---
phase: 31-fan-trap-detection
verified: 2026-03-14T19:00:00Z
status: passed
score: 11/11 must-haves verified
re_verification: false
gaps: []
---

# Phase 31: Fan Trap Detection Verification Report

**Phase Goal:** Users receive blocking errors when query structure risks inflating aggregation results due to one-to-many fan-out (user-approved change from original "warnings" to "blocking errors")
**Verified:** 2026-03-14
**Status:** PASSED
**Re-verification:** No — initial verification

---

## Goal Achievement

### Observable Truths

| #  | Truth | Status | Evidence |
|----|-------|--------|----------|
| 1  | Relationships can declare MANY TO ONE, ONE TO ONE, or ONE TO MANY after REFERENCES | VERIFIED | `parse_cardinality_tokens` in `src/body_parser.rs:617` handles all three; 6 parser unit tests pass |
| 2  | Omitting cardinality defaults to ManyToOne | VERIFIED | `parse_cardinality_tokens` returns `Ok(Cardinality::ManyToOne)` for empty token slice; `parse_relationship_without_cardinality_defaults` test passes |
| 3  | Old stored JSON without cardinality field deserializes as ManyToOne (backward compat) | VERIFIED | `#[serde(default)]` on `Join.cardinality`; `old_json_without_cardinality_defaults_to_many_to_one` test passes |
| 4  | Cardinality keywords are case-insensitive | VERIFIED | `parse_cardinality_tokens` calls `.to_ascii_uppercase()`; `parse_relationship_cardinality_case_insensitive` test; sqllogictest Test 9 uses lowercase `many to one` |
| 5  | Query aggregating a metric across a one-to-many boundary is BLOCKED with a descriptive error | VERIFIED | `ExpandError::FanTrap` variant returned from `check_fan_traps`; wired into `expand()` at line 1054; sqllogictest Tests 3, 5, 7 confirm `statement error` with `fan trap detected` |
| 6  | Error message names the specific relationship, metric, and tables involved | VERIFIED | `Display` impl at `src/expand.rs:128-145` includes view, metric, metric_table, dimension, dimension_table, relationship_name; `fan_trap_error_message_format` test asserts all fields present |
| 7  | Query NOT crossing a fan-out boundary succeeds normally | VERIFIED | `fan_trap_many_to_one_safe`, `fan_trap_one_to_one_safe`, `fan_trap_same_table_safe`, `fan_trap_no_joins_safe` unit tests pass; sqllogictest Tests 2, 4, 6, 9 return actual data |
| 8  | Derived metrics that transitively depend on base metrics in fan-out paths are also blocked | VERIFIED | `collect_derived_metric_source_tables` called for metrics with `source_table=None`; `fan_trap_derived_metric_blocked` unit test; sqllogictest Test 8 blocks `avg_revenue` (derived) with `fan trap detected` |
| 9  | Single-table views (no joins) never trigger fan trap detection | VERIFIED | Early return `Ok(())` when `def.joins.is_empty()`; `fan_trap_no_joins_safe` unit test passes |
| 10 | ONE TO ONE relationships never trigger fan trap detection (both directions safe) | VERIFIED | `check_path_up`/`check_path_down` skip `OneToOne` cardinality; `fan_trap_one_to_one_safe` unit test; sqllogictest Test 6 queries both directions on ONE TO ONE view |
| 11 | Transitive fan-out through chains of relationships is detected | VERIFIED | LCA-based path finding walks multi-hop paths; `fan_trap_transitive_chain` unit test with 3-table chain passes |

**Score:** 11/11 truths verified

---

### Required Artifacts

| Artifact | Expected | Status | Details |
|----------|----------|--------|---------|
| `src/model.rs` | `Cardinality` enum with ManyToOne (default), OneToOne, OneToMany; `cardinality` field on `Join` | VERIFIED | `pub enum Cardinality` at line 93; `pub cardinality: Cardinality` at line 144; `#[serde(default, skip_serializing_if = "Cardinality::is_default")]` |
| `src/body_parser.rs` | `parse_cardinality_tokens` helper; modified `parse_single_relationship_entry` | VERIFIED | `fn parse_cardinality_tokens` at line 617; token-split approach at line 740-741; `Cardinality::` used throughout |
| `src/expand.rs` | `FanTrap` variant in `ExpandError`; `check_fan_traps` function wired into `expand()` | VERIFIED | `FanTrap { ... }` variant at line 68; `fn check_fan_traps` at line 738; wired at line 1054 |
| `test/sql/phase31_fan_trap.test` | End-to-end sqllogictest for fan trap scenarios | VERIFIED | 9 test scenarios covering DDL with cardinality, blocking queries, safe queries, derived metrics, ONE TO ONE, case-insensitive; all pass |
| `test/sql/TEST_LIST` | `phase31_fan_trap.test` registered | VERIFIED | `phase31_fan_trap.test` present in TEST_LIST; runs as test 10/10 in sqllogictest |
| `tests/parse_proptest.rs` | Proptests for cardinality keyword variants | VERIFIED | `relationship_cardinality_keyword_variants` and `relationship_no_cardinality_defaults` proptests; both pass |

---

### Key Link Verification

| From | To | Via | Status | Details |
|------|----|-----|--------|---------|
| `src/body_parser.rs` | `src/model.rs` | `Cardinality::` used in `parse_single_relationship_entry` | WIRED | `use crate::model::Cardinality` import; `Cardinality::ManyToOne` etc. referenced in `parse_cardinality_tokens` |
| `src/model.rs` | serde | `#[serde(default)]` + `skip_serializing_if = "Cardinality::is_default"` on `Join.cardinality` | WIRED | Confirmed at line 143; `is_default()` method at line 105 |
| `src/expand.rs` | `src/model.rs` | Reads `Join.cardinality` in `check_fan_traps` | WIRED | `use crate::model::{Cardinality, ...}` at line 5; `j.cardinality` read into `card_map` at line 764 |
| `src/expand.rs` | `src/graph.rs` | `RelationshipGraph::from_definition` for path-finding | WIRED | `use crate::graph::RelationshipGraph` at line 4; called at line 748 inside `check_fan_traps` |
| `src/expand.rs` | `expand()` function | `check_fan_traps` called after `inline_derived_metrics`, before SQL generation | WIRED | Line 1054: `check_fan_traps(view_name, def, &resolved_dims, &resolved_mets)?;` — `?` propagates error, blocking the query |

---

### Requirements Coverage

| Requirement | Source Plan | Description (REQUIREMENTS.md) | Implementation | Status |
|-------------|------------|-------------------------------|----------------|--------|
| FAN-01 | 31-01-PLAN.md | Relationships can optionally declare cardinality type (one_to_one, one_to_many, many_to_one) | `Cardinality` enum on `Join`; parser accepts all three syntaxes; defaults to ManyToOne | SATISFIED |
| FAN-02 | 31-02-PLAN.md | [REQUIREMENTS.md text]: "warns when a metric aggregates across a one-to-many boundary" — [USER DECISION]: changed to BLOCKING ERROR | `ExpandError::FanTrap` returned from `expand()` via `check_fan_traps`; query never produces results when fan trap detected | SATISFIED (with approved deviation: blocking instead of warning) |
| FAN-03 | 31-02-PLAN.md | [REQUIREMENTS.md text]: "fan trap warnings do not block query execution" — [USER DECISION]: inverted to BLOCKING | Fan trap IS a blocking error; query returns error, no results produced | SATISFIED (with approved deviation: inverted semantics, now blocking) |

**Requirement deviation note:** FAN-02 and FAN-03 text in REQUIREMENTS.md describes "warnings" that do not block. The user explicitly changed this to blocking errors during the planning discussion (documented in `31-CONTEXT.md` lines 25-28: "Block the query when a metric aggregates across a one-to-many boundary... This is a hard error, not a warning"). The implementation correctly reflects the user's decision. REQUIREMENTS.md status is marked Complete for both, which is correct given the approved deviation.

---

### Anti-Patterns Found

| File | Pattern | Severity | Impact |
|------|---------|----------|--------|
| None found | — | — | — |

No `TODO`, `FIXME`, `HACK`, `PLACEHOLDER`, `todo!()`, or `unimplemented!()` markers in any of the three modified source files (`src/model.rs`, `src/body_parser.rs`, `src/expand.rs`). No stub implementations or empty returns.

---

### Quality Gate Results

Per `CLAUDE.md` requirements, all three test categories were run:

| Test Category | Command | Result |
|---------------|---------|--------|
| Rust unit + proptest + doc tests | `cargo test` | PASSED — 319 unit + 6 expand + 44 proptest + 5 vector + 1 doc = 411 total, 0 failed |
| SQL logic tests | `just build && just test-sql` | PASSED — 10/10 sqllogictests, including `phase31_fan_trap.test` [10/10] |
| DuckLake CI tests | (part of `just test-all` per SUMMARY) | PASSED — 6 ducklake CI tests (from SUMMARY.md: "6 ducklake CI") |

Phase 31 fan trap tests specifically:
- `fan_trap_one_to_many_blocked` — ok
- `fan_trap_many_to_one_safe` — ok
- `fan_trap_one_to_one_safe` — ok
- `fan_trap_same_table_safe` — ok
- `fan_trap_no_joins_safe` — ok
- `fan_trap_transitive_chain` — ok
- `fan_trap_derived_metric_blocked` — ok
- `fan_trap_error_message_format` — ok

Cardinality model tests:
- `cardinality_serde_roundtrip` — ok
- `join_with_cardinality_roundtrip` — ok
- `old_json_without_cardinality_defaults_to_many_to_one` — ok
- `definition_with_cardinality_joins_roundtrips` — ok

Parser tests:
- `parse_relationship_with_many_to_one` — ok
- `parse_relationship_with_one_to_one` — ok
- `parse_relationship_with_one_to_many` — ok
- `parse_relationship_without_cardinality_defaults` — ok
- `parse_relationship_cardinality_case_insensitive` — ok
- `parse_relationship_invalid_cardinality_rejected` — ok
- `parse_relationship_to_alias_not_polluted_by_cardinality` — ok

Proptests:
- `relationship_cardinality_keyword_variants` — ok
- `relationship_no_cardinality_defaults` — ok

---

### Human Verification Required

None — all behaviors are verifiable programmatically through unit tests and sqllogictest integration tests.

---

## Summary

Phase 31 goal is fully achieved. The codebase correctly implements blocking fan trap detection:

1. **Cardinality model (Plan 01):** `Cardinality` enum (ManyToOne/OneToOne/OneToMany) on `Join` struct with backward-compatible serde defaults. Parser correctly splits `to_alias` from cardinality keyword tokens after `REFERENCES`, case-insensitive. Invalid cardinality sequences rejected with clear error.

2. **Fan trap detection (Plan 02):** `ExpandError::FanTrap` variant with descriptive message naming view, metric, metric table, dimension, dimension table, and relationship. `check_fan_traps()` implements LCA-based tree path-finding, checking each edge for fan-out direction based on cardinality. Wired into `expand()` as a blocking gate before SQL generation. Derived metrics handled transitively. Single-table and ONE-TO-ONE cases correctly bypass detection.

3. **End-to-end coverage:** 9 sqllogictest scenarios confirm real DuckDB queries are blocked or succeed correctly through the full extension load pipeline.

The user-approved deviation from REQUIREMENTS.md (blocking errors instead of warnings for FAN-02/FAN-03) is intentional, documented in `31-CONTEXT.md`, and correctly implemented.

---

_Verified: 2026-03-14T19:00:00Z_
_Verifier: Claude (gsd-verifier)_
