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, niuji...@gmail.com 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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