All, Thanks for the 00 guidance. Your advice prompted me to read the Modeling Inheritance section of my 'EOF Developer's Guide' book for a refresher on OOing my RDB. I think I can take it from here. Thanks again. JohnR -----Original Message----- From: Chuck Hill [mailto:[EMAIL PROTECTED] Sent: Thursday, April 27, 2006 12:48 PM To: Rohrbaugh, John H Cc: webobjects-dev@lists.apple.com Subject: Re: Multi tables having a to-many relationship to the same table
Hi John, On Apr 27, 2006, at 8:03 AM, Rohrbaugh, John H wrote: First, a piece of advice. When you are working with EOF and start thinking thoughts like "how to specify the changeHistoryType part of the join", get up, take yourself outside, and give yourself a good talking to. Think Objects not SQL. Get the Objects right, the SQL will follow. > We have a change history logging table named 'ChangeHistory' and > would like several tables to have a to-many relationship to it. > > Student table has a to-many relationship to ChangeHistory named > 'toStudentChangeHistory' > > School table has a to-many relationship to ChangeHistory named > 'toSchoolChangeHistory' > > ChangeHistory includes an attribute named 'foreignKeyId' that will > contain the primary key of the table that points to it; ie > studentId or schoolId. That does not make any sense in OO or RDBMS terms. Your database will not allow you to have a FK to two different tables. In OO, you would not want a relationship to two different classes not in an inheritance hierarchy. At the very least your code would be littered with casts and instanceof tests. Ugly. Instead, let us consider this from a more OO perspective. You have a ChangeHistory class. This is an abstract class (defines the form and function of recording change history). There are two subclasses of ChangeHistory that track specific types of changes. These are SchoolChangeHistory and StudentChangeHistory. SchoolChangeHistory extends ChangeHistory with a relationship to the School that it records a change for. StudentChangeHistory extends ChangeHistory with a relationship to the Student that it records a change for. In EOModeler, we model a ChangeHistory entity and mark it as abstract. This entity gets one additional attribute not related to its use, type, a string of say length 10. We then subclass ChangeHistory into SchoolChangeHistory and identify the same table as ChangeHistory. Add an key attribute for the key of the School. Define the school relationship. On the section page of the Entity Inspector, we specify the restricting qualifier (type = 'School'). We then subclass ChangeHistory into StudentChangeHistory, again identifying the same table as ChangeHistory. Add an key attribute for the key of the Student. Define the student relationship. On the section page of the Entity Inspector, we specify the restricting qualifier (type = 'Student'). This will generate three Java classes and one database table. > Thought I would also have a ChangeHistory attribute named > 'changeHistoryType' that would distinguish the tables (value either > 'STUDENT' or 'SCHOOL') > > See type above, call it what you want. > toStudentChangeHistory join would be: > > Where Student.studentId = ChangeHistory.foreignKeyId and > ChangeHistory.changeHistoryType = 'STUDENT' Join Student to StudentChangeHistory in a to-many relationship based on the Student key stored in StudentChangeHistory. > toSchoolChangeHistory join would be: > > Where School.schoolId = ChangeHistory.foreignKeyId and > ChangeHistory.changeHistoryType = 'SCHOOL' Join Student to StudentChangeHistory in a to-many relationship based on the Student key stored in StudentChangeHistory. > I can not figure out how to specify the changeHistoryType part of > the join in the EOModeler. You need to use EOF inheritance as described above. > Thought of a few work-a-rounds, but would rather not add > additional attributes. For instance, could add the > changeHistoryType to both the Student and School tables. Just > seems like there should be a better way of doing this. There is! :-) Chuck -- Coming in late 2006 - an introduction to web applications using WebObjects and Xcode http://www.global-village.net/wointro Practical WebObjects - for developers who want to increase their overall knowledge of WebObjects or who are trying to solve specific problems. http://www.global-village.net/products/practical_webobjects _______________________________________________ Do not post admin requests to the list. They will be ignored. Webobjects-dev mailing list (Webobjects-dev@lists.apple.com) Help/Unsubscribe/Update your Subscription: http://lists.apple.com/mailman/options/webobjects-dev/archive%40mail-archive.com This email sent to archive@mail-archive.com
