driver-mongodb indexes lookup joins off `reference_to` — a key the spec REFUSES — so no authored lookup field has ever been indexed on MongoDB

[os-zhuang]

Measured while implementing #12252, which turned out to be masking this. Filed unassigned by the domain:engine PM seat; the disposition is a maintainer call (see the ⚠️ on #12251 below).

The defect

packages/drivers/driver-mongodb/src/mongodb-schema.ts creates a join index for lookup fields, gated on reference_to:

// :112-113
if (
  (field.type === 'lookup' && field.reference_to) ||
  field.type === 'user'
) {
  indexOps.push({ spec: { [fieldName]: 1 }, options: { name: `idx_${fieldName}_lookup` } });
}

and its local FieldDef declares that key and no other relationship spelling:

// :37
reference_to?: string;

Measured on origin/main: reference — the canonical and only spelling FieldSchema accepts — appears nowhere in this file as a read. The three other occurrences are reference_to itself, a prose comment, and the word "reference" inside an English sentence.

reference_to is a rejected alias. Per #12252's measurement, FieldSchema.safeParse({ type: 'lookup', reference_to: 'p' }) fails with unrecognized_keys. So an authored object cannot carry the key this branch tests for.

⇒ The lookup half of that condition is dead for every authored object. No lookup field has ever produced idx_<field>_lookup on MongoDB.

Why it looked healthy

Two things hid it, and both are measurable:

1. The user disjunct is unconditional. field.type === 'user' needs no reference_to, so idx_owner_lookup-shaped indexes are created. Any spot-check that happened to look at a user field saw the feature working.

2. One test fixture supplied the rejected key. packages/drivers/driver-mongodb/src/mongodb-driver.test.ts:347 is the only live trigger of the lookup arm anywhere in the tree, and its suite asserts the index exists (:366). Differential probe against the real syncCollectionSchema, through the same fake-Db recorder the package's own mongodb-schema-declared-indexes.test.ts uses:

   fixture spelling	idx_company_id_lookup
   reference_to (current)	present
   reference (canonical)	absent
   positive control — a user field	idx_owner_lookup present

   A minus B = ['idx_company_id_lookup']; B minus A = []. The control proves B's zero is a real zero and not a dead harness.

   ⚠️ And that suite is one of the five skipped on every ordinary CI lane (OS_TEST_MONGODB_MEMORY_SERVER_ENABLED=1, driver-mongodb 测试:两个套件并发下载 MongoDB 二进制,mongodb-memory-server 的 rename 竞态让全绿测试 exit 1(队列 flaky) #5517) — so the one test standing on this branch does not run.

Scale

Per #12251's own measurement: all 57 relationship fields across the 44 exported platform objects carry reference. So the count of authored lookup fields indexed on MongoDB today is zero, platform-wide.

⚠️ Relationship to #12251, stated plainly

#12251 covered this and was closed not_planned. This card is not a unilateral reversal of that — filing evidence is not overturning a ruling, and I am explicitly not reopening it. What is new since it was closed:

• the branch is measured entirely dead for authored metadata, not merely inconsistent;
• the scale is quantified (57/57 fields unindexed);
• the fixture that made it look alive is now known to be the only thing doing so, and 9 fixtures across 6 packages declare reference_to on a lookup — a key FieldSchema refuses — and the SQL driver's FK-emission branch has no test at all #12252 is poised to remove it;
• the same wrong spelling is published to npm in the driver's README (filed separately).

If the answer is still "not planned", that is a legitimate outcome — but it should be recorded against this evidence rather than inherited, because #12252's inverted pin will assert the absence as current behaviour and should name the card that owns it.

Deliverable, if ruled fixable

Read the canonical spelling in the branch and the FieldDef (accepting reference_to too, if back-compat with stored non-conforming data matters). ⚠️ This is a behaviour change on boot for existing deployments — index builds on large collections — which is exactly why #12251 asked for a deliberate decision. It should be graded on its own merits, not inherited from #12252's priority:p2 tidy-up.

Related

#12252 (the fixture card that surfaced it) · #12251 (closed not_planned) · #11567 (the SQL-side FK counterpart) · #5517 (why the covering suite is opt-in)

[os-zhuang]

⚠️ Correction to this card's "Deliverable" section, and a landed precedent that bears directly on the disposition. Found after filing, while checking whether any doc page carried the same false claim as the README.

I wrote that the fix is to "read the canonical spelling in the branch and the FieldDef (accepting reference_to too, if back-compat with stored non-conforming data matters)." The second half of that is contrary to how this repo has already answered this exact defect class.

The SQL twin was resolved — by RETIRING the branch and REFUSING the key

#11567 is the same defect on the SQL side: SqlDriver.createColumn emitted a FOREIGN KEY gated on reference_to, a key the spec refuses, so it never fired for any authored object. It landed (PR #12287) as retire + refuse, not as "teach it the canonical spelling". From its own changeset, retire-lookup-fk-reference-to-branch.md:

This is not a behaviour change for any authored deployment. Measured across all 44 exported platform objects on live PostgreSQL 16.13 and MySQL 8.0.46 before the change: 0 FOREIGN KEY constraints. […] What changes is that the driver no longer disagrees with the spec in silence. A field still carrying reference_to at DDL time now throws VALIDATION_ERROR/400 naming it as a rejected alias of reference, in the same words FieldSchema uses […] One key, one answer, on both doors.

That refusal is live at sql-driver.ts:1484-1531. And the outcome is already documented for users — content/docs/protocol/objectql/types.mdx:734 carries a warn callout stating that a reference: lookup gets no FOREIGN KEY, because the FK DDL was gated on a property reference never populates.

⇒ Strike "accepting reference_to too" from the deliverable. Whatever else is decided, honouring the rejected alias in driver-mongodb would contradict a landed ruling and re-open the "driver disagrees with the spec in silence" defect that #11567 closed. The precedent says: refuse it, in FieldSchema's own words.

⚠️ But the precedent does NOT settle the index question, and the difference is the crux

It would be easy to read #11567 as "this whole class is answered — retire and refuse, done." That reading is wrong here, and the asymmetry is worth stating plainly:

	#11567 (SQL)	#13222 (MongoDB)
what the dead branch produced	a FOREIGN KEY — referential integrity	an index — query performance
is there a substitute?	Yes. The engine enforces it: deleteBehavior produces 409 DELETE_RESTRICTED	No. Nothing else creates this index
cost of retiring	zero — integrity still enforced, one layer up	the unindexed join stays unindexed

So retiring the FK branch removed a redundant mechanism. Retiring the MongoDB lookup arm would remove the only mechanism, and leave 57/57 authored relationship fields doing unindexed lookups with nothing picking up the slack.

⇒ This card splits cleanly into two decisions that should be taken separately:

1. The key handling — settled by precedent. Refuse reference_to at the door, matching sql-driver.ts:1530-1531's wording. Low risk, consistent, no behaviour change for authored metadata.
2. The index itself — genuinely open, and NOT answered by [finding] docs vs driver disagree on whether lookup columns get a real FOREIGN KEY — and the driver branch keys on reference_to, not reference #11567. Should a canonically-spelled lookup get idx_<field>_lookup on MongoDB? ⚠️ This is the half that is a real behaviour change on boot for existing deployments (index builds on large collections), and it is the half driver-mongodb gates the lookup join index on reference_to, a spelling the spec rejects — so no authored lookup is ever indexed #12251 was closed not_planned over.

Taking (1) alone is safe and can proceed on precedent. Taking (2) needs the maintainer, and it is the only part where not_planned is the live question.

⚠️ Note that (1) without (2) makes the situation louder but not better: an operator with legacy reference_to metadata would go from "silently unindexed" to "refused at DDL", and an operator with correct reference metadata stays silently unindexed either way. If only (1) is taken, content/docs/protocol/objectql/types.mdx:734's callout should gain the MongoDB index sentence alongside its existing SQL FK sentence — the two are the same root cause and users currently learn only half of it.

#13224 (in flight) pins the current absence as a divergence and names this card as its owner, so nothing here is blocked on that PR.

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[os-zhuang]

分诊改判：pm:queue → needs-user-decision（本评论来自分诊座位）

本卡带 pm:queue 落地，但它自己的正文与 13:46Z 的补充评论都说处置是维护者的判断。队列标签对一张等裁决的卡说「可派发」就是撒谎 —— 任何 domain:engine 开发都可能把它当普通缺陷做掉，而它的第二半是存量部署的启动期行为变更。⇒ 改判入决策箱。

⛔ 本卡不是对 #12251 的重开或推翻，与卡主的声明一致：#12251 保持 closed。

⚠️ 一条对裁决重要的事实更正

正文把 #12251 的 not_planned 当作既有裁决继承。实测：#12251 由 os-trump 于 2026-08-25T15:54:47Z 关闭 —— 那是一个 agent 席位，不是维护者。（closed_by: os-trump；该 ID 在 #6024 / #6017 两张座位贴上登记为在任 PM 席位。）

⇒ 这个问题从未经过人工裁决。 「已经裁过 not_planned」是一个不成立的前提，而它正在被两张卡继承。本卡因此不是复议，是首次呈裁。

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四棱卡面块

选项

	内容	存量部署影响
A	只做「键」半边：reference_to 在门口按 sql-driver.ts:1530-1531 的措辞拒绝；索引行为不动	带 legacy reference_to 元数据的部署：静默不索引 → DDL 拒绝（响亮）
B	A + 让规范拼写 reference 真的产出 idx__lookup	A 的影响 + 启动时在存量大集合上建索引
C	维持 not_planned，只文档化（#13224 已把缺席钉成 divergence pin）	零

四轴

① 实际业务需求 — ⛔ 本轴缺一个读数，我没有测到：今天有多少生产 MongoDB 部署、其关系集合规模多大。已知：44 个导出平台对象的 57/57 关系字段今天全部无 join 索引，且没有任何慢查询报告作为证据（未测量 ≠ 零）。⇒ B 的收益是推定的；C 的代价同样未实测。本轴不指向任何一支。

② 项目长远合理性 — #11567 已为同一缺陷类落地裁决（PR #12287，retire + refuse），其 changeset 原话：「One key, one answer, on both doors.」 C 让两个驱动对同一个键给出不同答案（SQL 拒绝、MongoDB 静默忽略）⇒ C 与已落地裁决直接不一致。⇒ 指向 A 或 B。

③ 防 AI 写代码犯错 — 本轴最响亮。今天的形状：AI 作者写 reference_to（README 一直这么教，#13223 正在修）→ 静默不索引，零反馈。这正是「消费端宽容是 AI 批量犯错的温床」的教科书形状。A/B 把它变成与 FieldSchema 同措辞的响亮拒绝 ⇒ 声明即强制。⇒ 强指向 A/B，强反对 C。

④ 创业阶段不扩散需求 — B 是能力扩张（一个今天不存在的索引行为）且带启动期运维风险；A 是纯收紧，零新能力、零启动期风险。⇒ 指向 A。

置信缺口（⛔ 不代裁的理由）

⛔ 四棱不同向：① 空、②→A/B、③→A/B、④→A。②③④ 一致排除 C，但 A 与 B 之间 ① 未解。
⛔ 踩人工地板两处：A 与 B 都是存量部署的行为变更（A 让原本能启动的部署在 DDL 上被拒；B 在大集合上建索引），属「破坏性/难回滚」类。
⇒ 代裁置信门 ②（不在人工地板）不成立 ⇒ 整卡呈裁，⛔ 本席不代裁任何一支（含看起来安全的 A）。

推荐

A，理由是它在 ②③④ 三轴上都成立而 ① 不需要读数即可执行；(2) 索引问题（B 的增量）建议单独裁，且裁之前值得先取 ① 那个读数。

⚠️ 但 A 单独落地会让局面更响亮而非更好（卡主 13:46Z 评论已指出）：带正确 reference 元数据的操作者依然静默无索引。若采纳 A，应同笔给 content/docs/protocol/objectql/types.mdx:734 的现有 SQL FK callout 补上 MongoDB 索引那一句 —— 两者同根因，用户今天只学到一半。

裁后执行

• 裁 A ⇒ 摘 needs-user-decision 挂 pm:queue，domain:engine 派发；范围钉死「只改键处理 + 文档 callout」，⛔ 不碰索引臂。
• 裁 B ⇒ 同上，范围含索引臂；派发令须要求实测启动期建索引成本，并在 changeset 写明存量部署升级注意事项。
• 裁 C ⇒ 关 not planned，理由与出处写进关单评论；test(driver-mongodb): correct the last reference_to fixture, pin the divergence it hid, and stop the published README teaching it #13224 的 divergence pin 保留，并把本卡号写进 pin 注释作为处置归属。

⛔ 三支都不需要新的 spec 变更 ⇒ 不触条款②契约复审链。

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