SQLAlchemy: Referencing labels in SELECT subqueries - python-3.x

Question

I'm trying to figure out how to replicate the below query in SQLAlchemy
SELECT c.company_id AS company_id,
(SELECT policy_id FROM associative_table at WHERE at.company_id = c.company_id) AS policy_id_ref,
(SELECT `default` FROM policy p WHERE p.policy_id = policy_id_ref) AS `default`,
FROM company c;
Note that this is a stripped down, basic example of what I'm really dealing with. The actual schema supports data and relationship versioning that requires the subqueries to include additional conditions, sorting, and limiting, making it impractical (if not impossible) for them to be joins.
The crux of the problem is in how the second subquery relies on policy_id_ref -- the value obtained from the first subquery. In SQLAlchemy, this is effectively what I have now:
ct = aliased(classes.company)
at = aliased(classes.associative_table)
pt = aliased(classes.policy)
policy_id_ref = session.query(at.policy_id).\
filter(at.company_id == ct.company_id).\
label('policy_id_ref')
policy_default = session.query(pt.default).\
filter(pt.id == 'policy_id_ref').\
label('default')
query = session.query(ct.company_id,policy_id_ref,policy_default)
The pull from the "company" table works fine as does the first subquery that retrieves the "policy_id_ref" column. The problem is the second subquery that has to reference that "policy_id_ref" column. I don't know how to write its filter in such a way that it literally renders "policy_id_ref" in the resulting query, to match the label of the first subquery.
Suggestions?
Thanks in advance

Answer 1

You can write your query as
select(
Companies.company_id,
AssociativeTable.policy_id.label('policy_id_ref'),
Policy.default.label('policy_default'),
).select_from(
Companies,
).join(
AssociativeTable,
AssociativeTable.company_id == Companies.company_id,
).join(
Policy,
AssociativeTable.policy_id == Policy.id
)
but in case you need reference to label from subquery => use literal_column
from sqlalchemy import func, select, literal_column
session.query(
func.array_agg(
literal_column('batch_info'),
JSONB
).label('history')
).select_from(
select(
func.jsonb_build_object(
'batch_id', AccountingQueueBatch.id,
'batch_label', AccountingQueueBatch.label,
).label('batch_info')
).select_from(
AccountingQueueBatch,
)
)