如何确定关联表是否需要单独的实体和DAO？

Question

How to decide if a separate entity and DAO is needed for an association table?

Suppose I have 3 tables: user , role and user_role . user_role is the association table.

create table user (user_id int not null primary key, login varchar(50) not null unique key, password varchar(100))
create table role (role_id int not null primary key, description varchar(100))
create table user_role (user_role_id int not null identity(1,1) primary key, user_id int not null foreign key references user(user_id), role_id int not null foreign key references role(role_id))

Now I will have separate entities and DAOs for user and role tables.

public class User {
   private int userId;
   private String login;
   private String password;
   // getters and setters
}
public interface UserDAO {
   User getUserById(int userId);
   void insertUser(User user);
   void updateUserById(User user);
   void deleteUserById(User user);
}

public class Role {
   private int roleId;
   private String description;
   // getters and setters
}
public interface RoleDAO {
   User getRoleById(int roleId);
   void insertRole(Role role);
   void updateRoleById(Role role);
   void deleteRoleById(Role role);
}

I will have 3 screens:

• Screen 1: I will have a screen to maintain CRUD operations on user table.

• Screen 2: I will have a screen to maintain CRUD operations on role table.

• Screen 3: I will have a screen where users will be assigned to roles.

But I am confused about how to tackle the user_role table.

a) Should User entity have a list of associated Role . And thats enough?

public class User {
   private int userId;
   private String login;
   private String password;
   private Set<Role> roles;
   // getters and setters
}

b) Should Role entity have a list of associated User . And thats enough?

public class Role {
   private int roleId;
   private String description;
   private Set<User> users;
   // getters and setters
}

c) Should I have a separate entity and DAO for user_role table like this:

public class UserRole {
   private int userRoleId;
   private User user;
   private Role role;
}
public interface UserRoleDAO {
   User getUserRoleById(int userRoleId);
   void insertUserRole(UserRole userRole);
   void updateUserRoleById(UserRole userRole);
   void deleteUserRoleById(UserRole userRole);
}

Answer 1

I guess as it a bidirectional relationship between user and roles you should have to have set of roles in User.java and set of users in Roles.java. An separate entity user_roles is definitely required. If you have dao for user you can have below methods in userdao

 User getUserRoleById(int userRoleId);
 void insertUserRole(UserRole userRole);
 void updateUserRoleById(UserRole userRole);
 void deleteUserRoleById(UserRole userRole);