MyBatis系列之高级结果映射之一对一查询

1. 一对一关系

首先，我们拥有两张表，分别为Student和Address，每个学生都拥有一个地址。

Student表信息如下：

STUD_ID	NAME	E-MAil	PHONE	ADDR_ID

Address表信息如下：

ADDR_ID	STREET	CITY	STATE	ZIP	COUNTRY

public class Address {  
    private Integer addrId;  
    private String street;  
    private String city;  
    private String state;  
    private String zip;  
    private String country;  
    // setters & getters  
}  
public class Student {  
    private Integer studId;  
    private String name;  
    private String email;  
    private PhoneNumber phone;  
    private Address address;  
    //setters & getters  
}  

<resultMap type="Student" id="StudentWithAddressResult">  
    <id property="studId" column="stud_id"/>  
    <result property="name" column="name"/>  
    <result property="email" column="email"/>  
    <result property="phone" column="phone"/>  
    <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>  

<select id="selectStudentWithAddress" parameterType="int" resultMap="StudentWithAddressResult">  
    SELECT STUD_ID, NAME, EMAIL, 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>  

我们可以在<resultMap>的节点中，嵌套放入Address的信息。在<resultMap>中，学生的地址参数值是通过address.XX来获取的。像这样，我们可以将嵌套对象的属性引用到任何地方。我们可以使用如下的方法来调用嵌套对象：

public interface StudentMapper {  
    Student selectStudentWithAddress(int studId);  
} 

int studId = 1;  
StudentMapper studentMapper =  sqlSession.getMapper(StudentMapper.class);  
Student student = studentMapper.selectStudentWithAddress(studId);  
System.out.println("Student :"+student);  
System.out.println("Address :"+student.getAddress());  

上面展示了一种方法关于一对一的映射。可是问题来了，如果我们调用的Address对象中也包含了其它的对象，也就是说Address对象与其它对象也存在关系的话，那我们不就要做同样的嵌套工作。
Maybatis提供了更好的方法，那就是应用Nested ResultMap和Nested Select的声明。下面笔者将会介绍。

2. 使用用Nested ResultMap

我们可以获取Student的信息连带着Adress信息，我们可以运用到NestedResultMap，使用方法如下的例子

<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>  
<resultMap type="Student" id="StudentWithAddressResult">  
    <id property="studId" column="stud_id"/>  
    <result property="name" column="name"/>  
    <result property="email" column="email"/>  
    <association property="address" resultMap="AddressResult"/>  
</resultMap>  
<select id="findStudentWithAddress" parameterType="int" 
resultMap="StudentWithAddressResult"> SELECT STUD_ID, NAME, EMAIL, 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>

上面的例子中，<association>元素可能被使用加载一个对象的关联。在例子中，<association>>元素中还涉及到<resultMap>的节点，这个使用与前面的是一样的。我们还可使用如下的方法来使用<association>

<resultMap type="Student" id="StudentWithAddressResult">  
    <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>  

3. 使用 Nested Select

我们可以使用嵌套的Select语句来获取Student信息上连带着Adress的信息。

<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="findAddressById" parameterType="int"  
resultMap="AddressResult">  
SELECT * FROM ADDRESSES WHERE ADDR_ID=#{id}  
</select>  

<resultMap type="Student" id="StudentWithAddressResult">  
    <id property="studId" column="stud_id"/>  
    <result property="name" column="name"/>  
    <result property="email" column="email"/>  
    <association property="address" column="addr_id" select="findAddressById"/>  
</resultMap>  

<select id="findStudentWithAddress" parameterType="int"  
resultMap="StudentWithAddressResult">  
SELECT * FROM STUDENTS WHERE STUD_ID=#{Id}  
</select>  

在这个方法中，这个<association>元素的select有名称放到了<select>的id中。所以执行上面的语句将会执行两条SQL语句，一个是findStudentById,另一个是findAddressById，而且后者主要就是加载Addresss的信息。

我们可以用下面的方法来执行findStudentWithAddress.

StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);  
Student student = mapper.selectStudentWithAddress(studId);  
System.out.println(student);  
System.out.println(student.getAddress());