- joins along a relation() from a mapped

- joins along a relation() from a mapped
class to a mapped subclass, where the mapped
subclass is configured with single table
inheritance, will include an
IN clause which limits the subtypes of the
joined class to those requsted, within the
ON clause of the join.  This takes effect for
eager load joins as well as query.join().
Note that in some scenarios the IN clause will
appear in the WHERE clause of the query
as well since this discrimination has multiple
trigger points.