Added an option to the page model to be able to define the amount of subpages underneath a specific parent page

CHANGELOG.txt

@@ -20,6 +20,7 @@ Changelog
  * Added instructions on how to generate urls for `ModelAdmin` to documentation (LB (Ben Johnston), Andy Babic)
  * Added option to specify a fallback URL on `{% pageurl %}` (Arthur Holzner)
  * Add support for more rich text formats, disabled by default: `blockquote`, `superscript`, `subscript`, `strikethrough`, `code` (Md Arifin Ibne Matin)
+ * Added `max_count_per_parent` option on page models to limit the number of pages of a given type that can be created under one parent page (Wesley van Lee)
  * Fix: Set `SERVER_PORT` to 443 in `Page.dummy_request()` for HTTPS sites (Sergey Fedoseev)
  * Fix: Include port number in `Host` header of `Page.dummy_request()` (Sergey Fedoseev)
  * Fix: Validation error messages in `InlinePanel` no longer count towards `max_num` when disabling the 'add' button (Todd Dembrey, Thibaud Colas)

docs/reference/pages/model_reference.rst

@@ -203,6 +203,10 @@ In addition to the model fields provided, ``Page`` has many properties and metho
 
         Controls the maximum number of pages of this type that can be created through the Wagtail administration interface. This is useful when needing "allow at most 3 of these pages to exist", or for singleton pages.
 
+    .. attribute:: max_count_per_parent
+
+        Controls the maximum number of pages of this type that can be created under any one parent page.
+
     .. attribute:: exclude_fields_in_copy
 
         An array of field names that will not be included when a Page is copied.

docs/releases/2.5.rst

@@ -30,6 +30,7 @@ Other features
  * Added instructions on how to generate urls for ``ModelAdmin`` to documentation (LB (Ben Johnston), Andy Babic)
  * Added option to specify a fallback URL on ``{% pageurl %}`` (Arthur Holzner)
  * Add support for more rich text formats, disabled by default: ``blockquote``, ``superscript``, ``subscript``, ``strikethrough``, ``code`` (Md Arifin Ibne Matin)
+ * Added ``max_count_per_parent`` option on page models to limit the number of pages of a given type that can be created under one parent page (Wesley van Lee)
 
 
 Bug fixes

wagtail/core/models.py

@@ -343,6 +343,9 @@ class Page(AbstractPage, index.Indexed, ClusterableModel, metaclass=PageBase):
     # Define the maximum number of instances this page type can have. Default to unlimited.
     max_count = None
 
+    # Define the maximum number of instances this page can have under a specific parent. Default to unlimited.
+    max_count_per_parent = None
+
     # An array of additional field names that will not be included when a Page is copied.
     exclude_fields_in_copy = []
 
@@ -980,6 +983,9 @@ class Page(AbstractPage, index.Indexed, ClusterableModel, metaclass=PageBase):
         if cls.max_count is not None:
             can_create = can_create and cls.objects.count() < cls.max_count
 
+        if cls.max_count_per_parent is not None:
+            can_create = can_create and parent.get_children().type(cls).count() < cls.max_count_per_parent
+
         return can_create
 
     def can_move_to(self, parent):

wagtail/core/tests/test_page_model.py

@@ -17,8 +17,8 @@ from wagtail.tests.testapp.models import (
     AbstractPage, Advert, AlwaysShowInMenusPage, BlogCategory, BlogCategoryBlogPage, BusinessChild,
     BusinessIndex, BusinessNowherePage, BusinessSubIndex, CustomManager, CustomManagerPage,
     CustomPageQuerySet, EventCategory, EventIndex, EventPage, GenericSnippetPage, ManyToManyBlogPage,
-    MTIBasePage, MTIChildPage, MyCustomPage, OneToOnePage, PageWithExcludedCopyField, SimplePage,
-    SingleEventPage, SingletonPage, StandardIndex, TaggedPage)
+    MTIBasePage, MTIChildPage, MyCustomPage, OneToOnePage, PageWithExcludedCopyField, SimpleChildPage,
+    SimplePage, SimpleParentPage, SingleEventPage, SingletonPage, StandardIndex, TaggedPage)
 from wagtail.tests.utils import WagtailTestUtils
 
 
@@ -1167,6 +1167,23 @@ class TestSubpageTypeBusinessRules(TestCase, WagtailTestUtils):
         self.assertFalse(BusinessChild.can_create_at(SimplePage()))
         self.assertFalse(BusinessSubIndex.can_create_at(SimplePage()))
 
+    def test_can_create_at_with_max_count_per_parent_limited_to_one(self):
+        root_page = Page.objects.get(url_path='/home/')
+
+        # Create 2 parent pages for our limited page model
+        parent1 = root_page.add_child(instance=SimpleParentPage(title='simple parent', slug='simple-parent'))
+        parent2 = root_page.add_child(instance=SimpleParentPage(title='simple parent', slug='simple-parent-2'))
+
+        # Add a child page to one of the pages (assert just to be sure)
+        self.assertTrue(SimpleChildPage.can_create_at(parent1))
+        parent1.add_child(instance=SimpleChildPage(title='simple child', slug='simple-child'))
+
+        # We already have a `SimpleChildPage` as a child of `parent1`, and since it is limited
+        # to have only 1 child page, we cannot create anoter one. However, we should still be able
+        # to create an instance for this page at a different location (as child of `parent2`)
+        self.assertFalse(SimpleChildPage.can_create_at(parent1))
+        self.assertTrue(SimpleChildPage.can_create_at(parent2))
+
     def test_can_move_to(self):
         self.assertTrue(SimplePage().can_move_to(SimplePage()))
 

wagtail/tests/testapp/migrations/0042_simplechildpage_simpleparentpage.py

@@ -0,0 +1,35 @@
+# Generated by Django 2.1.7 on 2019-03-15 10:39
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('wagtailcore', '0041_group_collection_permissions_verbose_name_plural'),
+        ('tests', '0041_secretpage'),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='SimpleChildPage',
+            fields=[
+                ('page_ptr', models.OneToOneField(auto_created=True, on_delete=django.db.models.deletion.CASCADE, parent_link=True, primary_key=True, serialize=False, to='wagtailcore.Page')),
+            ],
+            options={
+                'abstract': False,
+            },
+            bases=('wagtailcore.page',),
+        ),
+        migrations.CreateModel(
+            name='SimpleParentPage',
+            fields=[
+                ('page_ptr', models.OneToOneField(auto_created=True, on_delete=django.db.models.deletion.CASCADE, parent_link=True, primary_key=True, serialize=False, to='wagtailcore.Page')),
+            ],
+            options={
+                'abstract': False,
+            },
+            bases=('wagtailcore.page',),
+        ),
+    ]

wagtail/tests/testapp/models.py

@@ -1327,3 +1327,15 @@ class SecretPage(PerUserPageMixin, Page):
     superuser_content_panels = basic_content_panels + [
         FieldPanel('secret_data'),
     ]
+
+
+class SimpleParentPage(Page):
+    # `BusinessIndex` has been added to bring it in line with other tests
+    subpage_types = ['tests.SimpleChildPage', BusinessIndex]
+
+
+class SimpleChildPage(Page):
+    # `Page` has been added to bring it in line with other tests
+    parent_page_types = ['tests.SimpleParentPage', Page]
+
+    max_count_per_parent = 1