Thanks a ton for your responses. Do all the normal columns of an aliased class need to match the ad-hoc select to which I map the alias?
> oh if the class doesn't have a mapper, then defintiely, just make ad-hoc subclasses of it and map to those. vastly easier that way. Mapping to a subclass would be different from mapping directly to an ad-hoc class? (Mostly I’m concerned whether the very act of adding a mapper to a class will keep the reference and prevent its garbage collection or in some other way modify the “main” mappers, especially if this is done via a thread. Like, would that modify the compiled mappers for the entire process... these are the things running through my head.) On Tue, Jan 26, 2021 at 8:18 PM Mike Bayer <mike...@zzzcomputing.com> wrote: > > > On Tue, Jan 26, 2021, at 7:31 PM, Kent Bower wrote: > > I should have given these details from the get-go: the use case is a > specialized select() (dynamically built) which would be extremely > convenient to map relationships against for convenience in subquery > loading, etc. So, the class would not already have a mapper. Can I pass > non_primary=True anyway, or won’t this work? > > > oh if the class doesn't have a mapper, then defintiely, just make ad-hoc > subclasses of it and map to those. vastly easier that way. > > > > > > On Tue, Jan 26, 2021 at 6:18 PM Mike Bayer <mike...@zzzcomputing.com> > wrote: > > > > > On Tue, Jan 26, 2021, at 3:16 PM, Kent wrote: > > Question: if I add a mapper to a class that is only needed temporarily, > does using the mapper compile it along side my "normal" mappers such that > I'll leak memory when I mean for the class to be garbage collected? > > Put another way, can I add a mapper to a class that doesn't influence my > "main mappers" and gets thrown away when the class is thrown away or is > that not possible? > > > Assuming the class already has a mapper, and this is an additional mapper > that maps the class in a different way, this feature is called a > "non-primary mapper" and requires that you pass the non_primary=True flag. > If you are using non-primary mappers, then yes they get garbage collected. > > This is also a deprecated feature that won't be in SQLAlchemy 2.0. The > only use case we can identify for "non-primary mappers" is when you want to > create a relationship() to one, and for that use case we now support the > "Relationship to AliasedClass" pattern. > > For anything else involving retrieving a class from an alternate > selectable of some kind, you should be able to use AliasedClass for, e.g. > sqlalchemy.orm.aliased(), so I would recommend using that instead unless > you are on a very old version of SQLAlchemy. > > > > > > > > > Thanks in advance, > Kent > > > -- > SQLAlchemy - > The Python SQL Toolkit and Object Relational Mapper > > http://www.sqlalchemy.org/ > > To post example code, please provide an MCVE: Minimal, Complete, and > Verifiable Example. 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