mybatis一对多查询resultMap

时间:2022-05-24 黑夜长行人气:0

问题描述：因为领导的一个需求，需要用到使用resultMap，很久没使用了，结果就除了点意外。就记录下这个问题
准备两个类：author(作者)和book（书），数据库创建对应的author->book一对多的数据

@Data
public class Author {
    private Integer id;
    private String name;
    private String phone;
    private String address;
    private List<Book> books;
}

@Data
public class Book {
    private Integer id;
    private String name;
    private String press;
    private BigDecimal price;
    private Integer authorId;
}

开始的Mapper.xml文件

<resultMap id="bookMap" type="com.example.demo.dto.Author">
        <id column="id" property="id"></id>
        <result column="name" property="name"></result>
        <result column="address" property="address"></result>
        <result column="phone" property="phone"></result>
        <collection property="books" ofType="com.example.demo.dto.Book">
            <id column="id" property="id"></id>
            <result column="name" property="name"></result>
            <result column="press" property="press"></result>
            <result column="price" property="price"></result>
            <result column="author_id" property="authorId"></result>
        </collection>
    </resultMap>
    <select id="queryAuthorInfo" parameterType="java.lang.String" resultMap="bookMap">
        select t1.*,t2.* from
        author t1 inner join book t2 on t1.id=t2.author_id
        where t1.id=#{id}
    </select>

使用postman执行查看结果：

{
    "code": "200",
    "msg": "成功",
    "data": {
        "id": 1,
        "name": "法外狂徒张三",
        "phone": null,
        "address": null,
        "books": [
            {
                "id": 1,
                "name": "法外狂徒张三",
                "press": "人民出版社",
                "price": 10.00,
                "authorId": 1
            }
        ]
    }
}

发现问题：本来author对应book有两条记录，结果books里面只返回了一条记录。
问题原因：2张表的主键都叫id，所以导致结果不能正确展示。
解决方法：1、主键使用不用的字段名。2、查询sql时使用别名
1、主键使用不用的字段名，涉及到更改数据库，只需要更改其中一个即可。这里演示将book的id更改为book_id

<resultMap id="bookMap" type="com.example.demo.dto.Author">
        <id column="id" property="id"></id>
        <result column="name" property="name"></result>
        <result column="address" property="address"></result>
        <result column="phone" property="phone"></result>
        <collection property="books" ofType="com.example.demo.dto.Book">
            <!---更改book类的id为bookId,数据库book的id更改为book_id-->
            <id column="book_id" property="bookId"></id>
            <result column="name" property="name"></result>
            <result column="press" property="press"></result>
            <result column="price" property="price"></result>
            <result column="author_id" property="authorId"></result>
        </collection>
    </resultMap>
    <select id="queryAuthorInfo" parameterType="java.lang.String" resultMap="bookMap">
        select t1.*,t2.* from
        author t1 inner join book t2 on t1.id=t2.author_id
        where t1.id=#{id}
    </select>

2、查询sql时使用别名。这里演示将查询book时id 更改别名为 bookId

<resultMap id="bookMap" type="com.example.demo.dto.Author">
        <id column="id" property="id"></id>
        <result column="name" property="name"></result>
        <result column="address" property="address"></result>
        <result column="phone" property="phone"></result>
        <collection property="books" ofType="com.example.demo.dto.Book">
            <!---这里将column值id更改为别名一致bookId-->
            <id column="bookId" property="id"></id>
            <result column="name" property="name"></result>
            <result column="press" property="press"></result>
            <result column="price" property="price"></result>
            <result column="author_id" property="authorId"></result>
        </collection>
    </resultMap>
    <select id="queryAuthorInfo" parameterType="java.lang.String" resultMap="bookMap">
        <!---这里新增了t2.id as bookId-->
        select t1.*,t2.id as bookId, t2.* from
        author t1 inner join book t2 on t1.id=t2.author_id
        where t1.id=#{id}
    </select>

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