hibernate 同一个类中出现 一对多 多对一 怎么处理啊

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<!-- 
    Mapping file autogenerated by MyEclipse Persistence Tools
-->
<hibernate-mapping>
    <class name="com.trgc.bean.TAdvertisementPosition" table="T_ADVERTISEMENT_POSITION" schema="TRGC_OUT">
        <id name="id" type="java.lang.Long">
            <column name="ID" precision="22" scale="0" />
            <generator class="sequence" />
        </id>
        <many-to-one name="TAdvertisementPosition" class="com.trgc.bean.TAdvertisementPosition" fetch="select">
            <column name="PARENT_ID" precision="22" scale="0" />
        </many-to-one>
        <property name="code" type="java.lang.String">
            <column name="CODE" length="24" />
        </property>
        <property name="name" type="java.lang.String">
            <column name="NAME" length="128" />
        </property>
        <property name="description" type="java.lang.String">
            <column name="DESCRIPTION" length="128" />
        </property>
        <set name="TAdvertisementPositions" inverse="true">
            <key>
                <column name="PARENT_ID" precision="22" scale="0" />
            </key>
            <one-to-many class="com.trgc.bean.TAdvertisementPosition" />
        </set>
    </class>
</hibernate-mapping>
package com.trgc.bean;import java.math.BigDecimal;
import java.util.HashSet;
import java.util.Set;/**
 * TAdvertisementPosition entity. @author MyEclipse Persistence Tools
 */public class TAdvertisementPosition implements java.io.Serializable { // Fields private Long id;
private TAdvertisementPosition TAdvertisementPosition;
private String code;
private String name;
private String description;
private Set TAdvertisementPositions = new HashSet(0); // Constructors /** default constructor */
public TAdvertisementPosition() {
} /** full constructor */
public TAdvertisementPosition(
TAdvertisementPosition TAdvertisementPosition, String code,
String name, String description, Set TAdvertisementPositions) {
this.TAdvertisementPosition = TAdvertisementPosition;
this.code = code;
this.name = name;
this.description = description;
this.TAdvertisementPositions = TAdvertisementPositions;
} // Property accessors public Long getId() {
return this.id;
} public void setId(Long id) {
this.id = id;
} public TAdvertisementPosition getTAdvertisementPosition() {
return this.TAdvertisementPosition;
} public void setTAdvertisementPosition(
TAdvertisementPosition TAdvertisementPosition) {
this.TAdvertisementPosition = TAdvertisementPosition;
} public String getCode() {
return this.code;
} public void setCode(String code) {
this.code = code;
} public String getName() {
return this.name;
} public void setName(String name) {
this.name = name;
} public String getDescription() {
return this.description;
} public void setDescription(String description) {
this.description = description;
} public Set getTAdvertisementPositions() {
return this.TAdvertisementPositions;
} public void setTAdvertisementPositions(Set TAdvertisementPositions) {
this.TAdvertisementPositions = TAdvertisementPositions;
}}
数据库里面有5个字段
比类里面多出来一个parent_id请问这种情况下该如何查询parent_id为空的对象呢？