@@ -0,0 +1,24 @@
+---
+'@objectstack/spec': minor
+'@objectstack/lint': minor
+'@object-ui/app-shell': minor
+'@object-ui/plugin-detail': minor
+'@object-ui/i18n': minor
+---
+
+feat(spec,lint,app-shell)!: a field group's `relatedList` names body tables only — a group naming a side card or a hidden list is refused, and Studio offers body tables only
+
+Clause-②: no (narrowing)
+Metadata-protocol: narrowed
+
+<!-- adr-0087: registered field-group-related-list-side-card-refused -->
+
+**BREAKING** for one shape: an object's `fieldGroups[i].relatedList` that names an inbound relationship which is not a body table of the record page. Ruling B on #27478: whether a related list is a body table is decided once, by the relationship's own keys on the child's FK (ruling B on #27055), and a field group only positions one.
+
+- **Before.** A group's `relatedList` naming any inbound relationship parsed, passed `defineStack` and published, but the record page draws only body tables at a group: a group naming a side card (`task` through a plain lookup, say) drew nothing at the group and the card stayed in the related column, silently. Studio's group inspector offered every inbound relationship.
+- **Now.** One predicate, `isBodyTableRelationship` (exported from `@objectstack/spec/data`), answers "is this inbound relationship a body table": its FK declares `relatedList: 'primary'`, or it is a `master_detail` FK that sets `inlineEdit` and declares no `relatedList`. A group whose `relatedList` names a relationship it does not count is refused where the child's fields are known: `defineStack` (`os validate`, `os build`) with a `STACK_CROSS_REFERENCE_INVALID` issue, and the object publish door (`PUT /api/v1/meta/object`, Studio, MCP) with `422 INVALID_METADATA`, rule `related-list-group-not-body` (`@objectstack/lint`, with an `os explain` entry). The sentence, written by `fieldGroupBodyTableRefusal` on both surfaces, names each FK that is not a body table (a side card, or hidden by `relatedList: false`) and the remedy. A bare child name names every FK of that child, so it is refused when any of them is not a body table, and the remedy points at `object.field` for the one that is. Children the name check leaves unjudged (another package's object) are left unjudged here too. `relatedLists` entries are not judged this way: they order body tables and side cards alike.
+- **The remedy.** Mark the relationship `relatedList: 'primary'` on the child's FK (or, on an inline-edited `master_detail`, remove an explicit `relatedList: true`), name only the body table as `object.field`, or remove the group's `relatedList`. There is no automatic conversion: which edit the author meant is not recorded. The upgrade guide entry is `field-group-related-list-side-card-refused`.
+- **One record page cell moves.** A `master_detail` FK that sets `inlineEdit` and declares `relatedList: true` is a side card, as `relatedList`'s description says ("write true to keep it a side card") and as Studio's relationship inspector writes it. The record page (`@object-ui/app-shell` `RecordDetailView`) used to draw it as a body table, because it kept its own copy of the rule that ignored `relatedList`. It now reads the spec predicate through `deriveRelatedLists`' `isPrimary`, so that list moves to the related column. `isMainContentList` (an internal helper of `@object-ui/app-shell`) is gone; its callers read `isBodyTableRelationship`.
+- **Studio.** The field-group inspector's related-table picker (`@object-ui/app-shell`) offers body tables only, refuses to write any other pick, and shows a stored value that is not a body table with a line saying why the page draws nothing for it (`@object-ui/i18n`, en and zh).
+- **Stored rows and built artifacts.** The object parse does not judge this, so a stored `sys_metadata` object row carrying such a group is not rewritten and still loads and registers unchanged; its record page draws what it drew before, except the one cell above. The next save of that object through the metadata write door is refused (`422`, at `fieldGroups[i].relatedList`) until it is fixed, and so is a write of the child that turns a body table a stored group places into a side card. A built artifact is judged when it is built. Census at the change: no example, platform object, dogfood fixture or HotCRM object (hotcrm `47a0666`) declares a field group `relatedList` at all; deployed metadata was not measured.
+- **`@object-ui/plugin-detail`.** Documentation only: `bodyTablePlacement` and the `isBody` option name the spec predicate.
@@ -315,7 +315,7 @@ const result = ApiMethod.parse(data);
 | **label** | `string` | ✅ | Group display label |
 | **icon** | `string` | optional | Icon name (Lucide/Material) for the group header |
 | **description** | `string` | optional | Optional description shown under the group header |
-| **relatedList** | `string` | optional | Places a related list's body table at this position in the record's details column instead of a section of fields: names one inbound relationship, as a relatedLists entry does (child object, or object.field when that child has more than one relationship to this object). A group with relatedList holds no fields: a field that names it as its group is refused. The name is judged as a relatedLists entry is: refused when its child is in the stack with no inbound lookup / master_detail field, or exists nowhere (by os validate / os build, which leave a sys_ / cloud_ / ai_ name to the runtime, and by a metadata write that names no package); a write into a package stores it, and the record page places nothing for it. relatedList: false on the relationship still hides the table. |
+| **relatedList** | `string` | optional | Places a related list's body table at this position in the record's details column instead of a section of fields: names one inbound relationship, as a relatedLists entry does (child object, or object.field when that child has more than one relationship to this object). A group with relatedList holds no fields: a field that names it as its group is refused. The name is judged as a relatedLists entry is: refused when its child is in the stack with no inbound lookup / master_detail field, or exists nowhere (by os validate / os build, which leave a sys_ / cloud_ / ai_ name to the runtime, and by a metadata write that names no package); a write into a package stores it, and the record page places nothing for it. It names a body table only (a FK declaring relatedList: 'primary', or a master_detail FK that sets inlineEdit and declares no relatedList): a group naming a side card or a list hidden by relatedList: false draws nothing, so it is refused where the child's fields are known (os validate / os build and the metadata write door), naming the FK and the remedy (set relatedList: 'primary' on it, name only the body table as object.field when a bare child name also names a side card, or remove the group's relatedList). |
 | **visibleWhen** | `string \| { dialect: Enum<'cel' \| 'cron' \| 'template'>; source: string; ast?: any; meta?: object }` | optional | Section visibility predicate (CEL) — the whole group (header included) is shown only when TRUE, else hidden (fail-closed). e.g. P`record.type == 'invoice'` |
 | **collapse** | `Enum<'none' \| 'expanded' \| 'collapsed'>` | optional (default: `"none"`) | [ADR-0085] Section collapse behaviour: 'none' (always open, no toggle), 'expanded' (collapsible, starts open), 'collapsed' (collapsible, starts closed). |
 | **defaultExpanded** | `boolean` | optional | [DEPRECATED → collapse] true → 'expanded', false → 'collapsed'. |
@@ -701,7 +701,7 @@ External datasource binding (ADR-0015)
 | **label** | `string` | ✅ | Group display label |
 | **icon** | `string` | optional | Icon name (Lucide/Material) for the group header |
 | **description** | `string` | optional | Optional description shown under the group header |
-| **relatedList** | `string` | optional | Places a related list's body table at this position in the record's details column instead of a section of fields: names one inbound relationship, as a relatedLists entry does (child object, or object.field when that child has more than one relationship to this object). A group with relatedList holds no fields: a field that names it as its group is refused. The name is judged as a relatedLists entry is: refused when its child is in the stack with no inbound lookup / master_detail field, or exists nowhere (by os validate / os build, which leave a sys_ / cloud_ / ai_ name to the runtime, and by a metadata write that names no package); a write into a package stores it, and the record page places nothing for it. relatedList: false on the relationship still hides the table. |
+| **relatedList** | `string` | optional | Places a related list's body table at this position in the record's details column instead of a section of fields: names one inbound relationship, as a relatedLists entry does (child object, or object.field when that child has more than one relationship to this object). A group with relatedList holds no fields: a field that names it as its group is refused. The name is judged as a relatedLists entry is: refused when its child is in the stack with no inbound lookup / master_detail field, or exists nowhere (by os validate / os build, which leave a sys_ / cloud_ / ai_ name to the runtime, and by a metadata write that names no package); a write into a package stores it, and the record page places nothing for it. It names a body table only (a FK declaring relatedList: 'primary', or a master_detail FK that sets inlineEdit and declares no relatedList): a group naming a side card or a list hidden by relatedList: false draws nothing, so it is refused where the child's fields are known (os validate / os build and the metadata write door), naming the FK and the remedy (set relatedList: 'primary' on it, name only the body table as object.field when a bare child name also names a side card, or remove the group's relatedList). |
 | **visibleWhen** | `string \| { dialect: Enum<'cel' \| 'cron' \| 'template'>; source: string; ast?: any; meta?: object }` | optional | Section visibility predicate (CEL) — the whole group (header included) is shown only when TRUE, else hidden (fail-closed). e.g. P`record.type == 'invoice'` |
 | **collapse** | `Enum<'none' \| 'expanded' \| 'collapsed'>` | optional (default: `"none"`) | [ADR-0085] Section collapse behaviour: 'none' (always open, no toggle), 'expanded' (collapsible, starts open), 'collapsed' (collapsible, starts closed). |
 | **defaultExpanded** | `boolean` | optional | [DEPRECATED → collapse] true → 'expanded', false → 'collapsed'. |
 
@@ -586,10 +586,12 @@ export type {
 // [#27069] Related-list names: each `relatedLists` entry and field-group
 // `relatedList` must match an inbound lookup / master_detail relationship.
 // `error`, the verdict `defineStack` gives, and on the runtime publish door,
-// which runs no `defineStack`.
+// which runs no `defineStack`. [#27478] A field group's `relatedList` must
+// name body tables only (`related-list-group-not-body`).
 export {
   validateRelatedListRefs,
   RELATED_LIST_REF_UNKNOWN,
+  RELATED_LIST_GROUP_NOT_BODY,
 } from './validate-related-list-refs.js';
 export type {
   RelatedListRefFinding,
 
@@ -405,6 +405,10 @@ export const REFERENCE_INTEGRITY_RULES: readonly ReferenceIntegrityRule[] = [
   // artifact's scope. MEASURED over the shipped object corpus: the two keys
   // are new and no shipped object declares either, so 0 findings; the door
   // controls in `runtime-gate.object-writes.test.ts` are the lit probe.
+  // [#27478] The same member refuses a field group naming a relationship that
+  // is not a body table (`related-list-group-not-body`), on the same children;
+  // a write of the child that turns a placed body table into a side card is
+  // refused too, the finding being new on the stored parent. Same census: 0.
   {
     name: 'validateRelatedListRefs',
     runtimeTypes: ['flow', 'object'],
 
@@ -3877,6 +3877,27 @@ const RELATED_LIST_REF_UNKNOWN_EXPLANATION: RuleExplanation = {
   ],
 };
 
+const RELATED_LIST_GROUP_NOT_BODY_EXPLANATION: RuleExplanation = {
+  rule: 'related-list-group-not-body',
+  covers: 'which related list a field group may place',
+  paragraphs: [
+    'A field group\'s `relatedList` places one related list\'s table among the field sections of the ' +
+      'record\'s details column. It only POSITIONS a list that is already a body table; it does not make ' +
+      'one. Whether a list is a body table is decided once, by the relationship\'s own FK field on the ' +
+      'child: `relatedList: \'primary\'`, or a `master_detail` FK that sets `inlineEdit` and declares no ' +
+      '`relatedList`. Every other list is a side card in the related column (`relatedList: true` or ' +
+      'absent), or hidden (`relatedList: false`).',
+    'A group naming a side card or a hidden list draws nothing at the group, silently, so it is an ' +
+      'error, the verdict `defineStack` gives on `os validate` and `os build` and the runtime publish ' +
+      'gate gives too. A bare child name names every FK of that child, so it is refused when any of ' +
+      'them is not a body table. The fix is on the relationship (mark it `relatedList: \'primary\'`), ' +
+      'in the name (`object.field` for the one FK that is a body table), or removing the group\'s ' +
+      '`relatedList`.',
+    'Judged on the children this rule can see, the same ones `related-list-ref-unknown` judges: a ' +
+      'child outside the stack (another package\'s object) is not judged here.',
+  ],
+};
+
 // ── Object-name references (`validate-object-references.ts`) ────────────────
 
 /** The sites judged — both object-reference ids. */
@@ -5338,6 +5359,7 @@ export const RULE_EXPLANATIONS: Readonly<Record<string, RuleExplanation>> = Obje
   [NAV_TARGET_UNRESOLVED_EXPLANATION.rule]: NAV_TARGET_UNRESOLVED_EXPLANATION,
   [OBJECT_FIELD_REF_UNKNOWN_EXPLANATION.rule]: OBJECT_FIELD_REF_UNKNOWN_EXPLANATION,
   [RELATED_LIST_REF_UNKNOWN_EXPLANATION.rule]: RELATED_LIST_REF_UNKNOWN_EXPLANATION,
+  [RELATED_LIST_GROUP_NOT_BODY_EXPLANATION.rule]: RELATED_LIST_GROUP_NOT_BODY_EXPLANATION,
   [OBJECT_REFERENCE_UNKNOWN_EXPLANATION.rule]: OBJECT_REFERENCE_UNKNOWN_EXPLANATION,
   [OBJECT_REFERENCE_UNREGISTERED_PLATFORM_EXPLANATION.rule]: OBJECT_REFERENCE_UNREGISTERED_PLATFORM_EXPLANATION,
   [CAPABILITY_REFERENCE_UNKNOWN_EXPLANATION.rule]: CAPABILITY_REFERENCE_UNKNOWN_EXPLANATION,
 
@@ -597,6 +597,7 @@ describe('the object door judges related-list names against the live universe (#
     fields: { title: { type: 'text' }, ...fields },
   });
   const withFk = childWith({ request_id: { type: 'lookup', reference: 'leave_request' } });
+  const withBodyFk = childWith({ request_id: { type: 'lookup', reference: 'leave_request', relatedList: 'primary' } });
   const withoutFk = childWith({});
   const gateWith = (item: unknown, child: unknown) =>
     runRuntimeAuthoringRules({ type: 'object', item, context: { objects: [...STORED, child] } });
@@ -625,7 +626,36 @@ describe('the object door judges related-list names against the live universe (#
     expect(refused.errors.map((e) => [e.rule, e.path])).toEqual([
       ['related-list-ref-unknown', 'objects.leave_request.fieldGroups[0].relatedList'],
     ]);
-    expect(gateWith(cleanObject({ fieldGroups: groups }), withFk).errors).toEqual([]);
+    expect(gateWith(cleanObject({ fieldGroups: groups }), withBodyFk).errors).toEqual([]);
+  });
+
+  // [#27478] A group only positions a BODY TABLE: the same group naming the
+  // stored child through a plain lookup (a side card) is refused; its control
+  // differs by the FK's `relatedList: 'primary'` alone.
+  it('REFUSES a field group naming a side card, and publishes it once the FK is primary', () => {
+    const groups = [{ key: 'lines', label: 'Lines', relatedList: 'leave_task' }];
+    const refused = gateWith(cleanObject({ fieldGroups: groups }), withFk);
+    expect(refused.errors.map((e) => [e.rule, e.path])).toEqual([
+      ['related-list-group-not-body', 'objects.leave_request.fieldGroups[0].relatedList'],
+    ]);
+    expect(refused.errors[0].message).toContain("'leave_task.request_id' is a side card");
+    expect(gateWith(cleanObject({ fieldGroups: groups }), withBodyFk).errors).toEqual([]);
+  });
+
+  // [#27478] The other half of the same verdict: a write of the CHILD that
+  // turns a body table a stored group places into a side card is refused too
+  // (the finding is new on the stored parent). Rewriting the child while it
+  // stays a body table publishes.
+  it('REFUSES a child write that makes a placed body table a side card; keeping it primary publishes', () => {
+    const parent = cleanObject({ fieldGroups: [{ key: 'lines', label: 'Lines', relatedList: 'leave_task' }] });
+    const gateChild = (child: unknown) =>
+      runRuntimeAuthoringRules({ type: 'object', item: child, context: { objects: [parent, withBodyFk] } });
+    const refused = gateChild(childWith({ request_id: { type: 'lookup', reference: 'leave_request', relatedList: true } }));
+    expect(refused.errors.map((e) => [e.rule, e.path])).toEqual([
+      ['related-list-group-not-body', 'objects.leave_request.fieldGroups[0].relatedList'],
+    ]);
+    expect(gateChild(childWith({ request_id: { type: 'lookup', reference: 'leave_request', relatedList: 'primary' }, note: { type: 'text' } })).errors)
+      .toEqual([]);
   });
 
   it('REFUSES a child that exists nowhere: with no closure in force the context is the whole collection', () => {