From getting started to giving up MyBatis 4: One-to-multiple association query, mybatis getting started

From getting started to giving up MyBatis 4: One-to-multiple association query, mybatis getting started
Preface

In the previous article, we learned one-to-one association query. In this article, we learned one-to-many Association queries. In one-to-multiple join queries, the key point is still configuring resultMap. Configure the collection attribute in resultMap. Do not ignore the ofType attribute.

 

Build a Development Environment

Create tables author and table blog to create one-to-many query scenarios.

Create author and blog model. In the author class, the attribute List <Blog> blogs is added.

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;    }}

Create alias in mybatis. xml and reference resource mapper. xml

1 <? Xml version = "1.0" encoding = "UTF-8"?> 2 <! DOCTYPE configuration 3 PUBLIC "-// mybatis.org//DTD Config 3.0 // EN" 4 "http://mybatis.org/dtd/mybatis-3-config.dtd"> 5 <configuration> 6 7 <! -- MyBatis configuration for SqlServer --> 8 <typeAliases> 9 <typeAlias alias = "User" type = "com. autohome. model. user "/> 10 <typeAlias alias =" Teacher "type =" com. autohome. model. teacher "/> 11 <typeAlias alias =" Student "type =" com. autohome. model. student "/> 12 <typeAlias alias =" Author "type =" com. autohome. model. author "/> 13 <typeAlias alias =" Blog "type =" com. autohome. model. blog "/> 14 </typeAliases> 15 <environments default =" development "> 16 <environment id =" development "> 17 <transactionManager type =" JDBC "/> 18 <dataSource type = "POOLED"> 19 <property name = "driver" value = "com. microsoft. sqlserver. jdbc. SQLServerDriver "/> 20 <property name =" url "value =" jdbc: sqlserver :/// localhost: 1433; databaseName = test "/> 21 <property name =" username "value =" sa "/> 22 <property name =" password "value =" 0 "/> 23 </dataSource> 24 </environment> 25 </environments> 26 27 28 <mappers> 29 <mapper resource = "mapper/Author. xml "/> 30 <mapper resource =" mapper/User. xml "/> 31 <mapper resource =" mapper/Student. xml "/> 32 </mappers> 33 </configuration>View Code

 

Create Mapper. xml (Author. xml)

 1 <?xml version="1.0" encoding="UTF-8"?> 2 <!DOCTYPE mapper 3         PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" 4         "http://mybatis.org/dtd/mybatis-3-mapper.dtd"> 5 <mapper namespace="com.autohome.mapper.Author"> 6     <resultMap id="authorResultMap" type="Author"> 7         <id property="id" column="id"/> 8         <result property="name" column="name"/> 9         <collection property="blogs" ofType="Blog">10             <id column="bid" property="id"/>11             <result column="title" property="title"/>12             <result column="category" property="category"/>13         </collection>14     </resultMap>15 16     <select id="getAuthorBlogsById" parameterType="int" resultMap="authorResultMap">17         SELECT a.id,name,b.id bid,title,category FROM t_author a18         LEFT JOIN t_blog b on a.id=b.author_id19         WHERE a.id=#{id}20 21     </select>22 </mapper>

 

Unit Test

@ Test public void getAuthorBlog () {SqlSession sqlSession = null; try {sqlSession = sqlSessionFactory. openSession (); Author author = sqlSession. selectOne ("com. autohome. mapper. author. getAuthorBlogsById ", 1); System. out. println ("author information id:" + author. getId () + ", name:" + author. getName (); System. out. println ("author's Blog:"); 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 ();}}

Attached unit test