This is very helpful.
How to return a list of scalar values in this case? Now the query returns a
list of tuples, and each tuple only has one value, which is what actually
needed. Is there a parameter to return a series of scalar value like this?
On Wednesday, November 10, 2021 at 12:05:27 PM UTC-8 Mike Bayer wrote:
> it has to do with how SQLAlchemy determines the FROM list in a select()
> statement.
>
> if you say select(table.c.id), it knows that "table" is the thing to
> select "from".
>
> however, if you say select(select(...).correlate(...).scalar_subquery()),
> that's assuming it's a SELECT from a scalar subquery, which normally does
> not imply a FROM clause.
>
> In this case, that you are saying correlate(C), we could argue that *does*
> imply a FROM clause of some kind, but SQLAlchemy doesn't make that leap
> right now. You might have said correlate(C, D) or correlate_except(Q) and
> then it's not as clear what the scalar subquery would be selected FROM.
>
> going further, there *could* be some new kind of API where a scalar
> subquery could suggest a FROM clause that should be used when this subquery
> is selected FROM. that would allow your column_property() to be built up
> such that it would be "self sufficient" when queried by itself. but again
> we don't have that right now.
>
> with all those options exhausted, for SQLAlchemy to be given
> select(select().scalar_subquery()), and then to produce "SELECT * FROM
> (SELECT ....) FROM <some table>", that FROM part has to be added on
> explicitly, i.e. select(select().scalar_subuqery()).select_from(<some
> table>).
>
>
>
> On Wed, Nov 10, 2021, at 2:35 PM, niuji...@gmail.com wrote:
>
> This works! Could you explain a little about this differences by using
> select_from here? I think this is very important and useful, really want to
> learn it right.
>
> On Wednesday, November 10, 2021 at 5:55:44 AM UTC-8 Mike Bayer wrote:
>
>
> try calling:
>
> query(C.symbol_from_a).select_from(C)
>
>
>
> On Wed, Nov 10, 2021, at 4:50 AM, niuji...@gmail.com wrote:
>
> class A(Base):
> primary_id = Column(Integer, prirmary_key=True)
> some_A_marker = Column(String)
>
> class B(Base):
> primary_id = Column(Integer, primary_key=True)
> referencing_A_id = Column(Integer, ForeignKey(A.primary_id))
>
> class C(Base):
> primary_id = Column(Integer, primary_key=True)
> basedOn_B_id = Column(Integer, ForeignKey(B.primary_id))
> symbol_from_A = column_property(
>
> sasql.select(A.some_A_marker).select_from(A).join(B).where(B.primary_id ==
> C.basedOn_B_id).correlate(C).scalar_subquery().label("symbol_from_A")
> )
>
>
> When I query this highlighted column_property along with the whole object,
> it works fine:
>
> >>> sess.query(C, C.symbol_from_A).all()
>
> However, when I query this column_property alone, it doesn't work:
>
> >>>sess.query(C.symbol_from_A).distinct().all()
>
> I noticed that somehow the correlated subquery wasn't executed property
> when query that column_property independently.
>
> How to solve this?
>
>
> --
> SQLAlchemy -
> The Python SQL Toolkit and Object Relational Mapper
>
> http://www.sqlalchemy.org/
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>
> http://www.sqlalchemy.org/
>
> To post example code, please provide an MCVE: Minimal, Complete, and
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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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