OK there's various small issues here but overall SQLAlchemy's new "nesting"
feature does not seem to work correctly yet for a RECURSIVE CTE with UNION,
meaning it's not very useful for RECURSIVE right now.
But here's the thing, I'm not as SQL expert as some people think but I don't
think it should be necessary for a CTE to actually be embedded in a subquery.
You can keep the CTE on top, where it can more efficiently create a set of all
rows, and then refer to it within a normal correlated subquery that's in the
column_property().
ive written the below MCVE we can work with going forward. see how close this
is, but the goal is, keep the CTE on top as it normally can be and do
correlations outside of it.
from sqlalchemy import Column
from sqlalchemy import ForeignKey
from sqlalchemy import func
from sqlalchemy import Integer
from sqlalchemy import select
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import backref
from sqlalchemy.orm import column_property
from sqlalchemy.orm import relationship
from sqlalchemy.orm import Session
Base = declarative_base()
class Staff(Base):
__tablename__ = "staff"
id = Column(Integer, primary_key=True)
selfreferencing_staff_id = Column(
Integer,
ForeignKey("staff.id", onupdate="CASCADE", ondelete="SET NULL"),
)
_enabling_factor = Column(Integer)
record_id = 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.label("anchor_id"),
anchor_s.c._enabling_factor,
anchor_s.c.selfreferencing_staff_id,
]
).select_from(anchor_s)
# this part we are going to move to the outside
# .where(anchor_s.c.id == Staff.id)
.cte(name="lineage_nodes", recursive=True)
)
recursive_cte_a = recursive_cte.alias()
lineage_nodes = recursive_cte.union_all(
select([s.c.id, s.c._enabling_factor, s.c.selfreferencing_staff_id])
.join(
recursive_cte_a, recursive_cte_a.c.selfreferencing_staff_id == s.c.id
)
.filter(recursive_cte_a.c._enabling_factor == None)
)
top_id = (
select(func.min(lineage_nodes.c.anchor_id))
# here's where we moved it
.where(lineage_nodes.c.anchor_id == Staff.id)
.correlate(Staff)
.scalar_subquery()
)
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.
s = Session()
q = s.query(Staff)
print(q)
On Sun, Sep 26, 2021, at 2:56 AM, niuji...@gmail.com wrote:
>
> 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, niuji...@gmail.com 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, 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/
>>>>>
>>>>> To post example code, please provide an MCVE: Minimal, Complete, and
>>>>> Verifiable Example. See http://stackoverflow.com/help/mcve for a full
>>>>> description.
>>>>> ---
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>>>>>
>>>>> <https://groups.google.com/d/msgid/sqlalchemy/a5e26719-3e5e-4f6e-92fc-c3319ad3ec6fn%40googlegroups.com?utm_medium=email&utm_source=footer>.
>>>>
>>>
>>>
>>> --
>>> 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
>>> description.
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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 Example. See http://stackoverflow.com/help/mcve for a full
> description.
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--
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http://www.sqlalchemy.org/
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