Error about CASCADE in Sql - Server 2008 - sql-server

Question

I posted this problem yesterday and didn't get answer.
I attach here sample of my problem. I want to create this DB:
CREATE DATABASE [TRYShemen];
GO
USE [TRYShemen]
GO
CREATE TABLE Persons(
ID VARCHAR(50) PRIMARY KEY,
FullName VARCHAR(50) NOT NULL
);
CREATE TABLE Class(
ClassNum VARCHAR(30) PRIMARY KEY,
Teacher VARCHAR(50) NOT NULL,
constraint Class_FK foreign key (Teacher) references Persons (ID) ON DELETE NO ACTION ON UPDATE CASCADE,
);
CREATE TABLE Students(
StudentID VARCHAR(50) ,
ClassNum VARCHAR(30)
constraint Students_PK PRIMARY KEY (StudentID, ClassNum),
constraint Students_FK foreign key (StudentID) references Persons(ID) ON DELETE NO ACTION ON UPDATE CASCADE ,
constraint Students_FK1 foreign key (ClassNum) references Class(ClassNum) ON DELETE NO ACTION ON UPDATE CASCADE
);
and I get an error:
Introducing FOREIGN KEY constraint 'Students_FK1' on table 'Students' may cause cycles or multiple cascade paths. Specify ON DELETE NO ACTION or ON UPDATE NO ACTION, or modify other FOREIGN KEY constraints.
I know that if i change the 'Students_FK1' to ON CASCADE NO ACTION, it will work, but it's not my target. I want that if the id of 'Person' will update , it will update in the Students table too.
how can I solve it?
Thank You!!

Answer 1

So what can you do about it?
Consider this structure as an alternative.
table-Persons
table-Students
table-Teachers
table-Classes
table-Classes_to_Teachers
table-Classes_to_Students
Then you can have Cascading references for
Persons-->Students & Students-->Classes_to_Students
Persons-->Teachers & Teachers-->Classes_to_Teachers
When you access your data, you will have more joins to get all the data for one class.
But the constraints should work out.

Answer 2

Note that it says "may cause cycles or multiple cascade paths." If you follow a delete action through your ER diagram, you'll probably find that the delete action cascades to the same table via multiple routes.
In this case, imagine that the same person is both the teacher and the student in a class. Yes, I know, that's a logic error, but if that condition were true, then an update to that person would cascade to the students table via two routes.