detached entity passed to persist 错误

报错的场景
使用Hibernate的JPA ，从BaseModel 的代码可以看出主键用到了自增

import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.MappedSuperclass;

import play.db.jpa.GenericModel;

/**
 * Base class for JPA model objects
 * Automatically provide a @Id Long id field
 */
@MappedSuperclass
public class BaseModel extends GenericModel {

    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    public Long id;

    public Long getId() {
        return id;
    }

    @Override
    public Object _key() {
        return getId();
    }

}

t_bids类继承BaseModel

package models;

import java.util.Date;

import javax.persistence.Entity;
import javax.persistence.Transient;

import com.hzjr.p2p.utils.DateUtil;

import constants.Constants;
import utils.Security;

/**
 * 标
 * 
 * @author bsr
 * @version 6.0
 * @created 2014-4-4 下午03:32:02
 */
@Entity
public class t_bids extends BaseModel {
	public long user_id;
	public Date time;
	public String bid_no;
	public String mer_bill_no;
	public String ips_bill_no;
	public long product_id;
	public String title;
	public long loan_purpose_id;
	public long repayment_type_id;
	public double amount;
	public int period;
	public double min_invest_amount;
	public double average_invest_amount;
	public int invest_period;
	public Date invest_expire_time;
	public Date real_invest_expire_time;
	public double apr;
	public long bank_account_id;
	public int bonus_type;
	public double bonus;
	public double award_scale;
	public String image_filename;
	public boolean is_quality;
	}

数据库也设置其为自增属性 在保存该数据时又给主键赋值了从而出现报错
这是由于@GeneratedValue 自增不能赋值，无论是创建还是更新时一旦有赋值就抛出异常。

解决办法：
1.在存储数据的时候,要记得把主键这一项设置为空.否则,是不可以存储的,因为已经identity了.
2.去掉@GeneratedValue自增