query update()/delete() pass to session.execute() without mapper

[Andrey]

Maybe not strictly a bug but it led to very buggy/unexpected results.

When query is getting connection it passes a mapper:

        conn = self._connection_from_session(
                        mapper=self._mapper_zero_or_none(),
                        clause=querycontext.statement,
                        close_with_result=True)

But delete (BulkDelete) does not:

        self.result = self.query.session.execute(delete_stmt,
                                    params=self.query._params)

We have a table that is not in binds list (session.__binds), so for the same table query works (because of mapper) but delete not and it was very hard to figure out what is the difference. It would be much better if delete used the same mechanism as query.

[Mike Bayer]

• The primary :class:.Mapper of a :class:.Query is now passed to the :meth:.Session.get_bind method when calling upon :meth:.Query.count, :meth:.Query.update, :meth:.Query.delete, as well as queries against mapped columns, :obj:.column_property objects, and SQL functions and expressions derived from mapped columns. This allows sessions that rely upon either customized :meth:.Session.get_bind schemes or "bound" metadata to work in all relevant cases. fixes #3227 fixes #3242 fixes #1326

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