mapping arbitrary selectable does not use primary_key argument

[Former user]

If a selectable does not contain a column that is primary key in a base table, then cannot use that selectable in a mapper.

This code

t2 = Table('t2', meta, Column('bar', Integer))
s2 = select([t2.c.bar](t2.c.bar))
class Two(object): pass
mapper(Two, s2, primary_key=[s2.c.bar](s2.c.bar))  # same error using [t2.c.bar](t2.c.bar)

raises exception

argumentError : Mapper Mapper|Two|%(31476816 anon)s could not assemble any primary key columns for mapped table '%(31476816 anon)s'

When 'bar' is a primary key for table 't2', the mapper is successful.

[Mike Bayer]

You missed a warning which states:

   util.warn("mapper %s creating an alias for the given "
                        "selectable.  References to the original selectable "
                        "may be misinterpreted by queries, polymorphic_on, etc. "
                        " Consider passing an explicit selectable.alias() construct instead." % self)

the only rationale for the alias() is that databases like PG and MySQL require it. so removed the ambiguity here by raising the warning to an error in cf6c66e70ea406a27c4a8d5b79f9c629a62320fc.

[Mike Bayer]

Removing milestone: 0.6.0 (automated comment)