<?php

namespace Fogs\TaggingBundle\Entity;

use \FPN\TagBundle\Entity\Tagging as BaseTagging;
use Doctrine\ORM\Mapping as ORM;
use Doctrine\ORM\Mapping\UniqueConstraint;

/**
 * Fogs\TaggingBundle\Entity\Tagging
 *
 * @ORM\Table(uniqueConstraints={@UniqueConstraint(name="tagging_idx", columns={"tag_id", "resource_type", "resource_id"})})
 * @ORM\Entity
 */
class Tagging extends BaseTagging
{
	/**
	 * @var integer $id
	 *
	 * @ORM\Column(name="id", type="integer")
	 * @ORM\Id
	 * @ORM\GeneratedValue(strategy="AUTO")
	 */
	protected $id;

	/**
	 * @ORM\ManyToOne(targetEntity="Tag")
	 * @ORM\JoinColumn(name="tag_id", referencedColumnName="id")
	 **/
	protected $tag;
}
<?php

namespace Fogs\TaggingBundle\Entity;

use \FPN\TagBundle\Entity\Tagging as BaseTagging;
use Doctrine\ORM\Mapping as ORM;
use Doctrine\ORM\Mapping\UniqueConstraint;

/**
 * Fogs\TaggingBundle\Entity\Tagging
 *
 * @ORM\Table(uniqueConstraints={@UniqueConstraint(name="tagging_idx", columns={"tag_id", "resource_type", "resource_id"})})
 * @ORM\Entity
 */
class Tagging extends BaseTagging
{
	/**
	 * @var integer $id
	 *
	 * @ORM\Column(name="id", type="integer")
	 * @ORM\Id
	 * @ORM\GeneratedValue(strategy="AUTO")
	 */
	protected $id;

	/**
	 * @ORM\ManyToOne(targetEntity="Tag", inversedBy="tagging")
	 * @ORM\JoinColumn(name="tag_id", referencedColumnName="id")
	 **/
	protected $tag;
}

Add 'inversedBy' attribute on ManyToOne association with Tag entity
Add 'inversedBy' attribute on ManyToOne association with Tag entity

Without this attribute the entity mapping information won't be considered valid.