理解mybaitis中的association和collection的區別

最近一直把collection和association弄混，所以為了增強自己的記憶，就擼一個關系出來算是總結罷了

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1. 關聯-association
2. 集合-collection

比如同時有User.java和Card.java兩個類

User.java如下:

public class User{

private Card card_one;

private List<Card> card_many;

}

 

在映射card_one屬性時用association標簽, 映射card_many時用collection標簽.

所以association是用於一對一和多對一，而collection是用於一對多的關系

下面就用一些例子解釋下吧

association-一對一

人和身份證的關系

下面是pojo

 

public class Card implements Serializable{
private Integer id;
private String code;
//省略set和get方法.
}

public class Person implements Serializable{
private Integer id;
private String name;
private String sex;
private Integer age;
//人和身份證是一對一的關系
private Card card;
//省略set/get方法.
}

 

下面是mapper和實現的接口

 

package com.glj.mapper;
 
import com.glj.poji.Card;
 
public interface CardMapper {
Card selectCardById(Integer id);
}

<?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.glj.mapper.CardMapper">
<select id="selectCardById" parameterType="int" resultType="com.glj.poji.Card">
select * from tb_card where id = #{id}
</select>
</mapper>

package com.glj.mapper;
 
import com.glj.poji.Person;
 
public interface PersonMapper {
Person selectPersonById(Integer id);
}

 

<?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.glj.mapper.PersonMapper">
<resultMap type="com.glj.poji.Person" id="personMapper">
<id property="id" column="id"/>
<result property="name" column="name"/>
<result property="sex" column="sex"/>
<result property="age" column="age"/>
<association property="card" column="card_id"
select="com.glj.mapper.CardMapper.selectCardById"
javaType="com.glj.poji.Card">
</association>
</resultMap>
<select id="selectPersonById" parameterType="int" resultMap="personMapper">
select * from tb_person where id = #{id}
</select>
</mapper>

 

 

PersonMapper.xml 還使用association的分步查詢。

同理多對一，也是一樣

只要那個pojo出現private Card card_one;

即使用association

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collection 一對多和association的多對一關系

學生和班級的一對多的例子

pojo類

 

package com.glj.pojo;
 
import java.io.Serializable;
import java.util.List;
 
public class Clazz implements Serializable{
private Integer id;
private String code;
private String name;
        //班級與學生是一對多的關系
private List<Student> students;
//省略set/get方法
}

package com.glj.pojo;
 
import java.io.Serializable;
 
public class Student implements Serializable {
private Integer id;
private String name;
private String sex;
private Integer age;
        //學生與班級是多對一的關系
private Clazz clazz;
//省略set/get方法
}

<?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.glj.mapper.ClazzMapper">
<select id="selectClazzById" parameterType="int" resultMap="clazzResultMap">
select * from tb_clazz where id = #{id}
</select>
<resultMap type="com.glj.pojo.Clazz" id="clazzResultMap">
<id property="id" column="id"/>
<result property="code" column="code"/>
<result property="name" column="name"/>
<!-- property: 指的是集合屬性的值, ofType：指的是集合中元素的類型 -->
<collection property="students" ofType="com.glj.pojo.Student"
column="id" javaType="ArrayList"
fetchType="lazy" select="com.glj.mapper.StudentMapper.selectStudentByClazzId">
<id property="id" column="id"/>
<result property="name" column="name"/>
<result property="sex" column="sex"/>
<result property="age" column="age"/>
</collection>
</resultMap>
</mapper>

package com.glj.mapper;
 
import com.glj.pojo.Clazz;
 
public interface ClazzMapper {
Clazz selectClazzById(Integer id);
}

 

ClazzMapper使用到了集合-collection 即為一對多，一個班級面對多個學生

 

<?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.glj.mapper.StudentMapper">
<select id="selectStudentById" parameterType="int" resultMap="studentResultMap">
select * from tb_clazz c,tb_student s where c.id = s.id and s.id = #{id}
</select>
<select id="selectStudentByClazzId" parameterType="int" resultMap="studentResultMap">
select * from tb_student where clazz_id = #{id}
</select>
<resultMap type="com.glj.pojo.Student" id="studentResultMap">
<id property="id" column="id"/>
<result property="name" column="name"/>
<result property="sex" column="sex"/>
<result property="age" column="age"/>
<association property="clazz" javaType="com.glj.pojo.Clazz">
<id property="id" column="id"/>
<result property="code" column="code"/>
<result property="name" column="name"/>
</association>
</resultMap>
</mapper>

package com.glj.mapper;
 
import com.glj.pojo.Student;
 
public interface StudentMapper {
Student selectStudentById(Integer id);
}

 

StudentMapper則是與班級為多對一關系，所以使用了關聯-association

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嗯，希望我以后又不記得二者的關系時,能感謝現在總結的自己