[finding][drivers] driver-mongodb cannot take a structured GroupByNode at all — the object stringifies into a "[object Object]" $group._id

Found while implementing #6401 (GroupByNode.alias on the SQL faces). Adjacent to it, not created by it. Filed unassigned, recording not claiming — driver-mongodb is inside the #5499 investment freeze.

The finding

GroupByNodeSchema declares a UNION: a bare field name, or a structured { field, dateGranularity?, alias? } node. driver-mongodb can only take the first half.

packages/drivers/driver-mongodb/src/mongodb-aggregation.ts:

groupBy?: string[];                       // :38 — half of the declared union
...
if (opts.groupBy && opts.groupBy.length > 0) {
  for (const field of opts.groupBy) {
    groupId[field] = `$${field}`;         // :66-69
  }
}
...
for (const field of opts.groupBy) {
  project[field] = `$_id.${field}`;       // :85-88 — mirrored in $project
}

A structured node is an object in that loop. groupId[field] stringifies it, so the $group._id key becomes the literal "[object Object]" and its value the literal "$[object Object]" — a field path that matches nothing. The $project stage mirrors the same key.

So the aggregation does not refuse and does not throw: it returns rows grouped by a nonexistent field path, under a column named [object Object].

Why `tsc` never saw it

packages/drivers/driver-mongodb/src/mongodb-driver.ts:512 passes the value through an any cast:

groupBy: (query as any).groupBy,

so the declared GroupByNode[] union never meets that string[] annotation. The annotation is a restatement of the protocol that drifted from it — the same shape #6212 closed for driver-turso's remote transport, which read groupBy as string[] and died on "[object Object]" as an unsafe identifier. That one at least failed loudly; this one answers.

Scope note — this is NOT the `alias` divergence

#6401 converged the three SQL faces onto alias ?? field for the projected column. driver-mongodb is not a fourth face of that divergence: the alias is unreachable here rather than ignored, because the whole structured half of the union is. Fixing alias alone would not help — the node has to be destructured first.

Recorded as a measured DEBT row in scripts/check-driver-conformance.mjs (the driver-mongodb × AGGREGATION_CASES cell) by #6401, so the conformance matrix carries the verdict rather than an omission.

Relationship to #6814

#6814 is the count_distinct disagreement for the same frozen pair — a wrong NUMBER from a lowering that exists. This is a different defect: a declared SHAPE that has no lowering at all. Same package, same freeze, separate cells; filing separately so the freeze is lifted against a measured list rather than one line item.

Verdict

Read from the source; not executed — this package has no server-free aggregation suite for the cell, which is part of the debt (the real-mongod suites are opt-in since #5517, so whatever closes this needs a server-free half like mongodb-filter-logic-translation.test.ts has).

Not fixed here: #5499 freezes the package, and #6401's ruling kept mechanical alignment out of the enforce change on principle — a frozen driver gets an honest row or a mechanical alignment, never a flip.

Related: #6401, #6814, #6212, #5499, ADR-0049.

Generated by Claude Code

Triage state repair: + finding (the title already declared it; the label was missing, which left the card invisible to the queue view AND the findings round — the half-labelled state the sweep disjuncts exist to catch).

Classification: observation-class as filed — the structured-GroupByNode half of the union has no lowering in a package under the [裁决] driver-memory / driver-mongodb 投入冻结 —— 维护者 2026-08-05 口径（跨单锚点） #5499 investment freeze, the defect is recorded as a measured DEBT row in the conformance matrix by [finding][drivers] GroupByNode.alias 只有内存分桶认，三个 SQL 面一律忽略 —— 同一个 aggregate 走下推与走回退拿到不同的列键 #6401, and the filing itself says recording-not-claiming. Grading stays with the findings round; the freeze is the standing hold reason.
Routing: domain:drivers (already on the card) is correct — fix face packages/drivers/driver-mongodb/src/mongodb-aggregation.ts:38 (groupBy?: string[]), :66-69 / :85-88 (the stringifying loops) and mongodb-driver.ts:512 (the as any pass-through) all verified live on origin/main @ 41610f6. Note per single-producer discipline the routing label should have been left to the triage seat; it happens to be right, so it stands.
Dedup: [drivers] driver-memory / driver-mongodb disagree with the SQL family on count_distinct — the two AGGREGATION_CASES cells #6409 left open #6814 (count_distinct, same frozen pair) and [finding][drivers] DriverQuery 收窄之外：aggregate / distinct / analyzeQuery 等驱动自有查询方法仍要求把对象名写两遍 #6212 (driver-turso's loud version of the same shape, closed) are siblings, not duplicates — the filing already delimits them.

本评论来自分诊座位 Routine（#5474 试点），不构成认领。

Generated by Claude Code

Findings triage round: held (finding stays) — first formal grading; the earlier comment was a label-state repair.

Premises re-verified on origin/main @ 3e8e669: mongodb-aggregation.ts:38 still declares groupBy?: string[] (half of the declared union) and mongodb-driver.ts:512 still passes the value through (query as any).groupBy, so a structured GroupByNode still stringifies into a "[object Object]" $group._id. The hold reason is unchanged: #5499 (investment freeze) is still open and standing, and the defect is carried as a measured DEBT row in the conformance matrix (driver-mongodb × AGGREGATION_CASES, per #6401) — the freeze will be lifted against a measured list, which is exactly what this card is a line of.

Restart conditions: #5499 lifts, or the maintainer green-lights the driver-mongodb AGGREGATION_CASES cell — the closing PR needs the structured-node destructure first (alias alone is unreachable), a server-free suite half per #5517, and the ledger-row deletion in the same PR.

本评论来自分诊座位 Routine（#5474 试点），不构成认领。

Generated by Claude Code

Claim: PM loop round 2 (drivers seat, post-unfreeze batch 1)
Session: session_013ZU38vNzLXCscgdFZjKd53
Branch: claude/issue-6850-mongodb-structured-groupby
Worktree: objectstack-issue-6850
Domain: domain:drivers
File surface: packages/drivers/driver-mongodb/src/mongodb-aggregation.ts, packages/drivers/driver-mongodb/src/mongodb-driver.ts (the :512 as any pass-through), a NEW server-free aggregation suite in the same package, scripts/check-driver-conformance.mjs (the driver-mongodb × AGGREGATION_CASES DEBT row), .changeset/* (stop on breach; explain in the report)
Container & model: M, mode:cloud, model: opus
Serial constraints cleared: file-disjoint from the #5346/#5376 dispatch riding in the same batch (filter faces vs aggregation faces — no shared file); hot-file sql-driver.ts untouched; no other in-flight claims in the lane.

Authorization: maintainer unfroze driver-mongodb 2026-08-11 (verbatim on #5499, comment 5249019855). The findings-round grading on this card named its restart condition as "#5499 lifts" — it has. This card rides with the mongodb half of #6814 (same file, same new suite, same conformance cell): PR carries Fixes #6850 + Part of #6814 (the driver-memory half of #6814 stays frozen).

Premises re-verified by the PM on origin/main @ 76d74ecb4: mongodb-aggregation.ts:38 still groupBy?: string[], :142 still $addToSet: fieldRef ?? null, mongodb-driver.ts:512 still (query as any).groupBy. Per the grading comment: the closing PR needs the structured-node destructure first (alias alone is unreachable), a server-free suite half per #5517, and the ledger-row deletion in the same PR.

Generated by Claude Code

os-dev report — #6850 + the driver-mongodb half of #6814

Branch claude/issue-6850-mongodb-structured-groupby pushed; draft PR #7550 open (Fixes #6850, Part of #6814 — deliberately NOT Fixes #6814, whose driver-memory half stays frozen).

Premises: all three held, and the card under-counted

Re-verified on origin/main @ 76d74ecb4 before the first edit:

Premise	Verdict
`mongodb-aggregation.ts:38` declares `groupBy?: string[]`	held
`:66-69` / `:85-88` stringify a structured node	held
`:142` lowers `count_distinct` via `$addToSet: fieldRef ?? null`	held
`mongodb-driver.ts:512` carries `(query as any).groupBy`	held
`check-driver-conformance.mjs` DEBT row, spelled as described	held (at `:446-486` on this tree)

One premise the cards did not carry: count(col) ignored field entirely and emitted { $sum: 1 } for both spellings, so count(stage) answered 6 where AGGREGATION_CASES says 4 — the number count(*) already has. Found by executing the case-set rather than reading the file, and it is mandatory for the cell: neither issue names it. This is the case-set's own stated purpose ("three different numbers over one column") catching the collapse it was built for.

What landed

normalizeGroupBy reads the declared union first (the destructure the grading comment required — alias alone was unreachable). $group._id keys on alias ?? field with the FIELD as value; the bare-string emission is byte-identical and pinned.
count_distinct sizing excludes null, in postProcessAggregation.
count(col) counts non-null values via $cond/$ifNull; count(*) unchanged.
dateGranularity → refused, NOT_IMPLEMENTED / 501, ADR-0112 envelope, message mirroring driver-sql/driver-turso sentence for sentence. A malformed entry → INVALID_QUERY / 400. Both refusal tests assert code and status, not toThrow() alone.
mongodb-driver.ts spells the declared type. Probed that this is a real guard: assigning DriverQuery['groupBy'] to string[] now fails tsc against the full union.
New server-free suite mongodb-aggregation-translation.test.ts; the driver-mongodb × AGGREGATION_CASES DEBT row deleted in the same commit.

Route choices (rejecting the card's own sketch, with reasons)

$ne: null before the $addToSet — rejected as unsound, not merely inferior: as a $match it drops the row from the whole pipeline, so a count(*) or sum() sharing that pipeline silently loses the null rows. Correct only for a single-aggregation pipeline, which the builder cannot assume.
$size of $setDifference against [null] — sound, and better on wire size; needs a $project stage the builder does not emit without groupBy, i.e. a pipeline-shape change nothing here can execute. Recorded in-code as the right move once the cell has a live half.
dateGranularity — measured what the SQL faces lower it to before deciding, per the suggestion. A faithful lowering must emit the engine's bucket LABELS ('2026-01', '2026-Q1', ISO '2026-W03'), publish supports.queryDateGranularity, and be held to date-bucket-parity.test.ts — a card of its own. Refusal chosen; silently ignoring a declared key was never an option.

Gates (local)

driver-mongodb tests 266 passed / 143 skipped (opt-in real-mongod halves) · driver-mongodb typecheck clean · spec typecheck clean · check:driver-conformance OK — 37 covered, 3 DEBT, 0 exempt (cell flipped to ok; driver-memory's row stands) · eslint on all changed files clean · build closure clean. CI: in_progress — not polled; the PM owns convergence.

Honest bound on the coverage this cell now claims

The suite holds the LOWERING to the table. It does not answer "does MongoDB agree?" — $cond / $ifNull / $addToSet are modelled from documentation, not observed, because this environment cannot fetch a mongod binary (proxy 403), and a suite nobody has executed is a claim rather than a check. That bound is written into the suite header, the AGGREGATION_CASES enrolment list and the ledger note rather than left to be discovered.

packages/drivers/driver-memory untouched. The dead string_agg arm untouched.

Generated by Claude Code

ACCEPT — PR #7550, MERGED at f067930 (record of the step-7 review that preceded the ready-flip).

Path surface: 6 files — the four claimed (aggregation source, driver cast, new server-free suite, conformance ledger) + changeset + one UNDECLARED file, packages/spec/src/data/aggregation-conformance.ts, verified comment-only by this seat (DEBT rows struck through, enrolled list updated, freeze paragraph corrected). Cross-seat declaration posted to the spec-surface seat per the UnknownFilterTokenError 漏诊:含非 word 字符的类占位符串({TODAY()}、{current-user-id}、{30 days ago})绕过诊断,原样下发按字面串比较(17.0.0-rc.2) #5586/fix(rest,objectql): the import dry run asks the engine for its verdict instead of predicting it (#4633) #6532 precedent ([PM seat] domain:spec-surface — 🔀 merged into #6017 #6298 comment 5249310176).
Beyond-the-cards find: count(col) ignored field and emitted { $sum: 1 } (6 where the standard says 4) — caught by EXECUTING the case-set, named by neither card, fixed as mandatory for the cell. The case-set doing what it was built for.
Two PM-sketch rejections reviewed and upheld: $ne:null pre-$addToSet rejected as unsound (a $match drops null rows from the whole shared pipeline); dateGranularity refused loudly (NOT_IMPLEMENTED/501, ADR-0112 envelope) rather than implemented or silently dropped — a faithful $dateTrunc lowering is measured as a card of its own (bucket labels + capability record + date-bucket-parity enrollment).
Gate rule honored: suite + DEBT-row deletion in the same PR; coverage bound (lowering-only, no live mongod — proxy blocks the binary) stated in the suite header, enrolment list and ledger note rather than left to be discovered.
CI: 26/26 completed; ESLint and TypeScript Type Check both completed: success read individually before the flip.

Part of #6814 behaved as intended — that card stays open and returns to pm:on-hold for its driver-memory half.

Generated by Claude Code

fix(driver-mongodb): take a structured GroupByNode, and answer count/…

docs(pm-dispatch): the draft-conversion reading splits by queue state…

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[finding][drivers] driver-mongodb cannot take a structured `GroupByNode` at all — the object stringifies into a `"[object Object]"` $group._id #6850

The finding

Why `tsc` never saw it

Scope note — this is NOT the `alias` divergence

Relationship to #6814

Verdict

Premises: all three held, and the card under-counted

What landed

Route choices (rejecting the card's own sketch, with reasons)

Gates (local)

Honest bound on the coverage this cell now claims

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[finding][drivers] driver-mongodb cannot take a structured GroupByNode at all — the object stringifies into a "[object Object]" $group._id #6850

Description

The finding

Why tsc never saw it

Scope note — this is NOT the alias divergence

Relationship to #6814

Verdict

Activity

os-zhuang commented on Aug 9, 2026

os-zhuang commented on Aug 9, 2026

huangyiirene commented on Aug 11, 2026

huangyiirene commented on Aug 11, 2026

Premises: all three held, and the card under-counted

What landed

Route choices (rejecting the card's own sketch, with reasons)

Gates (local)

Honest bound on the coverage this cell now claims

huangyiirene commented on Aug 11, 2026

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[finding][drivers] driver-mongodb cannot take a structured `GroupByNode` at all — the object stringifies into a `"[object Object]"` $group._id #6850

Why `tsc` never saw it

Scope note — this is NOT the `alias` divergence