MyBatis一對多，多對一，多對多

概述 MyBatis中的一對多、多對一和多對多，主要就是resultMap中：

association：一個複雜的類型關聯。許多結果將包成這種類型（一對多） collection：複雜類型的集合（多對一） 這2個屬性的使用，而一對多和多對一都是相互的，只是站的角度不一樣。 實例：

這個實例只說這2個屬性的使用方法。

首先是數據庫，以員工管理系統的數據庫爲例。

實體類：

這裏只列要實現映射功能用到的類user、role、department。

department和user，是一對多關係，一個部門能夠擁有多個員工。

role和user是多對多關係，一個角色可能有多個員工，而一個員工也可能有多個角色。而要實現多對多，在程序中則是拆分紅兩個一對多，詳情見下面的實體類註釋。

1）User.java

public class User {

private int user_id;

private String user_name;

private String user_gender;

private String user_email;

private String user_phone;

private String user_address;

private Date user_birthday;

private int department_id;

//一對多，一個用戶可能有多個角色。

private List roles;

//多對一，一個部門多個員工

private Department department;

public User() {

super();

}

public User(String user_name, String user_gender,

String user_email, String user_phone, String user_address,

Date user_birthday, int department_id) {

super();

this.user_name = user_name;

this.user_gender = user_gender;

this.user_email = user_email;

this.user_phone = user_phone;

this.user_address = user_address;

this.user_birthday = user_birthday;

this.department_id = department_id;

}

public int getUser_id() {

return user_id;

}

public void setUser_id(int user_id) {

this.user_id = user_id;

}

public String getUser_name() {

return user_name;

}

public void setUser_name(String user_name) {

this.user_name = user_name;

}

public String getUser_gender() {

return user_gender;

}

public void setUser_gender(String user_gender) {

this.user_gender = user_gender;

}

public String getUser_email() {

return user_email;

}

public void setUser_email(String user_email) {

this.user_email = user_email;

}

public String getUser_phone() {

return user_phone;

}

public void setUser_phone(String user_phone) {

this.user_phone = user_phone;

}

public String getUser_address() {

return user_address;

}

public void setUser_address(String user_address) {

this.user_address = user_address;

}

public Date getUser_birthday() {

return user_birthday;

}

public void setUser_birthday(Date user_birthday) {

this.user_birthday = user_birthday;

} public int getDepartment_id() {

return department_id;

}

public void setDepartment_id(int department_id) {

this.department_id = department_id;

}

public Department getDepartment() {

return department;

}

public void setDepartment(Department department) {

this.department = department;

}

public List getRoles() {

return roles;

}

public void setRoles(List roles) {

this.roles = roles;

} @Override

public String toString() {

return "User [user_id=" + user_id + ", user_name=" + user_name

• ", user_gender=" + user_gender + ", user_email=" + user_email

• ", user_phone=" + user_phone + ", user_address="

• user_address + ", user_birthday=" + user_birthday

• ", department_id=" + department_id + ", roles=" + roles

• ", department=" + department + "]\n"; }

}

2）Role.java

public class Role {

private int role_id;

private String role_name;

//一對多，1個角色也可能被多個用戶擁有,因此是list

private List users;

public Role() {

}

public Role(int role_id, String role_name) {

super();

this.role_id = role_id;

this.role_name = role_name;

} public int getRole_id() {

return role_id; } public void setRole_id(int role_id) {

this.role_id = role_id;

} public String getRole_name() {

return role_name; } public void setRole_name(String role_name) {

this.role_name = role_name; } public List getUsers() {

return users; } public void setUsers(List users) {

this.users = users; } @Override public String toString() {

return "Role [role_id=" + role_id + ", role_name=" + role_name

• ", users=" + users + "]"; } }

3）Department.java

public class Department {

private int department_id;

private String department_name;

//1對多，1個部門多個用戶

private List users;

public Department() {

super(); } public Department(int department_id, String department_name,

List users) {

super();

this.department_id = department_id;

this.department_name = department_name;

this.users = users;

} public int getDepartment_id() {

return department_id; } public void setDepartment_id(int department_id) {

this.department_id = department_id; } public String getDepartment_name() {

return department_name; } public void setDepartment_name(String department_name) {

this.department_name = department_name; } public List getUsers() {

return users; } public void setUsers(List users) {

this.users = users; } @Override

public String toString() {

return "Department [department_id=" + department_id

• ", department_name=" + department_name + ", users=" + users

• "]"+"\n"; } } 3.實體類的mapper文件

1）UserDepartmentMapper.xml：多對一，多個user對應一個department，使用association

select u.*,d.department_name from user u left join department d on u.department_id=d.department_id; 2）UserMapper.xml：一對多，使用collection，一個user對應多個role，同時也是多對多中user這一方的一對多。若是要實現多對多，還須要完成一個role對應多個user，即完成兩個一對多，寫法同下。 select * from user select u.*,r.* from user u left join user_role ur on u.user_id=ur.user_id left join role r on r.role_id=ur.role_id; 無論是一對多、多對一，仍是多對多，只須要知道這2個屬性哪一個表明多哪一個表明一，就能夠很好的在實體的mapper文件中配置出來。