MyBatis学习笔记（四）一多一关系

Student中包含Address

package com.skymr.mybatis.model;

public class Student {

    private int id;

    private String name;

    private int age;

    private Address address;

    /**
     * 必须要有无参构造器，有参构造器可有可无（至少我测试时是这样）
     * 如果没有无参构造器，只有有参构造器，会报错
     */
    public Student() {
    }
    public Student(String name, int age) {
        this.name = name;
        this.age = age;
    }

    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 int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public Address getAddress() {
        return address;
    }
    public void setAddress(Address address) {
        this.address = address;
    }
    public String toString(){
        return "["+name+","+age+","+address+"]";
    }

}

package com.skymr.mybatis.model;

public class Address {

    //省
    private String province;
    //市
    private String city;
    //区
    private String region;

    private int id;

    public String getProvince() {
        return province;
    }

    public void setProvince(String province) {
        this.province = province;
    }

    public String getCity() {
        return city;
    }

    public void setCity(String city) {
        this.city = city;
    }

    public String getRegion() {
        return region;
    }

    public void setRegion(String region) {
        this.region = region;
    }

    public int getId() {
        return id;
    }

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

    public String toString(){
        return "[" + id+","+this.province+","+this.city+","+this.region+"]";
    }
}

package com.skymr.mybatis.mappers;

import java.util.List;

import com.skymr.mybatis.model.Student;

public interface StudentMapper {

    public Student getStudent(int id);

    public List<Student> getAllStudents();
    //取得所有学生，带地址
    public List<Student> getAllStudentsWithAddr();
    //取得学生，带地址
    public Student getStudentWithAddr(int id);
}

方式一：对象集联

<!-- 一对一关系 -->
<select id="getAllStudentsWithAddr" resultMap="stuMapWidhAddr">
    select * from mybatis_Student t1, mybatis_address t2 where t1.address_id=t2.id
</select>
<resultMap type="Student" id="stuMapWidhAddr">
    <id property="id" column="id"/>
    <result property="name" column="name"/>
    <result property="age" column="age"/>
    <result property="address.id" column="address_id"/>
    <result property="address.province" column="province"/>
    <result property="address.city" column="city"/>
    <result property="address.region" column="region"/>
</resultMap>

<select id="getStudentWithAddr" resultMap="stuMapWidhAddr" parameterType="int">
    select * from mybatis_Student t1 left join mybatis_address t2 on t1.address_id=t2.id and t1.id=#{id}
</select>

这种方式不太好，重用性差．

方式二：

<!-- 一对一关系 -->
<select id="getAllStudentsWithAddr" resultMap="stuMapWidhAddr">
    select * from mybatis_Student t1, mybatis_address t2 where t1.address_id=t2.id
</select>
<resultMap type="Student" id="stuMapWidhAddr">
    <id property="id" column="id"/>
    <result property="name" column="name"/>
    <result property="age" column="age"/>
    <association property="address" resultMap="addressMap"></association>
</resultMap>
<!-- 将Address独立出来 -->
<resultMap type="Address" id="addressMap">
    <id property="id" column="id"/>
    <result property="province" column="province"/>
    <result property="city" column="city"/>
    <result property="region" column="region"/>
</resultMap>
<select id="getStudentWithAddr" resultMap="stuMapWidhAddr" parameterType="int">
    select * from mybatis_Student t1 left join mybatis_address t2 on t1.address_id=t2.id and t1.id=#{id}
</select>

将Address独立出来，然后使用association关联到Address的resultMap

方式三：

<resultMap type="Student" id="stuMapWidhAddr">
    <id property="id" column="id"/>
    <result property="name" column="name"/>
    <result property="age" column="age"/>
    <association property="address" javaType="Address">
        <result property="id" column="id"/>
        <result property="province" column="province"/>
        <result property="city" column="city"/>
        <result property="region" column="region"/>
    </association>
</resultMap>

方式四：推荐的方式

这种方式还是主要是应用association标签

首先，为Address类加入AddressMapper类与AddressMapper.xml

package com.skymr.mybatis.mappers;

import com.skymr.mybatis.model.Address;

public interface AddressMapper {

    public Address getAddress(int 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.skymr.mybatis.mappers.AddressMapper">
    <select id="getAddress" resultType="Address" parameterType="int">
        select * from mybatis_address where id=#{id}
    </select>
</mapper>

其次，修改StudentMapper.xml

<!-- 一对一关系 -->
<select id="getAllStudentsWithAddr" resultMap="stuMapWidhAddr">
    select * from mybatis_Student
</select>
<resultMap type="Student" id="stuMapWidhAddr">
    <id property="id" column="id"/>
    <result property="name" column="name"/>
    <result property="age" column="age"/>
    <association property="address" column="address_id" select="com.skymr.mybatis.mappers.AddressMapper.getAddress">
    </association>
</resultMap>
<select id="getStudentWithAddr" resultMap="stuMapWidhAddr" parameterType="int">
    select * from mybatis_Student where id=#{id}
</select>

使用了association标签，定义column与select属性

column:　传入mybatis_Student表的外键address_id.

select:　调用AddressMapper的getAddress方法

推演过程：查询一条student数据，再根据address_id查询address表的一条数据．

思考：不知道可不可以这样想，如果查询1000条student，再分别查address表的1000条数据，这样查询次数就是1001次了，不影响性能吗？