Nested subqueries with aliased queries/tables

Describe your question I’m trying to do multi-level nested query, but have some troubles I need to unnest several arrays in json fields and select some of them

Example

• table is check, it has id (int) and response (json) fields
• json example (contents of response column):

{
  "Results": [
    {
      "Name": "FooBar",
      "Field": "SomeValue"
    },
    {
      "Name": "SpamEggs",
      "Field": "AnotherValue"
    }
  ]
}

Here’s simplified SQL example which I’m trying to achieve:

SELECT result_elem -> 'Field' as field
FROM "check" AS check_, json_array_elements(
    (
      SELECT check_inside.response -> 'Results'
      FROM "check" as check_inside
      WHERE check_inside.id = check_.id
    )
) AS result_elem
WHERE result_elem ->> 'Name' = 'FooBar';

at first I’m trying to implement inner subquery

python code:

from sqlalchemy import func
from sqlalchemy.orm import aliased
from sqlalchemy.dialects import postgresql

check_inside = aliased(Check, name="check_inside")
check_ = aliased(Check, name="check_")

q_result_elems = session.query(
    check_inside.response["Results"],
).filter(
    check_inside.id == check_.id,
)
subq_result_elems = q_result_elems.subquery(name="subq_result_elems")

print(str(q_result_elems.statement.compile(compile_kwargs={"literal_binds": True}, dialect=postgresql.dialect())))

prints this:

SELECT check_inside.response -> 'Results' AS anon_1 
FROM "check" AS check_inside, "check" AS check_ 
WHERE check_inside.id = check_.id

it already differs on the FROM line then I’m completely lost. this is kinda pseudo-code, which doesn’t work at all:

q_foobar_fields = session.query(
    subq_result_elems.op("->")("Field").label("field"),
).select_from(
    check_,
    func.json_array_elements(
        subq_result_elems,
    ).label("result_elem")
).filter(
    subq_result_elems.op("->>")("Name") == "FooBar"
)

I tried a bit another way:

# added a label 'results' above and then creating a new query
q_foobar_fields = session.query(
    func.json_array_elements(subq_result_elems.c.results.op("->")("Field"))
).filter(
    subq_result_elems.c.results.op("->>")("Name") == "FooBar",
)

print(str(q_foobar_fields.statement.compile(compile_kwargs={"literal_binds": True}, dialect=postgresql.dialect())))

but it creates this SQL:

SELECT json_array_elements(subq_result_elems.results -> 'Field') AS json_array_elements_1
FROM (SELECT check_inside.response -> 'Results' AS results
      FROM "check" AS check_inside,
           "check" AS check_
      WHERE check_inside.id = check_.id) AS subq_result_elems
WHERE (subq_result_elems.results ->> 'Name') = 'FooBar';

and here ‘FROM’ is different from what I need (and it creates multiple rows for each check item)

How can I pass an aliased subquery and refer it in the ‘select’ part?

Thanks in advance!

Versions

• SQLAlchemy: 1.3.18
• Database: Postgres

Have a nice day!

[zzzeek]

the main thing you’re doing that’s not directly supported is a function with a query inside of it, which then goes into the FROM clause, which is very much a postgresql thing. you can probably find a recipe that can help if you take a look at #3566 which has a whole set of POCs for how to get these unusual PG syntaxes to work. This is stuff that we’d like to add as a feature but it never has gotten priority.

take a look through those and if you can’t get it working, I will add your example above to the list on that issue. basically when the feature gets added, all those comments will serve as test cases.

[mahenzon]

oh, that’s great! yeah, now it’s correct thank you very much!