一、引言

一直对association和collection有点混淆，现整理一篇文章，用于加强记忆。

二、association

association用于一对一、多对一场景使用。

现在有2个表book表、bookshelf书架表。

现有需求：查询根据书籍id查询书籍信息和所在书架编号。

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"/>
    <!--关联属性-->
    <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

应用场景为一对多关系，即实体里放集合。

表不变

现有需求：根据书架ID查询书架信息及书架存放的书籍信息。

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"/>
    <!--关联属性-->
    <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);