Attempts to update/insert into irrelevant tables when using a polymorphic mapper

[Former user]

I was trying the latest version of SQLAlchemy and (after some trouble with SQLite caused by the fact that my home area is on a network share), I found a problem in the new polymorphic mapper support.

Specifically, with a set-up like

pjoin = polymorphic_union({
    'user':      principals.join(users),
    'group':     principals.join(groups),
    'principal': principals,
    }, 'type', 'pjoin')

and {{{User}}}, {{{Group}}} and {{{Principal}}} classes with the obvious inheritance relationships, it's possible that creating a new {{{User}}} object will cause SQLAlchemy to attempt to insert into {{{groups}}} table(!)

This fails in my case because I have attributes on {{{Group}}} objects (and in the {{{groups}}} table) that are set to {{{nullable=False}}} and don't correspond to any attribute in the {{{users}}} table.

[Former user]

A test case plus a proposed fix for the problem.

[Mike Bayer]

we've worked this out in email, was a small user error