MyBatis映射之association和collection詳解

一、引言

一直對association和collection有點混淆，現整理一篇文章，用於加強記憶。

二、association

association用於一對一、多對一場景使用。

現在有2個表book表、bookshelf書架表。

BOOK
字段名稱	類型	備註
id	int	主鍵
name	varchar	書名
type	int	類型
shelf_id	int	書架id

Book_shelf
字段名稱	類型	備註
id	int	主鍵
number	varchar	書架編號
num	int	可存放數量

 

現有需求：查詢根據書籍id查詢書籍信息和所在書架編號。

PoJo

public class Book{
private Integer id;
private String name;
private String type;
private Integer shelfId;
private BookShelf bookShelfDto;
}

public class BookShelf {
private Integer id;
private String number;
private String num;
}

mapper

<resultMap id="bookResultMap" type="com.abc.Book">
    <id property="id" column="id"/>
    <result property="name" column="name"/>
    <result property="type" column="type"/>
    <!--關聯屬性-->
    <association property="bookShelfDto" javaType="com.abc.BookShelf">
        <id property="id" column="shelf_id"/>
        <result property="number" column="number"/>
        <result property="num" column="num"/>
    </association>
</resultMap>


<select id="getBookInfo" resultMap="bookResultMap">
select book.id,book.name,book.type,book.shelf_id,shelf.number,shelf.num
from book left join book_shelf shelf on book.shelf_id = shelf.id 
where book.id = #{id}
</select>

三、collection

應用場景爲一對多關係，即實體裏放集合。

表不變

現有需求：根據書架ID查詢書架信息及書架存放的書籍信息。

POJO

public class Book{
private Integer id;
private String name;
private String type;
private Integer shelfId;
}

public class BookShelf {
private Integer id;
private String number;
private String num;
private List<Book> bookList;
}

<resultMap id="bookShelfResultMap" type="com.abc.BookShelf">
    <id property="id" column="shelf_id"/>
    <result property="number" column="number"/>
    <result property="num" column="num"/>
    <!--關聯屬性-->
    <collection property="bookList" javaType="com.abc.Book">
         <id property="id" column="id"/>
         <result property="name" column="name"/>
         <result property="type" column="type"/>
    </collection>
</resultMap>


<select id="getBookShelfInfo" resultMap="bookShelfResultMap">
select book.id,book.name,book.type,book.shelf_id,shelf.number,shelf.num
from book left join book_shelf shelf on book.shelf_id = shelf.id 
where shelf.id = #{id}
</select>

Mapper.java
BookShelf getBookShelfInfo(Integer id);