@@ -0,0 +1,14 @@
+---
+'@objectstack/objectql': minor
+---
+
+A `validate()` preview binds the master-detail header the write binds, so an import dry run of a detail row whose field rule reads `parent` admits the rows the import admits (#22474)
+
+Clause-②: yes
+
+The preview's accept set widens: rows it refused, and the write admits, are now admitted. No key, export or error code is added, removed or renamed.
+
+- **Before.** A preview (`engine.validate(object, rows, { mode })`, `validateData`, and the import dry run) bound no master-detail header. A field `requiredWhen` that reads the header, such as `parent.status == 'sent'` on an invoice line, faulted there and the preview refused the row with `rule_violation` / `unevaluable`, in both modes, although the insert and the update resolve the header and admit it under a draft header.
+- **Now.** The preview binds `parent` as the write binds it: from the row's master-detail reference on an insert, and on an update from the reference the patch carries, else from the stored row the row's `id` names. An update that moves the row to another header also binds the header the stored row hangs off, which the write reads to decide whether the stored row already violated the rule. So a row under a draft header is admitted and a row that leaves the field empty under a sent header is refused with `required`, as the write refuses it. Only an object that declares a field `requiredWhen` reading `parent` reads a header, as on the write.
+- **Read under the caller's access.** The write reads the header with system access, kept to the caller's organization. The preview first reads it through the engine's read door, under the caller's own context: a header the caller cannot read gets the verdict a missing header gets. A header column is judged only where the read door served this caller that very value. A column the read door hid, or served transformed (a partially masked field, a masked secret), is judged as empty, so the verdict never depends on a header value the caller could not read in full. For such a caller the preview can therefore answer differently from the write, which judges the stored header.
+- **Unchanged.** An object with no field `requiredWhen` reading `parent` reads no header and gets the verdict it got before. The preview still runs no `readonlyWhen` strip, so an update whose `readonlyWhen` would keep the row on its stored header is judged against the header the patch names.
@@ -3207,18 +3207,20 @@ export interface EngineReadOptions {
   context?: ExecutionContext;
 }
 
-/** Merge read-path execution context from the query and the trailing options. */
 /**
  * [#22445] Did the read door serve this value AS STORED — can the caller read
  * the stored value in full? The `update`-mode preview keeps a stored column
- * only when this holds for it (see `storedRows` in `ObjectQL.validate`).
+ * only when this holds for it (see `storedRows` in `ObjectQL.validate`), and
+ * [#22474] the preview keeps a master-detail header column only when this
+ * holds for it (see `ObjectQL.resolvePreviewParents`).
  *
  * Both values come off the same driver read shape (`driver.findOne` under the
- * same `buildDriverOptions`), so a column nothing transformed compares equal
- * by construction; what differs is what the read path REWROTE for this caller
- * (a partial mask, a masked secret, a read hook's rewrite). Structural, not
- * identity: the two reads are two driver calls, so a `Date`, an array or a
- * JSON object arrives as two equal objects.
+ * same `buildDriverOptions`; for a header, the read door's `find` under the
+ * caller's context and under the write's elevated one), so a column nothing
+ * transformed compares equal by construction; what differs is what the read
+ * path REWROTE for this caller (a partial mask, a masked secret, a read hook's
+ * rewrite). Structural, not identity: the two reads are two driver calls, so a
+ * `Date`, an array or a JSON object arrives as two equal objects.
  *
  * ⛔ Fails CLOSED: a pair it cannot prove equal — another object kind (a
  * buffer, a class instance), a nesting past the depth bound — answers `false`,
@@ -3247,6 +3249,7 @@ function servedAsStored(served: unknown, stored: unknown, depth = 0): boolean {
     && servedKeys.every((k) => Object.prototype.hasOwnProperty.call(stored, k) && servedAsStored(served[k], stored[k], depth + 1));
 }
 
+/** Merge read-path execution context from the query and the trailing options. */
 function mergeReadContext(
   fromQuery?: ExecutionContext,
   fromOptions?: ExecutionContext,
@@ -8718,6 +8721,106 @@ export class ObjectQL implements IObjectQLEngine {
     };
   }
 
+  /**
+   * [#22474] The master-detail headers a `validate()` preview binds, per row:
+   * `parents[i]` is the header row `i` is judged under (`parent`), and
+   * `previousParents[i]` the header its stored row hangs off when the row
+   * REPOINTS it (`previousParent`, for the ADR-0113 pre-check), else
+   * `undefined`, which the evaluator reads as "the same header", as the
+   * write's own `undefined` is read.
+   *
+   * The header ids are read the way the write reads them ({@link masterIdOf}):
+   * `views[i]` is the judged view, the row on an insert and the stored row
+   * merged with the patch on an update, so the patch's FK wins over the stored
+   * one; `storedRows[i]` names the header a repoint leaves.
+   *
+   * ## Read under the caller's access, not elevated
+   *
+   * The write reads the header ELEVATED ({@link resolveMasterDetailParents}):
+   * the caller's right to write the detail was settled by the write's gates
+   * before the header is read. The preview runs none of those gates, so a
+   * header read elevated here would answer a predicate over a row the caller
+   * may not read. Two reads, each answering one question, as the preview's
+   * stored-row read does:
+   *
+   *  1. ⛔ MAY THIS CALLER SEE THE HEADER — the READ DOOR, under the caller's
+   *     own context, one `find` for the whole batch. A header it does not
+   *     return binds as a missing header binds: `null`, so a predicate that
+   *     reads `parent` refuses as unevaluable, whatever the header holds. A
+   *     failed read binds every header as missing, as the write's own failed
+   *     header read does.
+   *  2. WHAT THE WRITE WOULD JUDGE — the write's own header read
+   *     ({@link resolveMasterDetailParents}), for the headers read 1 returned
+   *     only.
+   *
+   * ⛔ A header column is kept only where the read door served this caller
+   * that very value ({@link servedAsStored}); every other column, hidden or
+   * served transformed (a partial mask keeps the key and replaces the value),
+   * is judged as empty: the header is materialised over the master's declared
+   * fields after the columns are kept, so such a column reads `null`. So the
+   * verdict never depends on a header value the caller could not have read in
+   * full.
+   */
+  private async resolvePreviewParents(
+    schema: unknown,
+    views: ReadonlyArray<Record<string, unknown>>,
+    storedRows: ReadonlyArray<Record<string, unknown> | undefined>,
+    context: ExecutionContext | undefined,
+  ): Promise<{
+    parents: Array<Record<string, unknown> | null>;
+    previousParents: Array<Record<string, unknown> | null | undefined>;
+  }> {
+    const rel = resolveMasterDetailRelation(schema as any);
+    const landingIds = views.map((view) => (rel ? masterIdOf(rel.fk, null, view) : undefined));
+    const priorIds = storedRows.map((stored) => (rel && stored ? masterIdOf(rel.fk, null, stored) : undefined));
+    // The write resolves a second header only for a repoint (`repointsMaster`).
+    const repoints = priorIds.map((prior, i) => prior != null && landingIds[i] !== prior);
+    const ids = new Set<string>();
+    landingIds.forEach((id) => { if (id != null) ids.add(String(id)); });
+    priorIds.forEach((id, i) => { if (repoints[i]) ids.add(String(id)); });
+
+    const headers = new Map<string, Record<string, unknown>>();
+    if (rel && ids.size > 0) {
+      // 1. The read door, under the caller's own context.
+      const served = new Map<string, Record<string, unknown>>();
+      try {
+        const rows = await this.find(rel.master, {
+          where: { id: { $in: [...ids] } },
+          context,
+        } as EngineQueryOptions) as Array<Record<string, unknown>>;
+        for (const row of Array.isArray(rows) ? rows : []) {
+          if (row?.id != null && ids.has(String(row.id))) served.set(String(row.id), row);
+        }
+      } catch (err) {
+        this.logger?.warn?.('validate preview: the caller\'s read of the master-detail header failed — parent stays unbound', {
+          object: rel.master, error: err,
+        });
+      }
+      // 2. The write's own header read, for the headers the caller can read.
+      if (served.size > 0) {
+        const asWritten = await this.resolveMasterDetailParents(
+          schema, null, [...served.keys()].map((id) => ({ [rel.fk]: id })), context,
+        );
+        for (const [id, servedRow] of served) {
+          const header = asWritten({ [rel.fk]: id });
+          if (!header) continue;
+          const kept: Record<string, unknown> = {};
+          for (const [key, value] of Object.entries(header)) {
+            if (Object.prototype.hasOwnProperty.call(servedRow, key) && servedAsStored(servedRow[key], value)) {
+              kept[key] = value;
+            }
+          }
+          headers.set(id, this.materializeParentHeader(rel.master, kept));
+        }
+      }
+    }
+    const headerOf = (id: string | number | undefined) => (id == null ? null : (headers.get(String(id)) ?? null));
+    return {
+      parents: landingIds.map(headerOf),
+      previousParents: priorIds.map((id, i) => (repoints[i] ? headerOf(id) : undefined)),
+    };
+  }
+
   /**
    * [#18682] Resolve the related rows this object's PREDICATE rules read one
    * hop through a reference field, for a whole batch of rows at once.
@@ -13046,10 +13149,24 @@ export class ObjectQL implements IObjectQLEngine {
    * refusal it is. A row with no address, or whose id names no row this
    * caller can read, is judged on the patch alone, as before.
    *
+   * ## The master-detail header is bound as the write binds it (#22474)
+   *
+   * Every write binds the detail row's master-detail header for a field
+   * `requiredWhen` that reads `parent`, and an update that repoints the row
+   * binds the header it leaves as `previousParent` for the ADR-0113
+   * pre-check. A preview that bound neither refused, as unevaluable, rows the
+   * write admits. So the preview binds both, from the same view and under the
+   * same gate as the write (an object with no parent-scoped `requiredWhen`
+   * reads no header). ⛔ Unlike the write, it reads the header under the
+   * caller's own access ({@link resolvePreviewParents}): a header the caller
+   * cannot read binds as a missing one, and a header column the read door
+   * served transformed or not at all is judged as empty.
+   *
    * Nothing is written and no sequence is consumed. Reads do happen: the
-   * stored row above, and the related rows a traversing rule names (see the
-   * `previewRelatedForRow` block below). Validation itself is in-process,
-   * which is what makes row-by-row dry run of a large import affordable.
+   * stored row above, the master-detail headers, and the related rows a
+   * traversing rule names (see the `previewRelatedForRow` block below).
+   * Validation itself is in-process, which is what makes row-by-row dry run
+   * of a large import affordable.
    */
   async validate(
     object: string,
@@ -13090,7 +13207,10 @@ export class ObjectQL implements IObjectQLEngine {
     // (see the `previewRelatedForRow` block below), and [#22445] an
     // `update`-mode row that carries its address issues two READS of the
     // stored row — the read door under the caller's own context, then the
-    // write's own prior-row read (the `storedRows` block below). Nothing is
+    // write's own prior-row read (the `storedRows` block below), and [#22474]
+    // an object with a parent-scoped `requiredWhen` issues two READS of the
+    // master-detail headers its rows name, the same two doors in the same
+    // order (the `previewParents` block below). Nothing is
     // written, no write hook runs (a read through the read door fires its read
     // hooks, as the related read always has), and the
     // RELATED read happens only for a caller who passes the arms the
@@ -13310,6 +13430,30 @@ export class ObjectQL implements IObjectQLEngine {
     // stored row.
     const judgedViews = rows.map((row, i) => (storedRows[i] ? { ...storedRows[i], ...row } : row));
 
+    // [#22474] THE MASTER-DETAIL HEADER a field `requiredWhen` reads as
+    // `parent`, bound as the write binds it and under the write's own gate:
+    // only an object that declares a parent-scoped `requiredWhen` reads a
+    // header (`hasParentScopedRequiredWhen`, the insert's gate and the
+    // update's `requiredWhen` half), so every other object's preview reads
+    // none. The header is named by the judged view, the write's
+    // payload-FK-first reading (`masterIdOf`): the row's own FK on an insert,
+    // and on an update the patch's FK, else the stored row's. A repointing
+    // update also binds the header the stored row hangs off as
+    // `previousParent`, the header the write's ADR-0113 pre-check reads.
+    //
+    // ⛔ Read under the caller's access, unlike the write, which reads the
+    // header elevated: see {@link resolvePreviewParents}. A header the caller
+    // cannot read binds as a missing header binds, and a header column the
+    // read door did not serve this caller as stored is judged as empty.
+    //
+    // Named limit: the update's `readonlyWhen` judgement of the FK itself
+    // (`settleMasterDetailLanding` ①) is not run, the limit `readonlyWhen`
+    // already holds in this preview, so a repoint that lock would take back
+    // out is judged against the header it names.
+    const previewParents = hasParentScopedRequiredWhen(schemaForValidation as any)
+      ? await this.resolvePreviewParents(schemaForValidation, judgedViews, storedRows, options?.context)
+      : undefined;
+
     // [#18682] The preview owes the SAME relationship resolution the real write
     // does. Without it a rule that reads one hop through a reference field
     // reports `valid: false` (unevaluable) against a row `insert()` happily
@@ -13391,6 +13535,11 @@ export class ObjectQL implements IObjectQLEngine {
           // update's `previous: priorRecord`. Absent, the record is the patch.
           ...(storedRows[i] ? { previous: storedRows[i] } : {}),
           logger: this.logger, currentUser, skipStateMachine, messages,
+          // [#22474] The header the write binds: the insert's
+          // `insertParentForRow`, the update's landing header and, for a
+          // repoint, `roWhenPreviousParent`.
+          parent: previewParents?.parents[i],
+          previousParent: previewParents?.previousParents[i],
           related: previewRelatedForRow(judgedViews[i]),
           permissions: previewPermissionsFor(row),
         });