generic modelling of data models; enforcing constraints dynamically...

Dear List,
I am trying to implement the following:

In a database I wish to implement a GENERIC datamodel, thus on a meta-level.
All RELATIONS (part of a MODEL) will be a view on some base (being a table)
JOINed with (an) extra column(s). Thus, this view consists of a number of
FIELDS. I whish to make this view editable (INSERT, UPDATE) using the RULE
system. Some constraints will apply; enforcing these is the problem I am
trying to solve by modeling these rules with a table "RELATION_CONSTRAINTS"
(see below).

Tables:

BASE(col1, col2, col3)

MODELS(name)

RELATIONS(modelname, name)

FIELDS(modelname, relation_name, name, datatype)

RELATION_CONSTRAINTS(modelname, relation_name, constraint_name,
constraining_expression)

I was thinking of implementing this using a FUNCTION that takes a
polymorphic record parameter (and the relation name); then checking this
record against the applicable constraint expression.
This whole idea may sound like a DBMS-in-a-DBMS kind of thing... What I am
trying is to write as little as table/view-specific code as would be
necessary, while still collecting all base data in one central table...

All suggestions are very much appreciated,
regards,

Rob