Hello, not sure whether this is TG or Sprox bug, but I patched it in Sprox.
When I added

 class Order(DeclarativeBase):

...

user = relation("User", backref=backref('orders',
order_by=order_id,cascade_backrefs=False),
cascade="",cascade_backrefs=False)

...

#standard TG User

class User(DeclarativeBase):

...

class __sprox__(object):
     omit_fields = ['orders','_password']

then the orders field won't show in html output in TG Admin, but
SAORMProvider.put() will try to update them, resulting in unwanted updates
or database constraint violation. My quick fix was to add the check

try:
if key in entity.__sprox__.omit_fields:
continue
except:
pass
into SAORMProvider.put() and SAORMProvider._remove_related_empty_params()
loops.

I can provide the patch for pulling if it's on right track. But, there were
some other trouble saving the orders even even if the field is not omitted
at all. I did not research this yet, maybe it relies on cascade_backrefs
being on.

Regards,

Juraj