Hello.
On 31.10.2014 18:33, Simon King wrote: > At a guess, I would say that the Python code "Foo.bar != bar" means > that you are looking for Foo objects that aren't related to bar. > This includes Foos that are related to a *different* Bar instance, > as well as Foos that aren't related to *any* bar instance. In sql > terms, foo.bar_id != bar.id OR foo.bar_id is NULL. > > Whereas in the other instance, you've asked for something more > specific, so that's what SA has given you. Thank you for the explanation. Seems logical. I have just never looked at it this way. The generated SQL has surprised me because in my model: * Foo.bar cannot be NULL by definition and * bar instance (the query parameter used in the filter()) already has an id. Also, the SQL SELECT with an OR condition might confuse the DB query planner to choose a suboptimal plan. I have learned new lesson: do not use SA object references when the ultimate goal is to create a SQL SELECT. Thank you once again to both of you, Ladislav Lenart PS: Sorry for the delay. I was offline during the weekend. >> On 31 Oct 2014, at 16:36, Ladislav Lenart <lenart...@volny.cz> wrote: >> >> Hello. >> >> I have just noticed (by accident) a semantic difference of the not-equal >> operator (!=), depending on the style used. For example a query >> >> session.query(Foo).filter(Foo.bar_id != bar.id) >> >> produces SQL like (expected) >> >> SELECT * >> FROM foo >> WHERE foo.bar_id != %(bar_id_1)s >> >> whereas a query >> >> session.query(Foo).filter(Foo.bar != bar) >> >> produces SQL like (NOT expected) >> >> SELECT * >> FROM foo >> WHERE ( >> foo.bar_id != %(bar_id_1)s >> OR foo.bar_id IS NULL >> ) >> >> I thought that >> >> Foo.bar != bar >> >> is just a shortcut for >> >> Foo.bar_id != bar.id >> >> Is this difference intentional? >> May I know why (i.e. the motivation)? >> >> >> Thanks in advance, >> >> Ladislav Lenart -- You received this message because you are subscribed to the Google Groups "sqlalchemy" group. To unsubscribe from this group and stop receiving emails from it, send an email to sqlalchemy+unsubscr...@googlegroups.com. To post to this group, send email to sqlalchemy@googlegroups.com. Visit this group at http://groups.google.com/group/sqlalchemy. For more options, visit https://groups.google.com/d/optout.
