Logical Model versus Physical Model in TDM, Why FKs do not migrate in LER?


Recently, we have received several questions on logical modeling in Toad Data Modeler:
Should I start with logical model or physical model?
What’s the difference between LER and PER in TDM?
Why Fks in logical model do not migrate to child tables?

Vaclav has written a short article describing the major differences. The document explains, shows some screenshots and offers recommendations on which model to choose.

You can find the article in our weblog at:

BTW I hope you’re all doing well just from the start of the new year. We’re happy to welcome new members on the community. Thanks for all the topics and questions you raise. All our team is back at work again, full of energy after the holiday season :wink: and ready to help. So, do not hesitate to write us.

Have a nice day.


Vladka + TDM Team

Message was edited by: vladka - fixed the link

Message was edited by: vladka

Remember the key difference is that the logical model is database agnostic and meant to show/verify with business users that the correct business reqs have been captured. And business people don’t know or care about FK’s - that’s a technical thing.

If you want to model the business reqs and then build a database for that - then always do logical model first. Think of it this way - logical model makes sure it’s correct (effective) and then physical is the implementation of that model for a specific database (efficient).

If you’re not effective, then efficiency means nothing :slight_smile:

Just a quick note: The document is available in PDF at: