Ejemplo de como usar herencia de una clase abstracta para desacoplar propiedades de una Entidad.

SocialBase.php

<?php

namespace UP\SocialBundle\Entity;

/**
 * UP\SocialBundle\Entity\SocialBase
 * @orm:Entity
 * @orm:HasLifecycleCallbacks
 * @orm:InheritanceType("JOINED")
 * @orm:DiscriminatorColumn(name="class_name", type="string")
 * @orm:DiscriminatorMap({
 *  "UP\SocialBundle\Entity\Friendship"     = "UP\SocialBundle\Entity\Friendship",
 *  "UP\SocialBundle\Entity\SocialFeedItem" = "UP\SocialBundle\Entity\SocialFeedItem"
 * })
 */
abstract class SocialBase {
    /** 
     * @orm:Id 
     * @orm:Column(type="integer") 
     * @orm:GeneratedValue
     */
    protected $id;
    
    /**
     * @orm:Column(type="datetime")
     */
    protected $created_at;
    
    /**
     * @orm:Column(type="datetime")
     */
    protected $updated_at;
    
    /**
     * @orm:ManyToMany(targetEntity="UP\SocialBundle\Entity\Tag")
     * @orm:JoinTable(name="social_tags",
     *      joinColumns={@orm:JoinColumn(name="social_id", referencedColumnName="id")},
     *      inverseJoinColumns={@orm:JoinColumn(name="tag_id", referencedColumnName="id")}
     *      )
     */
    protected $tags;
    
    /**
     * @orm:OneToMany(targetEntity="UP\SocialBundle\Entity\Comment", mappedBy="root")
     */
    protected $comments;

    public function __construct()
    {
        $this->tags = new \Doctrine\Common\Collections\ArrayCollection();
    }
    
    public function getId()
    {
        return $this->id;
    }
    
    public function addTag(\UP\SocialBundle\Entity\Tag $tag)
    {
        $this->tags[] = $tag;
    }

    public function getTags()
    {
        return $this->tags;
    }

    public function addComment(\UP\SocialBundle\Entity\Comment $comment)
    {
        $this->comments[] = $comment;
    }

    public function getComments()
    {
        return $this->comments;
    }
    
    public function setCreatedAt(\DateTime $created_at) {
        $this->created_at = $created_at;
    }
    
    public function getCreatedAt() {
        return $this->created_at;
    }
    
    public function setUpdatedAt(\DateTime $updated_at) {
        $this->updated_at = $updated_at;
    }
    
    public function getUpdatedAt() {
        return $this->updated_at;
    }
    
    /**
     * @orm:prePersist
     */
    public function prePersist() {
        $this->setCreatedAt(new \DateTime());
        $this->setUpdatedAt(new \DateTime());
    }
    
    /**
     * @orm:preUpdate
     */
    public function preUpdate() {
        $this->setUpdatedAt(new \DateTime());
    }
}

SocialFeedItem.php

<?php

namespace UP\SocialBundle\Entity;

use UP\SocialBundle\Entity\SocialBase;

/**
 * UP\SocialBundle\Entity\SocialFeedItem
 * @orm:Entity
 */
class SocialFeedItem extends SocialBase
{
    /**
     * @orm:Column(type="string", nullable="true")
     */
    protected $user_message;

    /**
     * @orm:Column(type="string")
     */
    protected $system_message;

    /**
     * @orm:ManyToOne(targetEntity="UP\UserBundle\Entity\User", inversedBy="id")
     */
    protected $source_user;

    /**
     * @orm:ManyToOne(targetEntity="UP\UserBundle\Entity\User", inversedBy="id")
     */
    protected $target_user;

    public function setUserMessage($userMessage)
    {
        $this->user_message = $userMessage;
    }

    public function getUserMessage()
    {
        return $this->user_message;
    }

    public function setSystemMessage($systemMessage)
    {
        $this->system_message = $systemMessage;
    }

    public function getSystemMessage()
    {
        return $this->system_message;
    }

    public function setSourceUser(\UP\UserBundle\Entity\User $sourceUser)
    {
        $this->source_user = $sourceUser;
    }

    public function getSourceUser()
    {
        return $this->source_user;
    }

    public function setTargetUser(\UP\UserBundle\Entity\User $targetUser = null)
    {
        $this->target_user = $targetUser;
    }

    public function getTargetUser()
    {
        return $this->target_user;
    }
}