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. See http://stackoverflow.com/help/mcve for a full
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--
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
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