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

[claude]

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.

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Generated by Claude Code

[os-zhuang]

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。

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Generated by Claude Code

[os-zhuang]

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。

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Generated by Claude Code