我使用类Hand来保存数据。它包含两个相同的内部类Card，也就是说两个Card组成一个Hand。如下所示：

public class Card {
    private Integer id;
    private Integer number;
    private Integer color;

    public Card() {}
    public Card(int number, int color) {
        this.number = number;
        this.color = color;
    }

public class Hand {

    private Integer id;

    private Card card1,card2;

    public Hand() {}
    public Hand(Card a, Card b) {
        card1 = a;
        card2 = b;
    }
}

然后我将Mapxml做成这样：

<resultMap id="handMap" type="com.company.handsmap.Hand">
        <id column="id" property="id"/>
        <association property="card1" resultMap="cardMap"/>
        <association property="card2" resultMap="cardMap"/>
    </resultMap>

    <resultMap id="cardMap" type="com.company.handsmap.Card">
        <id column="id" property="id"/>
        <result column="number" property="number"/>
        <result column="color" property="color"/>
    </resultMap>

    <select id="getAllHand" resultMap="handMap">
        SELECT
        *
        FROM Hand H
        LEFT JOIN Card c1 ON H.card1_id = c1.id
        LEFT JOIN Card c2 ON H.card2_id = c2.id
    </select>

然后我使用Map如下，它似乎得到一些错误：Debugging the bug
如所示，Hand获得了正确的Card 1，但在Card 2中获得了null
如果我跑了

SELECT
        *
        FROM Hand H
        LEFT JOIN Card c1 ON H.card1_id = c1.id
        LEFT JOIN Card c2 ON H.card2_id = c2.id

在数据库中，我可以得到正确的结果如下：SQL results
谁能告诉我问题出在哪里，我该怎么解决它？非常感谢!