Trying to reproduce https://github.com/symfony/symfony/issues/6727

DemoController.php

<?php

namespace Acme\DemoBundle\Controller;

use Symfony\Bundle\FrameworkBundle\Controller\Controller;
use Acme\DemoBundle\Entity;

use Sensio\Bundle\FrameworkExtraBundle\Configuration\Route;

class DemoController extends Controller
{
    /**
     * @Route("/")
     */
    public function indexAction()
    {
        $user = $this->findUser();
        $product = $this->findProduct();
        $like = new Entity\Like($user, $product);
        $errors = $this->get('validator')->validate($like);
        if (count($errors) === 0) {
            $this->getDoctrine()->getManager()->persist($like);
            $this->getDoctrine()->getManager()->flush($like);
        }
        var_dump($errors);
    }

    /**
     * @return Entity\User
     */
    protected function findUser()
    {
        $user = $this->getDoctrine()->getRepository('AcmeDemoBundle:User')->findOneBy(array());
        if (!empty($user)) {
            return $user;
        }

        $user = new Entity\User();
        $manager = $this->getDoctrine()->getManager();
        $manager->persist($user);
        $manager->flush($user);
        return $this->findUser();
    }

    /**
     * @return Entity\Product
     */
    protected function findProduct()
    {
        $product = $this->getDoctrine()->getRepository('AcmeDemoBundle:Product')->findOneBy(array());
        if (!empty($product)) {
            return $product;
        }

        $product = new Entity\Product();
        $manager = $this->getDoctrine()->getManager();
        $manager->persist($product);
        $manager->flush($product);
        return $this->findProduct();
    }
}

Like.php

<?php

namespace Acme\DemoBundle\Entity;

use Doctrine\ORM\Mapping as ORM;
use Symfony\Bridge\Doctrine\Validator\Constraints\UniqueEntity;

/**
 * @ORM\Entity
 * @ORM\Table(name="likes", uniqueConstraints={
 *     @ORM\UniqueConstraint(columns={"user_id", "product_id"})
 * })
 * @UniqueEntity(fields={"user", "product"})
 */
class Like
{
    /**
     * @ORM\Id
     * @ORM\Column(type="integer")
     * @ORM\GeneratedValue(strategy="AUTO")
     */
    private $id;

    /**
     * @ORM\ManyToOne(targetEntity="User", inversedBy="likes")
     */
    private $user;

    /**
     * @ORM\ManyToOne(targetEntity="Product", inversedBy="likes")
     */
    private $product;

    public function __construct(User $user, Product $product)
    {
        $this->user = $user;
        $this->product = $product;
    }
}

Product.php

<?php

namespace Acme\DemoBundle\Entity;

use Doctrine\Common\Collections\ArrayCollection;
use Doctrine\ORM\Mapping as ORM;

/**
 * @ORM\Entity
 */
class Product
{
    /**
     * @ORM\Id
     * @ORM\Column(type="integer")
     * @ORM\GeneratedValue(strategy="AUTO")
     */
    private $id;

    /**
     * @ORM\OneToMany(targetEntity="Like", mappedBy="product")
     */
    private $likes;

    public function __construct()
    {
        $this->likes = new ArrayCollection();
    }
}

User.php

<?php

namespace Acme\DemoBundle\Entity;

use Doctrine\Common\Collections\ArrayCollection;
use Doctrine\ORM\Mapping as ORM;

/**
 * @ORM\Entity
 */
class User
{
    /**
     * @ORM\Id
     * @ORM\Column(type="integer")
     * @ORM\GeneratedValue(strategy="AUTO")
     */
    private $id;

    /**
     * @ORM\OneToMany(targetEntity="Like", mappedBy="user")
     */
    private $likes;

    public function __construct()
    {
        $this->likes = new ArrayCollection();
    }
}