MyBatis mapping results (resultMap)

1.1.  Why use mapping results

　　Solve the table field names and object properties names are not the same situation (such as: many-to-table, many to many, one to one, one to many ) .

　　Queries related objects, in mybatis does not default to check out , need to query the results of their own and by resultMap to configure

 

1.2. The  association mapping classification

　　One pair a : only one employee ID number. Free to design a field party　　

　　Pairs a : multiple employees corresponds to a department. In general design of a multi-party property  staff inside the design department field

　　One pair of multi- : a department with more employees. Or in the maintenance of multi-subject relationship

　　Pairs and more :  an employee has multiple roles. A character belonging to multiple employees. Intermediate table to represent

 

　　　　In essence: Many, one is the same, just deal with an association (association). And many, many to many is the same process is a collection (collection)

1.3. The  association mapping approach

　　MyBatis provides two ways to deal with our associated objects, nested queries and nested results .

　　Nested Results : transmitting . 1 Article SQL, query all the information ( itself + associated objects )

　　Nested query: transmitting 1 + N article SQL .

　　Next, each of two many-way, and one-to-many, many-processing.

2.1. Many-to-one to one (find a teacher by the students)

　　2.1.1. Prepare a Studen.java student class, and a Teacher.java teacher class

　　Studen.java student class :

/**
 * Student class, multi-party
 */
public class Student {
    private Long id;
    private String name;
    private Integer age;
    private String hobby;

    /**
     * Teacher party
     */
    private Teacher teacher;

    // GET and set and tostring ... (get and set and tostring to, and not too long copy)       
}

　　Teacher.java teacher class

/**
 * Class teacher, a party
 */
public class Teacher {
    private Long id;
    private String name;
    private Integer age;
   
   // GET and set and tostring ... (not too long copy)   
}

　　2.1.2 Interface for students to write a class StudenMapper.java

import java.util.List;

public  interface StudentMapper {
     // Search student data corresponding to students and teacher data 
    List <Student> queryBy ();
}

　　2.1.2 for students to add Mapper configuration file StudentMapper.xml (emphasis here)

　　　2.1.2.1-- 2.1.2.3 optionally one

　　　　2.1.2.1. Nested Results

 

<!--
        1. Nested Results
         id = "studentMap": and select the following resultMap = "studentMap" corresponds to the value of the same double quotes
         property: java class attributes which
          <id property="id" column="id"></id>：就是主键
         column: check out the column names (took alias alias as write sql: t.name tname; that column = "tname")
         javaType: javatype type of attribute is specified teacher object (e.g., id, name, age)
    -->
    <resultMap id="studentMap" type="student">
        <id property="id" column="id"></id>
        <result property="name" column="name"></result>
        <result property="hobby" column="hobby"></result>
        <association property="teacher" javaType="teacher">
            <id property="id" column="tid"></id>
            <result property="name" column="tname"></result>
            <result property="age" column="tage"></result>
        </association>
    </resultMap>

    <!--
       id: id value to your StudenMapper.java interface queryBy () method of the same name
       resultMap: id values ​​above resultMap resultMap value corresponding to the same
    -->
    <select id="queryBy" resultMap="studentMap">
      SELECT
      s.id,
      s.name,
      s.hobby,
      t.id time,
      t.name tname,
      t.age tage
      FROM t_student s JOIN t_teacher t
      on s.tid = t.id
    </select>

 

　　　　2.1.2.2. The results expand nested　

<!--1.1嵌套结果-扩展-->
   <resultMap id="studentMap" type="Student">
        <id property="id" column="id"></id>
        <result property="name" column="name"></result>
        <result property="hobby" column="hobby"></result>
        <!--
            property = "teacher.id": You have a teacher in the field Studen.java student class and
                                   Teacher teacher field is based,
                                   Teacher.teacher fields can be assigned
        -->
        <result property="teacher.id" column="cid"></result>
        <result property="teacher.name" column="cname"></result>
        <result property="teacher.age" column="cage"></result>
    </resultMap>
    <select id="queryBy" resultMap="studentMap">
        SELECT s.id,s.name,s.hobby,t.id cid,t.name cname,t.age cage
        FROM t_student s JOIN t_teacher t
        ON s.id = t.id
    </select>

　　　　2.1.2.3 nested query

 

 <!--2.1嵌套查询-->
    <resultMap id="studenMap" type="Student">
        <id property="id" column="id"></id>
        <result property="name" column="name"></result>
        <result property="hobby" column="hobby"></result>
        <!--
            Properties explained:
            property: object corresponding field
            column: data tables returned, this column will be passed as a parameter, according to the method queryByT
            javaType: Specifies the type of property!
            select: query method, and its value to <select id = "queryByT" resultType = "Teacher"> ... </ select>
                    Id attribute values ​​match
        -->
        <association property="teacher" column="tid" javaType="Teacher" select="queryByT"></association>
    </resultMap>

    <select id="queryBy" resultMap="studenMap">
        SELECT id,name,hobby,tid FROM t_student
    </select>

    <select id="queryByT" resultType="Teacher">
        SELECT id,name,age FROM t_teacher WHERE id = #{id}
    </select>

 

 

 

 

Or saying a 2.1.2.1-- 2.1.2.3 Choose one

2.2 Test

MyBataisUtil.MYBATAIS.getSqlSession (); see below the package can be derived sqlSession

public class StudentMapperTest {

    @Test
    public  void queryBy () throws Exception {
         // MyBataisUtil.MYBATAIS.getSqlSession () themselves encapsulated SQLSESSION 
        the SqlSession SQLSESSION = MyBataisUtil.MYBATAIS.getSqlSession ();
        StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);
        List<Student> students = mapper.queryBy();
        for (Student student : students) {
            System.out.println(student);
        }
    }
}

 

Package sqlSession: 
　　create a MyBataisUtil.java class 

// Enumeration singleton pattern achieved package SQLSESSION 
public  enum MyBataisUtil {

    MYBATAIS;
    private static SqlSessionFactory sqlSessionFactory;

    static {

        the try {
             // read the configuration file 
            Reader = Resources.getResourceAsReader Reader ( "the Config.xml-MyBatis" );
             // Get sqlSessionFactory (sql session factory) 
            SqlSessionFactory = new new the SqlSessionFactoryBuilder () Build (Reader).;

        } catch (IOException e) {
            e.printStackTrace ();
        }
    }

    public SqlSession getSqlSession(){
        //获取sqlSession
        SqlSession sqlSession = sqlSessionFactory.openSession();
        return sqlSession;
    }
}