关于OneToMany中update某一个字段级联操作问题

代码如下：

package com.founder.tweb.entity.cms;

import java.util.HashSet;
import java.util.List;
import java.util.Set;

import javax.persistence.CascadeType;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;
import javax.persistence.OneToMany;
import javax.persistence.Table;
import javax.persistence.Transient;

import org.apache.commons.lang.builder.ToStringBuilder;
import org.hibernate.annotations.Cache;
import org.hibernate.annotations.CacheConcurrencyStrategy;

@Entity
//表名与类名不相同时重新定义表名.
@Table(name = "GROUPLIB")
//默认的缓存策略.
@Cache(usage = CacheConcurrencyStrategy.READ_WRITE)
public class GroupLib {
	
	private Integer groupId;
	
	private String groupName;
	
	private String groupDesc;
	
	private String groupType;
	
	private Integer groupOrder;
	
	private Integer activityId;
	
	private Set<MediaGroup> mediaGroups = new HashSet<MediaGroup>();
	
	private Set<ImageGroup> imageGroups = new HashSet<ImageGroup>();
	
	
    //默认构造函数
	public GroupLib() {
	}

	//带参数的构造函数
	public GroupLib(Integer groupId, String groupName, String groupDesc,
			String groupType, Integer groupOrder, Integer activityId) {
		super();
		this.groupId = groupId;
		this.groupName = groupName;
		this.groupDesc = groupDesc;
		this.groupType = groupType;
		this.groupOrder = groupOrder;
		this.activityId = activityId;
	}

	@Id
	@GeneratedValue(strategy = GenerationType.IDENTITY)
	public Integer getGroupId() {
		return groupId;
	}

	public void setGroupId(Integer groupId) {
		this.groupId = groupId;
	}



	public String getGroupName() {
		return groupName;
	}



	

	public void setGroupName(String groupName) {
		this.groupName = groupName;
	}



	public String getGroupDesc() {
		return groupDesc;
	}



	public void setGroupDesc(String groupDesc) {
		this.groupDesc = groupDesc;
	}



	public String getGroupType() {
		return groupType;
	}



	public void setGroupType(String groupType) {
		this.groupType = groupType;
	}



	public Integer getGroupOrder() {
		return groupOrder;
	}



	public void setGroupOrder(Integer groupOrder) {
		this.groupOrder = groupOrder;
	}



	public Integer getActivityId() {
		return activityId;
	}



	public void setActivityId(Integer activityId) {
		this.activityId = activityId;
	}
	
	
	@OneToMany
	@JoinColumn(name = "groupId",updatable=false)
	public Set<MediaGroup> getMediaGroups() {
		return mediaGroups;
	}

	public void setMediaGroups(Set<MediaGroup> mediaGroups) {
		this.mediaGroups = mediaGroups;
	}


	@OneToMany
	@JoinColumn(name = "groupId",updatable=false)
	public Set<ImageGroup> getImageGroups() {
		return imageGroups;
	}

	public void setImageGroups(Set<ImageGroup> imageGroups) {
		this.imageGroups = imageGroups;
	}

	@Override
	public String toString() {
		return ToStringBuilder.reflectionToString(this);
	}
}

  我的问题是当对表grouplib进行更新操作的时候，对应的mediaGroup中groupId就变成了null，我想改变这种影响，无论是否更新都不会影响mediagroup表，注意红色字体部分中的updatable=false，起了很关键的作用。