diff --git a/examples/app-showcase/src/apps/index.ts b/examples/app-showcase/src/apps/index.ts
index 81a13d99c..89939b9eb 100644
--- a/examples/app-showcase/src/apps/index.ts
+++ b/examples/app-showcase/src/apps/index.ts
@@ -23,6 +23,7 @@ export const ShowcaseApp = App.create({
         { id: 'nav_projects', type: 'object', objectName: 'showcase_project', label: 'Projects', icon: 'folder-kanban' },
         { id: 'nav_tasks', type: 'object', objectName: 'showcase_task', label: 'Tasks', icon: 'check-square' },
         { id: 'nav_accounts', type: 'object', objectName: 'showcase_account', label: 'Accounts', icon: 'building' },
+        { id: 'nav_invoices', type: 'object', objectName: 'showcase_invoice', label: 'Invoices', icon: 'receipt' },
         { id: 'nav_teams', type: 'object', objectName: 'showcase_team', label: 'Teams', icon: 'users' },
         { id: 'nav_categories', type: 'object', objectName: 'showcase_category', label: 'Categories', icon: 'list-tree' },
         { id: 'nav_field_zoo', type: 'object', objectName: 'showcase_field_zoo', label: 'Field Zoo', icon: 'shapes' },
diff --git a/examples/app-showcase/src/objects/index.ts b/examples/app-showcase/src/objects/index.ts
index 4ee837ed4..edcbeac80 100644
--- a/examples/app-showcase/src/objects/index.ts
+++ b/examples/app-showcase/src/objects/index.ts
@@ -5,4 +5,5 @@ export { Project } from './project.object.js';
 export { Task } from './task.object.js';
 export { Category } from './category.object.js';
 export { Team, ProjectMembership } from './team.object.js';
+export { Invoice, InvoiceLine } from './invoice.object.js';
 export { FieldZoo } from './field-zoo.object.js';
diff --git a/examples/app-showcase/src/objects/invoice.object.ts b/examples/app-showcase/src/objects/invoice.object.ts
new file mode 100644
index 000000000..0e4746850
--- /dev/null
+++ b/examples/app-showcase/src/objects/invoice.object.ts
@@ -0,0 +1,66 @@
+// Copyright (c) 2025 ObjectStack. Licensed under the Apache-2.0 license.
+
+import { ObjectSchema, Field } from '@objectstack/spec/data';
+
+/**
+ * Invoice + Invoice Line — the canonical master-detail "header + line items"
+ * shape. Unlike project↔task (a task is added to a project over time), an
+ * invoice is meaningless without its lines: you enter the header AND its lines
+ * together, in one atomic transaction. So `invoice_line.invoice` declares
+ * `inlineEdit: 'grid'` — every standard New/Edit Invoice form renders an
+ * editable line-item grid, and the invoice `total` rolls the line amounts up
+ * server-side. This is where inline master-detail entry belongs.
+ */
+export const Invoice = ObjectSchema.create({
+  name: 'showcase_invoice',
+  label: 'Invoice',
+  pluralLabel: 'Invoices',
+  icon: 'receipt',
+  description: 'A customer invoice entered together with its line items.',
+
+  fields: {
+    name: Field.text({ label: 'Invoice Number', required: true, searchable: true, maxLength: 60 }),
+    account: Field.lookup('showcase_account', { label: 'Account', required: true }),
+    status: Field.select({
+      label: 'Status',
+      required: true,
+      options: [
+        { label: 'Draft', value: 'draft', default: true, color: '#94A3B8' },
+        { label: 'Sent', value: 'sent', color: '#3B82F6' },
+        { label: 'Paid', value: 'paid', color: '#10B981' },
+        { label: 'Void', value: 'void', color: '#EF4444' },
+      ],
+    }),
+    issued_on: Field.date({ label: 'Issued On' }),
+    // Roll-up: recomputed server-side as line items are inserted/updated/deleted
+    // (child FK auto-detected: showcase_invoice_line.invoice).
+    total: Field.summary({
+      label: 'Total',
+      summaryOperations: { object: 'showcase_invoice_line', field: 'amount', function: 'sum' },
+    }),
+  },
+});
+
+/** Invoice line item — owned by its invoice, entered inline in the grid. */
+export const InvoiceLine = ObjectSchema.create({
+  name: 'showcase_invoice_line',
+  label: 'Invoice Line',
+  pluralLabel: 'Invoice Lines',
+  icon: 'list',
+  description: 'A single billable line on an invoice.',
+
+  fields: {
+    invoice: Field.masterDetail('showcase_invoice', {
+      label: 'Invoice',
+      required: true,
+      deleteBehavior: 'cascade',
+      // Thin, high-volume line items → the editable grid form factor.
+      inlineEdit: 'grid',
+      inlineTitle: 'Line Items',
+    }),
+    product: Field.text({ label: 'Product', required: true, maxLength: 200 }),
+    quantity: Field.number({ label: 'Qty', required: true, min: 0, defaultValue: 1 }),
+    unit_price: Field.currency({ label: 'Unit Price', scale: 2, min: 0 }),
+    amount: Field.currency({ label: 'Amount', scale: 2, min: 0 }),
+  },
+});
diff --git a/examples/app-showcase/src/objects/task.object.ts b/examples/app-showcase/src/objects/task.object.ts
index 8bb437cea..85d12264f 100644
--- a/examples/app-showcase/src/objects/task.object.ts
+++ b/examples/app-showcase/src/objects/task.object.ts
@@ -24,21 +24,15 @@ export const Task = ObjectSchema.create({
 
   fields: {
     title: Field.text({ label: 'Title', required: true, searchable: true, maxLength: 200 }),
-    // `inlineEdit` declares (in the data model) that tasks are entered inline
-    // within their project's form — so the standard New/Edit Project form
-    // auto-renders an atomic Tasks subtable, with no form view config and no
-    // bespoke page. `relatedList*` is the read-side mirror: the Project's
-    // record DETAIL page auto-renders a Tasks related list, with a focused
-    // column set — again, derived from the relationship, no page config.
+    // NOTE: no `inlineEdit` here. A task is added to a project over time, not
+    // entered together with it — so "New Project" must NOT force a Tasks
+    // subtable (that felt heavy/odd). Tasks are added later from the Project
+    // DETAIL page's Tasks related list (`relatedList*` below — the read side).
+    // Inline master-detail entry is reserved for true header+line shapes like
+    // Invoice + Invoice Line (see invoice.object.ts).
     project: Field.masterDetail('showcase_project', {
       label: 'Project',
       required: true,
-      // Pin the editable-grid form factor (fast bulk line-item entry, with the
-      // column chooser + per-row expand). Left at `true`, the smart default
-      // would pick `form` for this fat child — the right call for many apps;
-      // here we keep the grid demo. Use `'form'` to force the per-row form.
-      inlineEdit: 'grid',
-      inlineTitle: 'Tasks',
       relatedListTitle: 'Tasks',
       relatedListColumns: ['title', 'status', 'priority', 'assignee', 'due_date'],
     }),
diff --git a/examples/app-showcase/src/pages/project-workspace.page.ts b/examples/app-showcase/src/pages/project-workspace.page.ts
index 37c275446..20b5c3112 100644
--- a/examples/app-showcase/src/pages/project-workspace.page.ts
+++ b/examples/app-showcase/src/pages/project-workspace.page.ts
@@ -49,6 +49,11 @@ export const ProjectWorkspacePage: Page = {
             // columns are auto-derived from the child object's metadata — no
             // hand-authored columns block. Add `columns`/`relationshipField`
             // here only to override the derived defaults.
+            // `showcase_task` has rich fields (notes/location/cover), so the
+            // relationship's smart default resolves to the per-row `form`
+            // factor: a read-only Tasks list with an "Add task" button that
+            // opens the child's full form inline. (Thin children like invoice
+            // lines get the editable `grid` instead — see invoice.object.ts.)
             details: [
               { title: 'Tasks', childObject: 'showcase_task', addLabel: 'Add task' },
             ],