mybatis關系映射

resultMap 主要是 mybatis 幫助從數據庫中獲取列數據后封裝成對象。

1 一對一映射

比如每位學生有一個地址。

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
    }

我們根據學生 ID 選擇學生信息

方法一：使用句點符號表示嵌套對象的引用，

Student 的 address 屬性使用了圓點記法被賦上了 address 對應列的值。

<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>

 

方法二：使用mybatis提供的一對一映射，關鍵字：association

1）

<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>

元素<association>被用來導入“有一個”(has-one)類型的關聯。

如果內嵌的對象有對應的 resultMap 那么使用 <association property="address" resultMap="AddressResult" /> 比較方便。如果內嵌對象需要其它的查詢來生成對象，使用方法 3）比較好。

 

2）也可以使用<association 定義內聯的 resultMap，代碼如下所示：

<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）通過使用嵌套 select 查詢來獲取 Student 及其 Address 信息，代碼如下：

<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>

在此方式中，<association>元素的 select 屬性被設置成了 id 為 findAddressById 的語句。這里，兩個分開的SQL 語句將會在數據庫中執行，第一個調用 findStudentWithAddress 加載 student 信息，而第二個調用 findAddressById 來加載 address 信息。

Addr_id 列的值將會被作為輸入參數傳遞給 selectAddressById 語句；如果有多個參數可以通過逗號分隔。

 

2 一對多映射，關鍵詞使用 <collection> 

比如老師和課程的關系，老師可以講多門課程，每門課程有一個老師

public class Course
    {
        private Integer courseId;
        private String name;
        private String description;
        private Date startDate;
        private Date endDate;
        private Integer tutorId;
        //setters & getters
    }

public class Tutor
    {
        private Integer tutorId;
        private String name;
        private String email;
        private Address address;
        private List<Course> courses;
        // setters & getters
    }

1）使用內嵌結果 ResultMap 實現一對多映射。下面是配置文件

<resultMap type="Course" id="CourseResult">
    <id column="course_id" property="courseId" />
    <result column="name" property="name" />
    <result column="description" property="description" />
    <result column="start_date" property="startDate" />
    <result column="end_date" property="endDate" />
</resultMap>
<resultMap type="Tutor" id="TutorResult">
    <id column="tutor_id" property="tutorId" />
    <result column="tutor_name" property="name" />
    <result column="email" property="email" />
    <collection property="courses" resultMap="CourseResult" />
</resultMap>

<select id="findTutorById" parameterType="int" resultMap="TutorResult">
    SELECT T.TUTOR_ID, T.NAME AS TUTOR_NAME, EMAIL, C.COURSE_ID,
    C.NAME, DESCRIPTION, START_DATE, END_DATE
    FROM TUTORS T LEFT OUTER JOIN ADDRESSES A ON T.ADDR_ID=A.ADDR_ID
    LEFT OUTER JOIN COURSES C ON T.TUTOR_ID=C.TUTOR_ID
    WHERE T.TUTOR_ID=#{tutorId}
</select>

<collection>元素的 resultMap 屬性設置成了 CourseResult，CourseResult 包含了 Course 對象屬性與表列名之間的映射。

 

2）使用嵌套 Select 語句實現一對多映射，配置文件如下

<resultMap type="Course" id="CourseResult">
    <id column="course_id" property="courseId" />
    <result column="name" property="name" />
    <result column="description" property="description" />
    <result column="start_date" property="startDate" />
    <result column="end_date" property="endDate" />
</resultMap>