implement polymorphic get()-like behavior for concrete inheritance

[Mike Bayer]

currently, get() from the base mapper in concrete inheritance needs to issue a query in all cases; instance keys still use the actual class in their tuple so get() from the base mapper can never identify the object by instance key.

a composite-PK approach needs to be built into concrete such that the polymophic_on column is added to their PK; but also such that get() on query adds this value in automatically when its known.

[Mike Bayer]

this ticket makes no sense at all. what does :

sess.query(SomeBaseClass).get(5)

return ? id #5 could be in any or all of each concrete table. thats just a fact of how this works - with concrete you need to know the type. explicitly putting the polymorphic identity in the get() pk argument is redundant vs. just specifying the Query against the correct subtype.

[Mike Bayer]

Removing milestone: 0.4.xx (automated comment)