I thought I figured out another clever way, but it's not working working as I thought it might. Thought maybe using ".from_self" I could do the base full automatic ORM Model query to get everything, then do the .from_self to wrap it with an outer query and only return the columns I need.
q = (db.session.query(Task).from_self(*columns_i_want)) Unfortunately though, even though query(Task) alone can figure out the relationships/joins, once it's in this statement with the from_self version, you don't get them anymore. I was so excited - I thought that was definitely going to work. Any chance I'm just missing a step that could make that method work? I see the '.add_entity' option but that didn't seem to help and kind of defeats the purpose of my simplicity goal anyway. On Monday, June 3, 2019 at 3:57:18 PM UTC-4, Gmoney wrote: > > 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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