difference between collection and association mapping in mybatis 3

Mybatis处理“一对多”的关系时，需要用到associasion元素。处理”多对一“用collection元素来实现（这两个元素在之前mapper文件中提到过）。

本例子中，假设一名User可以有多个Orders，用associasion来实现关联关系

首先数据库表结构

CREATE TABLE `user` (
  `id` int(8) NOT NULL AUTO_INCREMENT,
  `username` varchar(20) COLLATE utf8_bin NOT NULL,
  `usernumber` varchar(20) COLLATE utf8_bin NOT NULL,
  `loginname` varchar(20) COLLATE utf8_bin NOT NULL,
  `loginpassword` varchar(20) COLLATE utf8_bin NOT NULL,
  `sex` varchar(4) COLLATE utf8_bin DEFAULT NULL,
  `birthday` date DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=20 DEFAULT CHARSET=utf8 COLLATE=utf8_bin;

//orders表中为user_id添加外键，指向user表的id
CREATE TABLE `orders` (
  `oid` int(8) NOT NULL AUTO_INCREMENT,
  `orderid` varchar(20) COLLATE utf8_bin NOT NULL,
  `message` varchar(20) COLLATE utf8_bin DEFAULT NULL,
  `user_id` int(8) NOT NULL,
  PRIMARY KEY (`oid`),
  KEY `1001` (`user_id`),
  CONSTRAINT `1001` FOREIGN KEY (`user_id`) REFERENCES `user` (`id`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB AUTO_INCREMENT=6 DEFAULT CHARSET=utf8 COLLATE=utf8_bin;

Orders实体类

        public int oid;
        public String orderid;
        public String message;
        public User user;
       //省略get/set方法

Order对象的sql映射文件，order.xml

<mapper namespace="com.mybaits.dao.impl.OrdersImpl">
        <resultMap type="com.mybaits.bean.User" id="userResult">
                <id property="id" column="id" />
                <result property="username" column="username"/>
                <result property="usernumber" column="usernumber"/>
                <result property="loginname" column="loginname"/>
                <result property="loginpassword" column="loginpassword"/>
                <result property="sex" column="sex"/>
                <result property="birthday" column="birthday" />
        </resultMap>

        <resultMap type="com.mybaits.bean.Orders" id="orderResult">
            <id property="oid" column="oid"/>
            <result property="orderid" column="orderid"/>
            <result property="message" column="message"/>
　　　　　　　　
            <association property="user" column="user_id" javaType="com.mybaits.bean.User" jdbcType="INTEGER" resultMap="userResult">
            </association>
        </resultMap>

        <select id="findAllOrders" resultMap="orderResult">
            select * from orders o left join user u on o.user_id=u.id
        </select>

        <insert id="saveOrder" parameterType="Orders">
            insert into orders (orderid,message,user_id) values(#{orderid},#{message},#{user.id})
        </insert>

</mapper><!--使用resultMap属性引用上面的User实体映射-->

OrderTest类

@Test
    public void Test2(){
        List<Orders> l=order.findOrders();
        for(int i=0;i<l.size();i++){
            System.out.println(l.get(i));
        }
    }
订单编号：10001    订单信息：订单1    下单用户：BN
订单编号：10002    订单信息：订单2    下单用户：BN
订单编号：10003    订单信息：订单3    下单用户：qwe
订单编号：10004    订单信息：订单4    下单用户：qwe
订单编号：10005    订单信息：订单5    下单用户：JAVA

利用collection在User对象中关联

1.在User中添加 List<Orders> orders 属性

private List<Orders> orders=new ArrayList<Orders>();//get/set方法

2.User的sql映射文件user.xml

   <resultMap type="com.mybaits.bean.User" id="userResult">
                <id property="id" column="id" />
                <result property="username" column="username"/>
                <result property="usernumber" column="usernumber"/>
                <result property="loginname" column="loginname"/>
                <result property="loginpassword" column="loginpassword"/>
                <result property="sex" column="sex"/>
                <result property="birthday" column="birthday" />
                 //collection元素映射 user对象中order的集合属性，resultMap指向下面的order的resultMap
                <collection property="orders" ofType="Orders" resultMap="orderResult"></collection>
        </resultMap>
       <resultMap type="com.mybaits.bean.Orders" id="orderResult">
            <id property="oid" column="oid"/>
            <result property="orderid" column="orderid"/>
            <result property="message" column="message"/>
        </resultMap>
　　　　　　<select id="getUOBN" parameterType="string" resultMap="userResult">
             select * from user u left join orders o on o.user_id=u.id where u.usernumber=#{usernumber}
          </select>

3.测试类

@Test
    public void Test11(){
        User u=userDao.findUserOrders("01111001");
        System.out.println("用户："+u.getUsername()+"\n订单总数："+u.getOrders().size());
        for(Orders o:u.getOrders()){
            System.out.println("订单号："+o.getOrderid()+"\t订单信息："+o.getMessage());
        }

    }

用户：BN
订单总数：2
订单号：10001    订单信息：订单1
订单号：10002    订单信息：订单2

http://www.voidcn.com/blog/fqf_520/article/p-4973660.html

I am going to assume that you have a many to many relationship between Projects and Employees, which is why you created a Project Assignment table. This Project Assignment table / object may only have two fields/columns: a mapping of project id to employee id - a classic "bridge table" (aka "join" or "junction" table).

When you map this model to an object graph, you have three options:

A Project object can have a list of all employees assigned to it
An Employee object can have a list of projects s/he is assigned to
Create a Project Assignment object that has a mapping of each projects to its employee and each employee to his/her project.
In your example you chose the last option.

Association

An association is a single mapping for a "has-one" relationship.

Suppose an Employee can only be assigned to one Project at a time. Some models call this a "has-one" or "belongs to" relationship. If you want to make Employee your "primary" focus in the object graph, then you would map it with an association to his/her Project:

<resultMap id="employeeResultMap" type="Employee">
<constructor>
<idArg column="employee_id" javaType="_integer"/>
</constructor>
<result property="firstName" column="first_name"/>
<result property="lastName" column="last_name"/>
<!-- etc. for other simple properties of Employee -->

<!-- Project is a "complex property" of Employee, so we use an -->
<!-- association to grab all of the Projects properties also -->
<association property="assignedProject" resultMap="projectResultMap"/>
</resultMap>
In this case your objects would look like this:

public Employee {
int id;
String firstName;
String lastName
Project assignedProject;
}

public Project {
int id;
String name;
String abc;
}

Collection

An collection is a "list" or "set" of associations.

Now model the inverse - we make Project the primary focus. A Project has a "has-many" relationship with Employee, so it will have a list or collection of those, so we use a "collection" mapping:

<resultMap id="projectResultMap" type="Project">
<constructor>
<idArg column="project_id" javaType="_integer"/>
<arg column="name" javaType="String"/>
</constructor>
<result property="abc" column="abc"/>

<!-- This tells mybatis that there can be multiple Employees -->
<!-- to look up and get their properties -->
<collection property="employees" ofType="Employee">
<constructor>
<idArg column="employee_id" javaType="_integer"/>
</constructor>
<result property="firstName" column="first_name"/>
<result property="lastName" column="last_name"/>
</collection>
</resultMap>
Now your objects would look like this:

public Employee {
int id;
String firstName;
String lastName
}

public Project {
int id;
String name;
String abc;
List<Employee> employees;
}

Project Association

To have a Project Association object, you would either need:

A single Project Association object that maps all projects to employees and vice versa
One Project Association object per project, mapping a project to its employees
One Project Association object per employee, mapping an employee to his/her projects
The first option is rather complex and messy - you would be trying to do relational mapping with object graphs (hash tables most likely).

I would choose to make one of the entities (Project or Employee) the primary focus and then model it as I showed above. The one case I didn't cover is if Employee is your primary focus and an Employee can be on multiple projects, then make that a "has-many" relationship using a collection rather than the association I used above.

Final Note: if it would help to see examples of using a "has-one" association and a "has-many" collection, see the MyBatis Koans I created: https://github.com/midpeter444/mybatis-koans. Koans 10 and 11 demonstrate this.

http://stackoverflow.com/questions/12425384/difference-between-collection-and-association-mapping-in-mybatis-3
http://www.mybatis.org/mybatis-3/zh/sqlmap-xml.html

https://github.com/ShawnyXiao/SpringBoot-MyBatis
https://github.com/oneone1995/M-Volunteer-SpringBoot

待确定：
https://github.com/sunlightcs/renren-security