use DoctrineCommonCollectionsArrayCollection;
    use DoctrineORMMappingColumn;
    use DoctrineORMMappingTable;
    use DoctrineORMMappingEntity;
    use DoctrineORMMappingId;
    use DoctrineORMMappingGeneratedValue;
    use DoctrineORMMappingManyToOne;
    use DoctrineORMMappingOneToMany;
    use DoctrineORMMappingJoinColumn;

    use JMSSerializerAnnotation as JMS;

    /**
     * Document
     *
     * @Table(name="document")
     * @Entity(repositoryClass="AcmeBundleDocumentRepository")
     */

    class Document
    {

        /**
         * @var string
         *
         * @Column(name="id", type="string")
         * @Id
         * @GeneratedValue(strategy="UUID")
         */
        protected $id;

        /**
         * @var string
         * @Column(name="name", type="string", length=255)
         */
        protected $name;

        /**
         * @var string
         * @Column(name="type", type="string", length=255)
         */
        protected $type;

        /**
         * @var boolean
         * @Column(name="has_attachments", type="boolean")
         */
        protected $hasAttachments;

        /**
         * @ManyToOne(targetEntity="Delivery")
         * @JoinColumn(name="delivery_id", referencedColumnName="id", nullable=false)
         * @JMSExclude()
         */
        protected $delivery;

        /**
         * @OneToMany(targetEntity="Extension", mappedBy="document", cascade={"persist","remove"})
         **/
        protected $extensions;

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

        /* getter and setters */

}

use DoctrineORMMappingTable;
use DoctrineORMMappingEntity;

    /**
     * Note
     *
     * @Table(name="note")
     * @Entity(repositoryClass="NoteRepository")
     */
    class Note extends Document
    {

    }

/**
 * @MappedSuperclass
 */
class DocumentSuperClass
{
    ...
}

/**
 * @Table(name="document")
 * @Entity(repositoryClass="AcmeBundleDocumentRepository")
 */
class Document extends DocumentSuperClass
{
    ...
}

/**
 * @Table(name="note")
 * @Entity(repositoryClass="NoteRepository")
 */
class Note extends DocumentSuperClass
{
    ...
}