\Column(type="integer")
     */
    protected $userId;
    
    /**
     * @ORM\ManyToOne(targetEntity="User")
     * @ORM\JoinColumn(name="userId", referencedColumnName="id")
     */
    protected $user;
    
    public function getId()
    {
        return $this->id;
    }

    public function setUserId($userId)
    {
        $this->userId = $userId;
    }

    public function getUserId()
    {
        return $this->userId;
    }

    public function setChallengeId($challengeId)
    {
        $this->challengeId = $challengeId;
    }

    public function getChallengeId()
    {
        return $this->challengeId;
    }

    public function setUser(User $user)
    {
        $this->user = $user;
    }

    public function getUser()
    {
        return $this->user;
    }
}

Rival.php

<?php

use Doctrine\ORM\Mapping as ORM;
use Symfony\Component\Validator\Constraints as Assert;

/**
 * @ORM\Entity
 * @ORM\Table(name="rivals")
 */
class Rival extends Participant
{
    /**
     * @ORM\OneToOne(targetEntity="Challenge", mappedBy="rival")
     */
    protected $challenge;
    
    public function setChallenge(Challenge $challenge)
    {
        $this->challenge = $challenge;
    }

    public function getChallenge()
    {
        return $this->challenge;
    }
}

User.php

<?php

use DateTime;
use FOS\UserBundle\Entity\User as BaseUser;
use Doctrine\ORM\Mapping as ORM;

/**
 * @ORM\Entity
 * @ORM\HasLifecycleCallbacks
 * @ORM\Table(name="users")
 */
class User extends BaseUser
{
    /**
     * @ORM\Id
     * @ORM\Column(type="integer")
     * @ORM\GeneratedValue(strategy="AUTO")
     */
    protected $id
}