I think it's starting to make sense. I did misunderstand exactly when the .expression SQL was used - I had thought even queries for ORM models would use it, but I see it does work as you describe.
My goal is to return a subset of fields and make that query building operation be dynamic and "simple" for downstream services. My trouble seems to be that as I move away from the baseline full "query(MyORMClass)", the relationship configured joins no longer automatically come along, so I have to re-specify them in the query. Thanks for the response, I will consider the column_property alternative - it might be better for our situation. As far as limiting fields returned then though - I think these are my options, if I understand them right: query(ORM_Class) -Returns list of ORM Model instances -The only option that auto-includes all joins defined in ORM Model relationships. -Does not calculate hybrid_property in SQL -Referencing hybrid_property on the returned ORM Model evaluates hybrid_property in python. load_only(ORM_Class.column_attr,... etc) -Returns list of ORM Models (sparse, lazily populated) -Will *NOT* add necessary joins from relationship definitions in Model. Must join manually in query. -Can't ask for hybrid_property in query -Referencing hybrid_property on the returned ORM Model still evaluates hybrid_property in python. query(ORM_Class.column_attr, ORM_Class.hybrid_property) -Returns Tuple -Will *NOT* add necessary joins from relationship definitions in Model. Must join manually in query. -Uses the .expression form of the hybrid_property to SELECT/calculate it in SQL with_entities(ORM_Class.column_attr,... etc) -Returns Tuple -Will *NOT* add necessary joins from relationship definitions in Model. Must join manually in query. -Uses the .expression form of the hybrid_property to SELECT/calculate it in SQL On Monday, June 3, 2019 at 2:21:41 PM UTC-4, Mike Bayer wrote: > > well hybrids don't apply to a load_only operation because they are not > included in the query that emits when the ORM SELECTs for an object. > that is, @hybrid_property.expression gives you a SQL expression, but that's > never included when you say session.query(Task). It's only if you said, > session.query(Task).filter(Task.hybrid_test == 'x'). > > There's a construct called column_property() that *is* included in the > columns when you say query(Task). This is actually the kind of property > that's used for all the regular columns but it also accomodates arbitrary > SQL expressions. to that extent there's some overlap between > column_property and hybrids which is why they are both listed at > https://docs.sqlalchemy.org/en/13/orm/mapped_sql_expr.html. > <https://docs.sqlalchemy.org/en/13/orm/mapped_sql_expr.html> > > let me know if that makes sense. > > On Mon, Jun 3, 2019, at 2:17 PM, Gmoney wrote: > > I should at least clarify that I have @hybrid_test.expression in place for > that property and it works when I just do the base 'get me everything' > query. > > > On Monday, June 3, 2019 at 2:13:17 PM UTC-4, Gmoney wrote: > > I'm trying to use load_only and was able to get a really basic example to > work. My problem is that once I try to load one of my hybrid properties, > it fails. > > This code works for any number of basic Column properties but fails once I > add the hybrid property: > > columns = (Task.msg_id, Task.hybrid_test) > > q = (db.session.query(Task).options(load_only(*columns).lazyload('*'))) > > AttributeError: Neither 'hybrid_property' object nor 'ExprComparator' > object associated with Task.hybrid_test has an attribute 'property' > > > Just wondering if I should be able to get this to work if I keep digging > or post a more thorough code snippet, or is it not possible using this > approach? > > > -- > 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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