difference between collection and association mapping in mybatis 3

Mybatis處理“一對多”的關系時，需要用到associasion元素。處理”多對一“用collection元素來實作（這兩個元素在之前mapper檔案中提到過）。

本例子中，假設一名User可以有多個Orders，用associasion來實作關聯關系

首先資料庫表結構

Orders實體類

Order對象的sql映射檔案，order.xml

OrderTest類

利用collection在User對象中關聯

1.在User中添加 List<Orders> orders 屬性

2.User的sql映射檔案user.xml

3.測試類

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 {

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">

<idArg column="project_id" javaType="_integer"/>

<arg column="name" javaType="String"/>

<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">

</collection>

Now your objects would look like this:

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