Hibernater由于外键设置不为空，导致在主键级联删除时失败解决办法！

媒体表
@Entity
public class Media implements Serializable {
	......略
	@OneToMany(cascade=CascadeType.REMOVE)
	@JoinColumn(name = "FK_Media_id",updatable=false)
	private Set<MediaComment> mediaCommetnSet; // 媒体评论表

}
媒体评论表
@Entity
public class MediaComment implements Serializable {
	......略
	@ManyToOne
	@JoinColumn(name = "FK_Media_id")
	private Media media;
}
媒体表
@Entity
public class Media implements S