MyBatis从入门到放弃四：一对多关联查询

前言

      上篇学习了一对一关联查询，这篇我们学习一对多关联查询。一对多关联查询关键点则依然是配置resultMap，在resultMap中配置collection属性，别忽略了ofType属性。

搭建开发环境

     创建表author、表blog，构建一对多的查询场景。

     创建author、blog model。author类中主要是添加属性List<Blog> blogs属性。

public class Author {
    private int id;
    private String name;
    private List<Blog> blogs;

    public List<Blog> getBlogs() {
        return blogs;
    }

    public void setBlogs(List<Blog> blogs) {
        this.blogs = blogs;
    }
    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

　

public class Blog {
    private int id;
    private String title;
    private String category;
    private int author_id;

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }

    public String getCategory() {
        return category;
    }

    public void setCategory(String category) {
        this.category = category;
    }

    public int getAuthor_id() {
        return author_id;
    }

    public void setAuthor_id(int author_id) {
        this.author_id = author_id;
    }
}

　　在mybatis.xml创建alias、引用resource mapper.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE configuration
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>

    <!-- MyBatis针对SqlServer进行的配置  -->
    <typeAliases>
        <typeAlias alias="User" type="com.autohome.model.User"/>
        <typeAlias alias="Teacher" type="com.autohome.model.Teacher" />
        <typeAlias alias="Student" type="com.autohome.model.Student" />
        <typeAlias alias="Author" type="com.autohome.model.Author" />
        <typeAlias alias="Blog" type="com.autohome.model.Blog" />
    </typeAliases>
    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="com.microsoft.sqlserver.jdbc.SQLServerDriver"/>
                <property name="url" value="jdbc:sqlserver://localhost:1433;DatabaseName=test"/>
                <property name="username" value="sa"/>
                <property name="password" value="0"/>
            </dataSource>
        </environment>
    </environments>


    <mappers>
        <mapper resource="mapper/Author.xml"/>
        <mapper resource="mapper/User.xml"/>
        <mapper resource="mapper/Student.xml"/>
    </mappers>
</configuration>

创建Mapper.xml（Author.xml）

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.autohome.mapper.Author">
    <resultMap id="authorResultMap" type="Author">
        <id property="id" column="id"/>
        <result property="name" column="name"/>
        <collection property="blogs" ofType="Blog">
            <id column="bid" property="id"/>
            <result column="title" property="title"/>
            <result column="category" property="category"/>
        </collection>
    </resultMap>

    <select id="getAuthorBlogsById" parameterType="int" resultMap="authorResultMap">
        SELECT a.id,name,b.id bid,title,category FROM t_author a
        LEFT JOIN t_blog b on a.id=b.author_id
        WHERE a.id=#{id}

    </select>
</mapper>

单元测试

 @Test
    public void getAuthorBlog(){
        SqlSession sqlSession=null;
        try{
            sqlSession=sqlSessionFactory.openSession();

            Author author = sqlSession.selectOne("com.autohome.mapper.Author.getAuthorBlogsById",1);
            System.out.println("作者信息 id:"+author.getId()+",name:"+author.getName());
            System.out.println("作者博客:");
            for(Blog blog:author.getBlogs()){
                System.out.println("id:"+blog.getId()+",title:"+blog.getTitle()+",category:"+blog.getCategory());
            }
        }catch(Exception e){
            e.printStackTrace();
        }finally {
            sqlSession.close();
        }
    }

附单元测试截图