Hi Mike,
Yes the SQL code runs as desired, but the Python code doesn't,
unfortunately.
After reading the references you pointed out, my Python code looks like
this:
class Staff(Base):
id = Column(Integer, primary_key=True)
selfreferencing_staff_id = Column(
Integer,
ForeignKey('staff_table.id',
onupdate="CASCADE",
ondelete='SET NULL'))
_enabling_factor = Column(Integer)
team_members = relationship('Staff',
backref=backref(
'supervisor',
remote_side='Staff.id'))
anchor_s = Staff.__table__.alias(name="anchor_s")
s = Staff.__table__.alias(name="s")
final_s = Staff.__table__.alias(name="final_s")
recursive_cte = select([
anchor_s.c.id, anchor_s.c._enabling_factor,
anchor_s.c.selfreferencing_staff_id
]).select_from(anchor_s).where(anchor_s.c.id == Staff.id).cte(
name="lineage_nodes", recursive=True)
lineage_nodes = recursive_cte.union_all(
select([
s.c.id, s.c._enabling_factor, s.c.selfreferencing_staff_id
]).join(recursive_cte, recursive_cte.c.selfreferencing_staff_id
==
s.c.id).filter(recursive_cte.c._enabling_factor == None))
top_id = select(sasql.func.min(lineage_nodes.c.id))
Staff.effective_enabling_factor =
column_property(select(final_s.c._enabling_factor).where(
final_s.c.record_id == top_id).scalar_subquery()) # This is where
I define the desired column_property.
The problems seems to be that this code doesn't generate `correlated
subquery` for each `Staff` row ( as in the SQL code `WHERE anchor_s.id =
outer_s.id`). Could you take a look?
On Saturday, September 25, 2021 at 8:26:46 PM UTC-7 Mike Bayer wrote:
> well __class__.__table__ isn't going to be there inside the class body,
> just to make things simple you will want to add this column_property()
> after the Staff class is fully defined; then you make use of Staff.<col> to
> get at columns.
> https://docs.sqlalchemy.org/en/14/orm/mapped_sql_expr.html#using-column-property
>
> will show how to assign a new column_property to the class after the fact.
>
> next, the query you have is correlating, but i really dont have any idea
> if SQLAlchemy is going to handle a correlated column deep inside of a CTE
> like that. The form you have, with the CTE embedded in the parenthesis,
> is only possible with the most recent SQLAlchemy 1.4.24 release where
> someone contributed a new parameter called "nesting", which means the CTE
> should not be moved to the top level of the SELECT. still, not really
> sure what a CTE will do inside a correlated subquery like that. For an
> example of how to use "nesting" see example four at
> https://docs.sqlalchemy.org/en/14/core/selectable.html?highlight=hascte#sqlalchemy.sql.expression.HasCTE.cte
>
> . for the correlate, when you write out the select() that refers to
> "outer_s", add .correlate(Staff) to it, which means Staff isn't added to
> the FROM list, it's assumed to be on the outside.
>
> give those a try but im not totally sure CTEs work as correlated
> subqueries right now, it's not been tried. I assume you've confirmed this
> query actually runs, im surprised you can even correlate inside of a CTE
> like that.
>
>
>
>
>
> On Sat, Sep 25, 2021, at 12:04 AM, [email protected] wrote:
>
> Hi Mike, thanks for pointing out the direction.
> I've worked out the SQL, but failed when converting to SQLAlchemy
> construct.
>
> My SQL query looks like this:
>
> SELECT id, (
> WITH lineage_nodes (id, _enabling_factor,
> selfreferencing_staff_id) AS
> (
> SELECT anchor_s.id, anchor_s._enabling_factor,
> anchor_s.selfreferencing_staff_id
> FROM staff_table AS anchor_s
> WHERE anchor_s.id = outer_s.id
>
> UNION ALL
>
> SELECT s.id, s._enabling_factor, s.selfreferencing_staff_id
> FROM lineage_nodes AS l
> INNER JOIN staff_table AS s
> ON l.selfreferencing_staff_id = s.id
> WHERE l._enabling_factor IS NULL
> ),
>
> top_node_id (top_id) AS
> (
> SELECT MIN(id) AS top_id FROM lineage_nodes
> )
>
> SELECT staff_table._enabling_factor
> FROM staff_table
> INNER JOIN top_node_id
> ON staff_table.id = top_node_id.top_id
> ) AS effective_enabling_factor
> FROM staff_table AS outer_s;
>
>
>
>
> My Python codes looks like this:
>
>
> class Staff(Base):
> id = Column(Integer, primary_key=True)
> selfreferencing_staff_id = Column(
> Integer,
> ForeignKey('staff_table.id',
> onupdate="CASCADE",
> ondelete='SET NULL'))
> _enabling_factor = Column(Integer)
>
> ## codes below doesn't work:
> anchor_s = __class__.__table__.alias(name="anchor_s")
> s = __class__.__table__.alias(name="s")
>
> recursive_cte = select([
> id, _enabling_factor, selfreferencing_staff_id
> ]).select_from(anchor_s).where(anchor_s.c.id == id).cte(
> name="lineage_nodes", recursive=True)
>
> lineage_nodes = recursive_cte.union_all(
> select([
> s.c.id, s.c._enabling_factor, s.c.selfreferencing_staff_id
> ]).join(recursive_cte,
> recursive_cte.c.selfreferencing_staff_id ==
> s.c.id).filter(recursive_cte.c._enabling_factor == None))
>
> top_id = select(sasql.func.min(lineage_nodes.c.id))
> effective_enabling_factor = column_property(...) # I have trouble
> in this line here.
> ## codes above has a NameError: name '__class__' is not defined
>
> team_members = relationship('Staff',
> backref=backref(
> 'supervisor',
> remote_side='Staff.id'))
> On Friday, September 24, 2021 at 1:00:01 PM UTC-7 Mike Bayer wrote:
>
>
> this is a hefty query to dig in to but column_property() subqueries have
> to be formed in terms of a correlated subquery. So instead of injecting a
> particular primary key into it, you set it to point to the Staff.id column.
>
> correlated subqueries are not terrific performers and the construct can be
> a little bit clumsy in the ORM as well, however, the second example at
> https://docs.sqlalchemy.org/en/14/orm/mapped_sql_expr.html#using-column-property
>
> shows the general idea.
>
> a first step to understanding might be to write out the SQL you think you
> want when you SELECT some Staff rows, where one of the columns in the row
> is the "effective_enabling_factor". that column needs to be written as a
> correlated subquery for it to be compatible with column_property().
>
>
>
> On Fri, Sep 24, 2021, at 1:47 AM, [email protected] wrote:
>
> class Staff(Base):
> id = Column(Integer, primary_key=True)
> selfreferencing_staff_id = Column(
> Integer,
> ForeignKey('staff_table.id',
> onupdate="CASCADE",
> ondelete='SET NULL'))
> _enabling_factor = Column(Integer)
> effective_enabling_factor = column_property(...) # I have trouble in
> this line here.
> team_members = relationship('Staff',
> backref=backref(
> 'supervisor',
> remote_side='Staff.id'))
>
>
> This is a self-referencing lineage. Each staff has one supervisor above
> them. Each staff has a `_enabling_factor`, which can be either a Integer,
> or Null. A staff's `effective_enabling_factor` is either their own
> `_enabling_factor` value, or their supervisor's `effective_enabling_factor`
> if their own is Null.
>
> This seems to be a case to use recursive CTE.
> I can construct the query for a certain staff member, e.g. staff #5:
>
> recursive_cte = select([Staff.id, Staff._enabling_factor,
> Staff.selfreferencing_staff_id]).where(Staff.id==5).cte(recursive=True)
>
> lineage_nodes = recursive_cte.union_all(select([Staff.id,
> Staff._enabling_factor,
> Staff.selfreferencing_staff_id]).join(recursive_cte,
> recursive_cte.c.selfreferencing_staff_id==Staff.record_id).filter(recursive_cte.c._enabling_factor
>
> == None))
>
> marker_carrying_supervisor_id = select(sasql.func.min(lineage_nodes.c.id
> )).scalar_subquery()
>
>
> select(Staff._enabling_factor).where(Staff.id==marker_carrying_supervisor_id)
>
>
> However, I don't see how I can write this recursive CTE as a
> column_property on the `Staff` class. Instead of giving specific primary
> key (e.g. #5), I need to somehow reference current row as the anchor.
>
> 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/
>
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http://www.sqlalchemy.org/
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