MyBatis3一对多关系映射

1. MyBatis3关系映射，一对多关系

创建t_grade数据表
create table t_grade(
id int(11) NOT NULL AUTO_INCREMENT,
gradeName varchar(20) DEFAULT NULL,
PRIMARY KEY(id)
) ENGINE=InnoDB AUTO_INCREMENT=0 DEFAULT CHARSET=utf8;

插入数据
insert into t_grade(id, gradeName) values(1, '大学一年级');
insert into t_grade(id, gradeName) values(2, '大学二年级');

在t_student表增加gradeId关联字段
alter table t_student add gradeId INT(11);

2. 使用mybatis查询关联表使用collection 方式，一对多关系

<resultMap type="Grade" id="GradeResult">
    <result property="id" column="id"/>
    <result property="gradeName" column="gradeName"/>
    <collection property="students" column="id" select="com.andrew.mappers.StudentMapper.findByGradeId"></collection>
</resultMap>

3. 使用mybatis查询关联表，一对多关系

package com.andrew.model;
import java.util.List;
public class Grade {
    private Integer id;
    private String gradeName;
    private List<Student> students;
    @Override
    public String toString() {
        return "Grade [id=" + id + ", gradeName=" + gradeName +", student=" + students + "]";
    }
    // getter and setter
}

package com.andrew.mappers;
import com.andrew.model.Grade;
public interface GradeMapper {
    public Grade findById(Integer id);
}

<?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.andrew.mappers.GradeMapper">
    <resultMap type="Grade" id="GradeResult">
        <result property="id" column="id"/>
        <result property="gradeName" column="gradeName"/>
        <collection property="students" column="id" select="com.andrew.mappers.StudentMapper.findByGradeId"></collection>
    </resultMap>
    <select id="findById" parameterType="Integer" resultMap="GradeResult">
        select * from t_grade where id=#{id}
    </select>
</mapper>

package com.andrew.mappers;
import java.util.List;
import com.andrew.model.Student;
public interface StudentMapper {
    public int add(Student student);
    public int update(Student student);
    public int delete(Integer id);
    public Student findById(Integer id);
    public List<Student> find();
    public Student findStudentWithAddress(Integer id);
    public Student findByGradeId(Integer gradeId);
}

<?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.andrew.mappers.StudentMapper">
    <resultMap type="Student" id="StudentResult">
        <id property="id" column="id"/>
        <result property="name" column="name"/>
        <result property="age" column="age"/>
    </resultMap>
    <resultMap type="Student" id="StudentAddressResult">
        <id property="id" column="id"/>
        <result property="name" column="name"/>
        <result property="age" column="age"/>
        <association property="address" column="addressId" select="com.andrew.mappers.AddressMapper.findById"></association>
        <association property="grade" column="gradeId" select="com.andrew.mappers.GradeMapper.findById"></association>
    </resultMap>
    <insert id="add" parameterType="Student">
        insert into t_student values(null, #{name}, #{age})
    </insert>
    <update id="update" parameterType="Student">
        update t_student set name=#{name},age=#{age} where id=#{id}
    </update>
    <delete id="delete" parameterType="Integer">
        delete from t_student where id=#{id}
    </delete>
    <select id="findById" parameterType="Integer" resultType="Student">
        select * from t_student where id=#{id}
    </select>
    <select id="find" resultMap="StudentAddressResult">
        select * from t_student
    </select>
    <select id="findStudentWithAddress" resultMap="StudentAddressResult" parameterType="Integer">
        select * from t_student t1,t_address t2 where t1.addressId=t2.id and t1.id=#{id}
    </select>
    <select id="findByGradeId" resultMap="StudentAddressResult" parameterType="Integer">
        select * from t_student where gradeId=#{gradeId}
    </select>
</mapper> 

package com.andrew.model;
public class Student {
    private Integer id;
    private String name;
    private Integer age;
    private Address address;
    private Grade grade;
    public Student() {
        super();
    }
    public Student(String name, Integer age) {
        super();
        this.name = name;
        this.age = age;
    }
    public Student(Integer id, String name, Integer age) {
        super();
        this.id = id;
        this.name = name;
        this.age = age;
    }
    @Override
    public String toString() {
        return "Student [id=" + id + ", name=" + name + ", age=" + age
                + ", address=" + address + "]";
    }
    // getter and setter
}

package com.andrew.service;
import org.apache.ibatis.session.SqlSession;
import org.apache.log4j.Logger;
import org.junit.After;
import org.junit.Before;
import org.junit.Test;
import com.andrew.mappers.GradeMapper;
import com.andrew.model.Grade;
import com.andrew.util.SqlSessionFactoryUtil;
public class StudentGradeTest {
    private static Logger logger = Logger.getLogger(StudentTest.class);
    private SqlSession sqlSession = null;
    private GradeMapper gradeMapper = null;
    @Before
    public void setUp() throws Exception {
        sqlSession = SqlSessionFactoryUtil.openSession();
        gradeMapper = sqlSession.getMapper(GradeMapper.class);
    }
    @After
    public void tearDown() throws Exception {
        sqlSession.close();
    }
    @Test
    public void testFindGradeWithStudents() {
        logger.info("查询年级和学生");
        Grade grade = gradeMapper.findById(2);
        System.out.println(grade);
    }
}

运行结果：
Grade [id=2, gradeName=大学二年级, student=[Student [id=7, name=赵六3, age=23, address=Address [id=1, sheng=北京市, shi=北京市, qu=东城区]], Student [id=9, name=赵六5, age=25, address=Address [id=1, sheng=北京市, shi=北京市, qu=东城区]]]]

注意：
一对多关系不能a调用b的同时b调用a，否则会有死循环。
可以不使用默认的toString方法，自定义不同的方法。