Grails的Many-to-Many改良

标准的M-M做法：

class User{
    static hasMany=[roles:Role]
    String name
}

class Role{
    static belongsTo = User
    static hasMany=[users:User]
    String name
}

使用方法：

Role roleUser=new Role(name: "ROLE_USER").save()
Role roleAdmin==new Role(name: "ROLE_ADMIN").save()

User user1= new User(name: "bruce_lin_chn")
user1.addToRoles(roleUser)
user1.save()

存在的问题：
[list]

hasMany会在User和Role中创建Set(org.hibernate.collection.PersistentSet)

为了唯一性检查，增加一个Role到User要求加载所有用户角色的实例和所有其他拥有该权限的用户

存在乐观锁异常的风险

解决方案：

class User{
    String name
}

class Role{
    String name
}

class UserRole implements Serializable{
    User user
    Role role
    static mapping = {
        table 'user_role_mapping'
        version false
        id composite: ['user', 'role']
    }
    
    static Set<Role> getRoles(User user){
        UserRole.findAllByUser(user).collect{it.role} as Set
    }

    static boolean hasRole(User user, Role role){
        return UserRole.countByUserAndRole(user,role)>0
    }

    static beforeDeleteUser(User user){
        
    }

    static beforeDeleteRole(Role role){

    }
}