AssertionError with CROSS

[juankx-bodo]

PyDough code succeeds with pydough.explain_llm(), but fails with pydough.to_sql().

Error generating SQL:

Traceback (most recent call last):
  File "/home/juankx/svc/PyDough/pydough_test.py", line 182, in <module>
    sql = pydough.to_sql(unqualified_node, session=tester.session)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/juankx/svc/PyDough/pydough/evaluation/evaluate_unqualified.py", line 166, in to_sql
    relational: RelationalRoot = convert_ast_to_relational(
                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/juankx/svc/PyDough/pydough/conversion/relational_converter.py", line 1892, in convert_ast_to_relational
    output: TranslationOutput = rel_translator.rel_translation(
                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/juankx/svc/PyDough/pydough/conversion/relational_converter.py", line 1519, in rel_translation
    context = self.rel_translation(*preceding_hybrid)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/juankx/svc/PyDough/pydough/conversion/relational_converter.py", line 1519, in rel_translation
    context = self.rel_translation(*preceding_hybrid)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/juankx/svc/PyDough/pydough/conversion/relational_converter.py", line 1519, in rel_translation
    context = self.rel_translation(*preceding_hybrid)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  [Previous line repeated 2 more times]
  File "/home/juankx/svc/PyDough/pydough/conversion/relational_converter.py", line 1615, in rel_translation
    result = self.handle_children(result, hybrid, pipeline_idx)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/juankx/svc/PyDough/pydough/conversion/relational_converter.py", line 939, in handle_children
    assert isinstance(key_expr, HybridChildRefExpr)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError

Minimal reproducible example (TPCH):

a = orders.CALCULATE(year=YEAR(order_date)).PARTITION(name="a_years", by=year).CALCULATE(
    year=year, total_a=SUM(orders.total_price)
)
b = orders.CALCULATE(year=YEAR(order_date)).PARTITION(name="b_years", by=year).CALCULATE(
    other_year=year, total_b=COUNT(orders)
)
result = a.CROSS(b).WHERE(year == other_year).CALCULATE(year, total_a, total_b)

Additional Context / Workarounds

The bug can be prevented by doing b.CROSS(a) instead of a.CROSS(b)

TPCH collections used:

  orders
  ├── order_date
  └── total_price

[hadia206]

The problem is that a PARTITION's own key is silently shadowed by an ancestor's term of the same name.

In a, the key stays exposed as year; in b, it's renamed to other_year. So a.CROSS(b) makes b a descendant of a and since a still exposes year, it collides with and shadows b's own year key, which is what triggers the crash.

b.CROSS(a) avoids it not because of CROSS order per se, but because b (now the ancestor) only exposes other_year, not year, so there's nothing for a's own key to collide with.

I'll raise a clear error at the qualification step.
To work around it today, rename year=year to something else (e.g. a_year=year) in a, and update the WHERE/final CALCULATE to use the new name