mybatis plus 实现关联查询

MyBatis-Plus-Join可以帮助我们轻松实现关联查询，具体步骤如下：

1.定义实体类之间的关系，例如：

public class User {
    private Long id;
    private String name;
    private Integer age;
    // 省略getter和setter方法
}

public class Order {
    private Long id;
    private Long userId;
    private BigDecimal amount;
    // 省略getter和setter方法
}

2.使用注解或XML配置关联查询，例如：

@TableName("user")
public class User {
    @TableId
    private Long id;
    private String name;
    private Integer age;
    @TableField(exist = false)
    private List<Order> orders;
    // 省略getter和setter方法
}

@TableName("order")
public class Order {
    @TableId
    private Long id;
    private Long userId;
    private BigDecimal amount;
    // 省略getter和setter方法
}

<!-- userMapper.xml -->
<select id="getUserWithOrders" resultMap="userMap">
    SELECT u.*, o.id AS order_id, o.amount
    FROM user u
    LEFT JOIN order o ON u.id = o.user_id
    WHERE u.id = #{id}
</select>

<resultMap id="userMap" type="com.example.User">
    <id column="id" property="id"/>
    <result column="name" property="name"/>
    <result column="age" property="age"/>
    <collection property="orders" ofType="com.example.Order">
        <id column="order_id" property="id"/>
        <result column="amount" property="amount"/>
    </collection>
</resultMap>

3.调用关联查询方法，例如：

User user = userMapper.getUserWithOrders(1L);
List<Order> orders = user.getOrders();