cant join to a polymorphic mapper that uses straight joins for select_table; cant use aliased joins because PKs get duped

[Mike Bayer]

polymorph.py, should be able to map as:

person_join = people.outerjoin(engineers).outerjoin(managers).outerjoin(boss).select(folded_equivalents=True, use_labels=True).alias()

but this incorrectly gen's a composite PK

when this is fixed, mapper should automatically apply .select(folded_equivalents=True, use_labels=True).alias() to all incoming "joins" used for select_table so that we can join to it. test cases in test/orm/inheritance/query.py

[Mike Bayer]

forget about folded_equivalents, I was wrong about what that does. 9eced72c035a9e0424ca8a77c9f657783a5a94dd gives us a nice new reduce() function which seems to work great and has simpified Mapper as well as Join in determining the minimal set of PK columns.

[Mike Bayer]

Removing milestone: 0.4.3 (automated comment)