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Using Hibernate with a link table

Posted on 2004-09-29
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Last Modified: 2008-01-09
Hi.
I have a pretty complex problem and i'm not sure how to map it.

I have a standard tomcat User & UserRoles tables setup.
I also have what's going to be an event/class scheduler with the tables Event & EventInstructor

the schema looks like this:

 ----------       -------------------
|   User   |     |   User_Role    |
 ----------       -------------------
| ID (int) |     | ID (int)           |
| Name   |     | user_id (int)   |
| ...        |     | role (varchar) |
 ----------       -------------------
 

 ----------       -----------------------
|   Event  |     |  Event_Instructor |
 ----------       -----------------------
| ID (int) |     | instructor_id (int) |
| Name   |     | event_id (int)       |
| ...        |      -----------------------
 ----------


Problem #1:
How do i map the Event & User using the Event_Instrucotr table?
I've looked at some posts about many-to-many, but my problem is that and event may or may not have an instructor, but if it does it can only have 1.

Problem #2:
An instructor is a User who has the role "instructor" in the User_Role table.
Not sure how to get only these Users to map to the Event_Instructor table.

Again, and event may or may not have up to 1 instructor, but and instructor can have many events.

Can anyone help me map this?

Thanks!

- Jonathan
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Question by:jdoklovic
2 Comments
 
LVL 142

Expert Comment

by:Guy Hengel [angelIII / a3]
ID: 12199371
Problem #1:
How do i map the Event & User using the Event_Instrucotr table?
I've looked at some posts about many-to-many, but my problem is that and event may or may not have an instructor, but if it does it can only have 1.

I assume that INSTRUCTORID should have the value of USER id, which shall be implemented by defining INSTRUCTORID being a foreign key to userid (in table user). The value can be null, which indicates no instructor.

Problem #2:
An instructor is a User who has the role "instructor" in the User_Role table.
Not sure how to get only these Users to map to the Event_Instructor table.

IN RDBMS like SQL Server and Oracle, I would use contraints or triggers to validate this requirements. In MySQL, there are no triggers unless you use MaxDB by MySQL...

So unless you have a DBMS Version that allows triggers, you need to solve this on application level.

CHeers
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LVL 7

Accepted Solution

by:
petoskey-001 earned 500 total points
ID: 12199891
----------              -------------------
|   User   |           |   User_Role    |
 ----------             -------------------
| ID (int) | <---    | ID (int)           |
| Name   |       |--| user_id (int)   |
| ...        |           | role (varchar) |
 ----------             -------------------
 

 ----------       -----------------------
|   Event  |     |  Event_Instructor |
 ----------       -----------------------
| ID (int) |     | instructor_id (int) |
| Name   |     | event_id (int)       |
| ...        |      -----------------------



Problem #1:
How do i map the Event & User using the Event_Instrucotr table?
I've looked at some posts about many-to-many, but my problem is that and event may or may not have an instructor, but if it does it can only have 1.

Problem #2:
An instructor is a User who has the role "instructor" in the User_Role table.
Not sure how to get only these Users to map to the Event_Instructor table.

Again, and event may or may not have up to 1 instructor, but and instructor can have many events.
-----------------------------------------------------------------
As stated you can't enforce specific links based on data without triggers to check things.  You can't force Event_Instroctor to link to User_Role with a value of Instructor.  You'll have to put that logic in your middleware or application.

BUT it appears to me that you don't really need the Event_Instructor table.  Each event would point to the User_Role table for their instructor.  Again you would have to check that they were an instructor if code elsewhere, but you don't really need a separate table.  Just put any extra fields from the event_instructor table into the event.

So the linking would look like this...  

  USERS <--- USER_ROLES <--- EVENT

That sounds like it would work based on what you've mentioned so far.  
 
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