The update and delete methods on query are for performance optimization and
potentially for atomicity in some situations, and thus dont assume much
about how the UPDATE should proceed. Adding the discriminator for simple
cases is not that hard to implement but would be backwards incompatible and
also I'm not sure how easy it would be for the feature to work under more
complex edge cases, such as when one seeks to update only the "joined"
table and not the base table.
As Simon pointed out, these caveats are very prominently documented.
On Jan 11, 2018 12:14 PM, "Simon King" <si...@simonking.org.uk> wrote:
> On Thu, Jan 11, 2018 at 4:51 PM, Mischa S <mis...@jetbridge.com> wrote:
> > In [6]: ClassificationTask.query.update(values={'completed': True})
> > INFO [sqlalchemy.engine.base.Engine] base.py:679 BEGIN (implicit)
> > INFO [sqlalchemy.engine.base.Engine] base.py:1140 UPDATE
> schwartz_task
> > SET completed=%(completed)s
> > INFO [sqlalchemy.engine.base.Engine] base.py:1143 {'completed':
> True}
> > Out[6]: 71
> >
> >
> > In [7]: ClassificationTask.query.count()
> > INFO [sqlalchemy.engine.base.Engine] base.py:1140 SELECT count(*) AS
> > count_1
> > FROM (SELECT [...]
> > FROM schwartz_task
> > WHERE schwartz_task.task_type_id IN (%(task_type_id_1)s)) AS anon_1
> > INFO [sqlalchemy.engine.base.Engine] base.py:1143
> {'task_type_id_1': 10}
> > Out[7]: 14
> >
> > If I have a polymorphic discriminator, it's automatically added as a
> > constraint when doing a SELECT query. When I do an update query, it's not
> > automatically added. This is surprising and dangerous. Is this a bug?
> >
> >
> > (semi-related thread:
> > https://groups.google.com/forum/#!searchin/sqlalchemy/
> polymorphic_on$20join$20mischa%7Csort:date/sqlalchemy/U7RkHnM3R2U/
> 332NjxnLAAAJ)
> >
>
> This is mentioned in the docs, in the "Additional Caveats for bulk
> query updates:
>
> http://docs.sqlalchemy.org/en/latest/orm/query.html#
> sqlalchemy.orm.query.Query.update
>
> * The polymorphic identity WHERE criteria is not included for single-
> or joined- table updates - this must be added manually, even for
> single table inheritance.
>
> So I guess it's not strictly a bug, and there's probably a good reason
> why it's not being done. It might be simply that it's a bit
> complicated to implement so it hasn't been done yet.
>
> Simon
>
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