Finally, I figured out what causes the 'ModelController' error in the TG2.1 Admin System Page. Thanks to Eclipse (Aptana Studio 3) and PyDev, I was able to debug and spot the cause of the trouble.
I was kind of surprised that Sprox, the module that is responsible for form-model binding, could not handle unique constraint in one to many relationship.
Due to this, all other kind of constraints on relationship will probably fail too. Perhaps UniqueConstraint also get effected.
So, mainly all other constraints that are not applied directly to the fields of the sqlalchemy models could not be handled properly by TG2.1 (sprox).
Since, unique constraint is very common in DB design, I really hope that this functionality could be handled not just in simple field but also in relationship and multi-columns unique contraint.
As for the 'elegant' solution, I have not yet find a way to do it. Perhaps, I could add validator in the relationship field/attribute (in sqlalchemy declarative model class), but no clue on how to implement it. Another thing is that even if I could do that, I am not sure it would work, since I am not even sure if relationship field would be registered to be checked by the framework.
It would be a good thing if in sprox the relationship field and the foreignkey field both are being checked as one.
As for now, I just need to do a work around to prevent adding duplicate one to many relationship.
Pretty impressed with TG2.1. This is by far the most flexible web framework that I have worked with. My suggestion to those who would use this framework is, be prepared to swim and dive in the source codes, as you definitely need to extend functionalities by overriding methods and sub-classing.