Query.union producing query with polymorphic discriminator when not appropriate

[Ollie Rutherfurd]

a.union(b) is unnecessarily including polymorphic discriminator when a and b contain columns from a polymorphic subclass, whereas union(a, b) does not. The result is cartesian product from a.union(b). A demo file is attached.

[Mike Bayer]

Disable single-inheritance critera on the outside of UNION

Fixed bug related to 3177, where a UNION or other set operation emitted by a :class:.Query would apply "single-inheritance" criteria to the outside of the union (also referencing the wrong selectable), even though this criteria is now expected to be already present on the inside subqueries. The single-inheritance criteria is now omitted once union() or another set operation is called against :class:.Query in the same way as :meth:.Query.from_self.

Change-Id: I0fd1331c7ba85a758a1c15e06c271914f2c717f3 Fixes: #3856

→ <<cset 7de0d1785335>>

[Mike Bayer]

thanks for the report

[Ollie Rutherfurd]

Thanks for the quick fix!