fix(core): reject collapsed relationship joins

[primorLee]

Summary

• Detect different DatasetLink values when calculated-column graphs merge the same model pair.
• Return a planning error containing both join conditions instead of silently keeping whichever relationship was merged first.
• Preserve deduplication when the incoming relationship is genuinely identical.

This is the short-term containment described in #2688. It prevents plausible-but-wrong SQL while full first-class relationship aliases remain a larger architectural change.

What failure does this repair?

The regression test builds bill-to and ship-to edges from orders to addresses with different join conditions, then merges the two graphs.

Before this change:

cargo test -p wren-semantic-core --lib rejects_distinct_relationships_between_the_same_models -- --nocapture

failed with:

distinct relationships must not collapse into one join: ()

The unit value shows merge_graph returned Ok and silently discarded the incoming ship-to edge. In the role-playing-dimension query from #2688, that causes both calculated cities to use the bill-to join.

After this change, the same graph merge returns a plan error that names both conflicting conditions.

How is it tested?

• Targeted regression: 1 passed.
• Full core library suite: 153 passed, 0 failed.
• cargo clippy -p wren-semantic-core --all-targets -- -D warnings: passed.
• The test is a normal core library unit test and runs in the default Rust test target.

Duplicate check

Searched open PRs for #2688, relationship handles, and distinct join aliases; the issue still has no linked PR. #976 was also reviewed and is an unrelated, service-only semantics-enrichment alias change that does not touch wren-core or relationship planning.

Addresses #2688.

Summary by CodeRabbit

• Bug Fixes
  • Improved query planning when multiple relationships connect the same datasets.
  • Distinct join conditions are now preserved and reported as planning errors instead of being silently ignored.

[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 Plus

Run ID: 49dde44a-a85c-439e-9ec8-c4f009604fd2

📥 Commits

Reviewing files that changed from the base of the PR and between f2841bc and 5eed46a.

📒 Files selected for processing (1)

• core/wren-core/core/src/logical_plan/analyze/plan.rs

Included review availability: Your plan provides up to 10 included reviews per hour; 9 remain after this review.

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Walkthrough

merge_graph now preserves identical relationship edges and reports conflicting relationships between the same dataset nodes. A regression test verifies that the error includes both distinct join conditions.

Changes

Relationship merge validation

Layer / File(s)	Summary
Detect and test conflicting dataset relationships core/wren-core/core/src/logical_plan/analyze/plan.rs	merge_graph skips identical relationships and returns a planning error for different relationships between the same dataset nodes. A unit test verifies that both join conditions appear in the error.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Merge Risk: ⚪ Minimal · up to 5eed4

This change rejects conflicting relationship joins instead of silently producing potentially incorrect SQL, while preserving identical-relationship deduplication. No actionable merge-blocking risk remains after normal checks and review.

Suggested reviewers: goldmedal

Poem

I’m a rabbit guarding every link,
No hidden joins shall slip or shrink.
Same paths merge with a gentle hop,
Conflicting paths make planning stop.
Two join clues now shine in sight—
Thump, thump, the graph is right!

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the core fix: rejecting collapsed relationship joins.
Description check	✅ Passed	The description includes the required summary, observed failure and output, testing details, and duplicate check.
Docstring Coverage	✅ Passed	Docstring check was indeterminate for this PR — some files could not be analyzed in time. Not blocking.
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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