fix(osi): skip empty expressions when picking a dialect expression

[Bartok9]

What

_pick_expression (OSI → MDL importer) resolves a SQL expression for a field/metric in priority order: preferred dialect → ANSI_SQL → first available. The membership checks used in by_dialect, so a present-but-empty expression for the preferred dialect (or for ANSI_SQL) counted as "found" and was returned verbatim — shadowing a real expression sitting in another dialect entry.

This skips blank expressions at every resolution tier and falls through to the first non-empty expression.

Why it's a bug

An empty string carries no SQL. When the preferred dialect entry is blank, the field is genuinely calculated elsewhere in the dialects list, but the importer would emit it as an empty/identity column — silently dropping the calculation.

Evidence

Before (unpatched) — new tests fail:

test_pick_expression_skips_empty_preferred_and_uses_ansi      FAILED
test_pick_expression_skips_empty_ansi_and_uses_first_nonempty FAILED
  assert "" == "SUM(x)"   /   assert "" == "[x]"

After (patched) — whole OSI suite green:

$ pytest tests/unit/test_osi.py
74 passed

Scope

• core/wren/src/wren/osi.py — _pick_expression skips empty entries per tier (1 change)
• core/wren/tests/unit/test_osi.py — 2 new cases (empty-preferred→ANSI, empty-preferred+empty-ANSI→first non-empty)

Summary by CodeRabbit

• Bug Fixes
  • Improved expression selection so blank dialect-specific values are skipped instead of being treated as valid results.
  • Prevented empty calculated fields from being emitted when the preferred or fallback dialect entry has no expression.
• Tests
  • Added coverage for empty-expression handling and fallback behavior across dialects.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro

Run ID: 39bb4817-69ec-4f09-a0f4-99605649f996

📥 Commits

Reviewing files that changed from the base of the PR and between 0c031d6 and 5d70451.

📒 Files selected for processing (2)

• core/wren/src/wren/osi.py
• core/wren/tests/unit/test_osi.py

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Walkthrough

_pick_expression in osi.py is updated to treat empty-string expression values as absent rather than terminal, continuing to the next non-empty dialect entry. Two unit tests are added to cover the cases where the preferred dialect or both preferred and ANSI_SQL expressions are empty.

Changes

Empty expression fallback fix

Layer / File(s)	Summary
_pick_expression fix and unit tests core/wren/src/wren/osi.py, core/wren/tests/unit/test_osi.py	Empty-string expressions for the preferred dialect and ANSI_SQL are now skipped in favor of the next non-empty dialect entry. Two new tests assert this behavior for the preferred-dialect-empty and both-empty cases.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Poem

A blank expression tried to slip through,
But the rabbit said "Nope, that won't do!"
We skip past the empty, hop to the next,
A non-empty answer now passes the test. 🐇✨

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 20.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: skipping empty dialect expressions when selecting an expression.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[goldmedal]

Thanks @Bartok9 LGTM