<?php
namespace JoModel;

/**
 * @Entity
 * @InheritanceType("SINGLE_TABLE")
 * @DiscriminatorColumn(name="resource_type", type="string")
 * @DiscriminatorMap({"article" = "JoModelArticleArticleVote", "comment" = "JoModelArticleCommentVote"})
 */
class Vote
{
    /**
     * @Id
     * @Column(type="integer")
     * @GeneratedValue(strategy="AUTO")
     */
    protected $id;

    /**
     * @ManyToOne(targetEntity="JoModelUserUser")
     */
    protected $user;


    /**
     * @Column(type="integer")
     */
    protected $weight;

    public function setWeight($weight)
    {
        $this->weight = $weight;

        return $this;
    }

    public function getWeight()
    {
        return $this->weight;
    }
}

<?php
namespace JoModelArticle;
use JoModel;
/**
 * @Entity
 */

class CommentVote extends ModelVote
{
    /**
     * @ManyToOne(targetEntity="Comment")
     */
    protected $comment;

    public function setComment(Comment $comment)
    {
        $this->comment = $comment;

        return $this;
    }

    public function getComment()
    {
        return $this->comment;
    }
}

CREATE TABLE Vote (
   id INT AUTO_INCREMENT NOT NULL,
   user_id INT DEFAULT NULL,
   article_id INT DEFAULT NULL,
   comment_id INT DEFAULT NULL,
   weight INT NOT NULL,
   resource_type VARCHAR(255) NOT NULL,
INDEX IDX_FA222A5AA76ED395 (user_id),
INDEX IDX_FA222A5A62922701 (article_id),
INDEX IDX_FA222A5AF8697D13 (comment_id),
PRIMARY KEY(id)
) ENGINE = InnoDB;

$commentVote = new CommentVote();
$commentVote->setComment($comment); // $comment instance of Comment
$commentVote->setWeight(1);
$em->persist($commentVote);
$em->flush();

Message: SQLSTATE[23000]: Integrity constraint violation: 1048 Column 'resource_type' cannot be null

resource_type VARCHAR(255) NOT NULL

/**
 * @Column(type="string", length=255", nullable=true)
 */
private $resource_type;