【持久化框架MyBatis3四】MyBatis3一对一关联查询

当两个实体具有1对1的对应关系时，可以使用One-To-One的进行映射关联查询

One-To-One示例数据

以学生表Student和地址信息表为例，每个学生都有都有1个唯一的地址(现实中，这种对应关系是不合适的，因为人和地址是多对一的关系)，这里只是演示目的

学生表

CREATE TABLE STUDENTS 
(
  STUD_ID INT(11) NOT NULL AUTO_INCREMENT,
  NAME VARCHAR(50) NOT NULL,
  EMAIL VARCHAR(50) NOT NULL,
  PHONE VARCHAR(15) DEFAULT NULL,  
  DOB DATE DEFAULT NULL,
  GENDER VARCHAR(6) DEFAULT NULL, 
  BIO LONGTEXT DEFAULT NULL,
  PIC BLOB DEFAULT NULL,
  ADDR_ID INT(11) DEFAULT NULL, 
  PRIMARY KEY (STUD_ID),
  UNIQUE KEY UK_EMAIL (EMAIL),
  CONSTRAINT FK_STUDENTS_ADDR FOREIGN KEY (ADDR_ID) REFERENCES ADDRESSES (ADDR_ID)
) ENGINE=INNODB AUTO_INCREMENT=1 DEFAULT CHARSET=UTF-8;

地址表：

CREATE TABLE ADDRESSES 
(
  ADDR_ID INT(11) NOT NULL AUTO_INCREMENT,
  STREET VARCHAR(50) NOT NULL,
  CITY VARCHAR(50) NOT NULL,
  STATE VARCHAR(50) NOT NULL,
  ZIP VARCHAR(10) DEFAULT NULL,
  COUNTRY VARCHAR(50) NOT NULL,
  PRIMARY KEY (ADDR_ID)
) ENGINE=INNODB AUTO_INCREMENT=1 DEFAULT CHARSET=UTF-8;

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样例数据：

INSERT INTO ADDRESSES (ADDR_ID,STREET,CITY,STATE,ZIP,COUNTRY) VALUES 
 (1,'4891 Pacific Hwy','San Diego','CA','92110','San Diego'),
 (2,'2400 N Jefferson St','Perry','FL','32347','Taylor'),
 (3,'710 N Cable Rd','Lima','OH','45825','Allen'),
 (4,'5108 W Gore Blvd','Lawton','OK','32365','Comanche');

-- Sample data for table STUDENTS

INSERT INTO STUDENTS (STUD_ID,NAME,EMAIL,PHONE,DOB,BIO,PIC,ADDR_ID) VALUES 
 (1,'Timothy','[email protected]','123-123-1234','1988-04-25',NULL,NULL,3),
 (2,'Douglas','[email protected]','789-456-1234','1990-08-15',NULL,NULL,4);

从上面的建表的sql中，可以看到，STUDENTS表通过外键ADDR_ID与ADDRESSES表建立1:1的映射关系

建立Address-Mapper.xml映射文件

<?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.mybatis3.mappers.AddressMapper">
	<!--Address是在MyBatis主配置文件中定义的类型别名-->
        <!--AddressResult这个resultMap对ADDRESSES表进行了SQL和Model的映射-->
  	<resultMap type="Address" id="AddressResult">
  		<id property="addrId" column="addr_id"/>
		<result property="street" column="street"/>
		<result property="city" column="city"/>
		<result property="state" column="state"/>
		<result property="zip" column="zip"/>
		<result property="country" column="country"/>
  	</resultMap>
  	<!--查询指定的Address-->
  	<select id="selectAddressById" parameterType="int" resultMap="AddressResult">
  		select * from addresses where addr_id=#{addrId}
  	</select>
  	
</mapper>

建立Student-Mapper.xml映射文件

<mapper namespace="com.mybatis3.mappers.StudentMapper">
	
	<resultMap type="Student" id="StudentResult">
		<id 	property="studId" column="stud_id"/>
		<result property="name" column="name" />
		<result property="email" column="email"/>
		<result property="phone" column="phone"/>
	</resultMap>
  	
  	<resultMap type="Student" id="StudentWithAddressExtResult" extends="StudentResult">
		<result property="address.addrId" column="addr_id"/>
		<result property="address.street" column="street"/>
		<result property="address.city" column="city"/>
		<result property="address.state" column="state"/>
		<result property="address.zip" column="zip"/>
		<result property="address.country" column="country"/>
	</resultMap>
	
  	<resultMap type="Student" id="StudentWithAddressNestedSelect">
		<id 	property="studId" column="stud_id"/>
		<result property="name" column="name"/>
		<result property="email" column="email"/>
		<association property="address" column="addr_id" select="com.mybatis3.mappers.AddressMapper.selectAddressById"/>
	</resultMap>
	
	<resultMap type="Student" id="StudentWithAddressNestedResultMap">
		<id 	property="studId" column="stud_id"/>
		<result property="name" column="name"/>
		<result property="email" column="email"/>
		<association property="address" javaType="Address">
			<id property="addrId" column="addr_id"/>
			<result property="street" column="street"/>
			<result property="city" column="city"/>
			<result property="state" column="state"/>
			<result property="zip" column="zip"/>
			<result property="country" column="country"/>
		</association>
	</resultMap>
	
	<select id="findAllStudents" resultMap="StudentResult">
    	select * from Students
  	</select>
        <select id="findStudentWithAddressExtResult" parameterType="int" resultMap="StudentWithAddressExtResult">
            SELECT STUD_ID, NAME, EMAIL, PHONE, A.ADDR_ID, STREET, CITY,  STATE, ZIP, COUNTRY
            FROM STUDENTS S LEFT OUTER JOIN ADDRESSES A ON S.ADDR_ID=A.ADDR_ID
            WHERE STUD_ID=#{studId}
        </select>
  	
  	<select id="findStudentWithAddressNestedSelect" parameterType="int" resultMap="StudentWithAddressNestedSelect">
    	select * from Students where stud_id=#{studId}
  	</select>
  	  	  	
  	<select id="findStudentWithAddressNestedResultMap" parameterType="int" resultMap="StudentWithAddressNestedResultMap">
  		select stud_id, name, email,phone, a.addr_id, street, city, state, zip, country
  		FROM students s left outer join addresses a on s.addr_id=a.addr_id
		where stud_id=#{studId}
  	</select>
	
	  	
</mapper>

三种One-To-One映射的写法

从Student-Mapper.xml中，可以看到，One-To-One的映射关系可以有三种写法

findStudentWithAddressExtResult
findStudentWithAddressNestedSelect
findStudentWithAddressNestedResultMap

One-To-One映射：使用resultMap扩展(不推荐，了解即可)

1. resultMap定义

	<resultMap type="Student" id="StudentResult">  <!--定义Student表和Student之间的映射关系-->
		<id 	property="studId" column="stud_id"/>
		<result property="name" column="name" />
		<result property="email" column="email"/>
		<result property="phone" column="phone"/>
	</resultMap>
  	
  	<resultMap type="Student" id="StudentWithAddressExtResult" extends="StudentResult"> <!--扩展Student表和Student之间的映射关系-->
		<result property="address.addrId" column="addr_id"/> <!--property的写法使用.语法，类似OGNL表达式语言-->
		<result property="address.street" column="street"/> <!--column从哪里来的？这是在SQL语句中定义-->
		<result property="address.city" column="city"/>
		<result property="address.state" column="state"/>
		<result property="address.zip" column="zip"/>
		<result property="address.country" column="country"/>
	</resultMap>

2.SQL-Mapping定义

        <select id="findStudentWithAddressExtResult" parameterType="int" resultMap="StudentWithAddressExtResult">
            SELECT STUD_ID, NAME, EMAIL, PHONE, A.ADDR_ID, STREET, CITY,  STATE, ZIP, COUNTRY
            FROM STUDENTS S LEFT OUTER JOIN ADDRESSES A ON S.ADDR_ID=A.ADDR_ID
            WHERE STUD_ID=#{studId}
        </select>

在SQL-Mapping中定义了StudentWithAddressExtResult定义的列名，这里看出，列名的定义，可以不关心是来自哪个表，MyBatis只是将得到的列名与属性名进行匹配，这样就有个问题：假如Student表和Address表都有一个相同的列name，如下所示，那么如果为两个NAME设置对应的值？

        <select id="findStudentWithAddressExtResult" parameterType="int" resultMap="StudentWithAddressExtResult">
            SELECT STUD_ID, NAME, EMAIL, PHONE, A.ADDR_ID, A.NAME, STREET, CITY,  STATE, ZIP, COUNTRY
            FROM STUDENTS S LEFT OUTER JOIN ADDRESSES A ON S.ADDR_ID=A.ADDR_ID
            WHERE STUD_ID=#{studId}
        </select>

One-To-One映射：使用association + select属性（直观简洁，推荐用法）

1.resultMap定义

 	<resultMap type="Student" id="StudentWithAddressNestedSelect">
		<id 	property="studId" column="stud_id"/>
		<result property="name" column="name"/>
		<result property="email" column="email"/>
		<association property="address" column="addr_id" select="com.mybatis3.mappers.AddressMapper.selectAddressById"/>
	</resultMap

2. association定义

assocation用于MyBatis定义1对1的关联关系

<association property="address" column="addr_id" select="com.mybatis3.mappers.AddressMapper.selectAddressById"/>

com.mybatis3.mappers.AddressMapper.selectAddressById是一个查询操作标识符，定义在Address-Mapper.xml中

<?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.mybatis3.mappers.AddressMapper">
	
  	<resultMap type="Address" id="AddressResult">
  		<id property="addrId" column="addr_id"/>
		<result property="street" column="street"/>
		<result property="city" column="city"/>
		<result property="state" column="state"/>
		<result property="zip" column="zip"/>
		<result property="country" column="country"/>
  	</resultMap>
  	
  	<select id="selectAddressById" parameterType="int" resultMap="AddressResult">
  		select * from addresses where addr_id=#{addrId}
  	</select>
  	
</mapper>

3. SQL查询语句

    <select id="findStudentById" parameterType="int" resultMap="StudentWithAddressNestedSelect">
    	select * from STUDENTS where stud_id=#{studId}
    </select>

One-To-One映射：使用association内部定义映射（无法重用，不推荐）

1. resultMap定义

	<resultMap type="Student" id="StudentWithAddressNestedResultMap">
		<id 	property="studId" column="stud_id"/>
		<result property="name" column="name"/>
		<result property="email" column="email"/>
		<association property="address" javaType="Address">
			<id property="addrId" column="addr_id"/>
			<result property="street" column="street"/>
			<result property="city" column="city"/>
			<result property="state" column="state"/>
			<result property="zip" column="zip"/>
			<result property="country" column="country"/>
		</association>
	</resultMap>

2. assocation内部定义映射关系，javaType的Address是一个类型别名，引用了Address类

		<association property="address" javaType="Address">
			<id property="addrId" column="addr_id"/>
			<result property="street" column="street"/>
			<result property="city" column="city"/>
			<result property="state" column="state"/>
			<result property="zip" column="zip"/>
			<result property="country" column="country"/>
		</association>

3. SQL语句的写法：

    <select id="selectStudentWithAddress" parameterType="int" resultMap="StudentWithAddressNestedResultMap">
  		select stud_id, name, email,phone, a.addr_id, street, city, state, zip, country
  		FROM STUDENTS s left outer join ADDRESSES a on s.addr_id=a.addr_id
		where stud_id=#{studId}
  	</select>

懒加载配置

要在第二种方式(assocation+select的方式)使用懒加载的方式，需要在MyBatis的主配置文件中配置懒加载选项：

<settings>
 <setting name="lazyLoadingEnabled" value="true"/>
 <setting name="aggressiveLazyLoading" value="false"/>
</settings>

总结

1. 实现1对1映射的三种方式

resultMap继承, join查询，不能懒加载
assocation+select，简单查询(N+1)，可以懒加载
assocation+内嵌的resultMap映射，join查询，不能懒加载

2. 实际工作中，尽量使用assocation+select，加懒加载的方式

3. 工作中，数据库设计的不是那么的严格,比如不设置外键；对于这种关联关系的查询，使用多次的查询，每次执行简单语句，这是Bad Practices