Prevent deletion of collections that are non-empty

Since the collections infrastructure itself does not know about the object types that
can exist within collections, this requires a new hook `describe_collection_contents`
to allow collection contents to be discovered.

docs/reference/hooks.rst

@@ -91,6 +91,23 @@ Hooks for building new areas of the admin interface (alongside pages, images, do
         menu_items[:] = [item for item in menu_items if item.name != 'explorer']
 
 
+.. _describe_collection_contents:
+
+``describe_collection_contents``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+  Called when Wagtail needs to find out what objects exist in a collection, if any. Currently this happens on the confirmation before deleting a collection, to ensure that non-empty collections cannot be deleted. The callable passed to this hook will receive a ``collection`` object, and should return either ``None`` (to indicate no objects in this collection), or a dict containing the following keys:
+
+``count``
+  A numeric count of items in this collection
+
+``count_text``
+  A human-readable string describing the number of items in this collection, such as "3 documents". (Sites with multi-language support should return a translatable string here, most likely using the ``django.utils.translation.ungettext`` function.)
+
+``url`` (optional)
+  A URL to an index page that lists the objects being described.
+
+
 .. _register_admin_menu_item:
 
 ``register_admin_menu_item``

wagtail/wagtailadmin/templates/wagtailadmin/collections/delete_not_empty.html

@@ -0,0 +1,24 @@
+{% extends "wagtailadmin/base.html" %}
+{% load i18n %}
+{% block titletag %}{{ view.page_title }}{% endblock %}
+
+{% block content %}
+    {% include "wagtailadmin/shared/header.html" with title=view.page_title subtitle=view.get_page_subtitle icon=view.header_icon %}
+
+    <div class="nice-padding">
+        <p>
+            {% trans 'This collection cannot be deleted, because it is not empty. It contains:' %}
+        </p>
+        <ul>
+            {% for item_type in collection_contents %}
+                <li>
+                    {% if item_type.url %}
+                        <a href="{{ item_type.url }}">{{ item_type.count_text }}</a>
+                    {% else %}
+                        {{ item_type.count_text }}
+                    {% endif %}
+                </li>
+            {% endfor %}
+        </ul>
+    </div>
+{% endblock %}

wagtail/wagtailadmin/tests/test_collections_views.py

@@ -4,6 +4,7 @@ from django.core.urlresolvers import reverse
 
 from wagtail.tests.utils import WagtailTestUtils
 from wagtail.wagtailcore.models import Collection
+from wagtail.wagtaildocs.models import Document
 
 
 class TestCollectionsIndexView(TestCase, WagtailTestUtils):
@@ -131,6 +132,7 @@ class TestDeleteCollection(TestCase, WagtailTestUtils):
     def test_get(self):
         response = self.get()
         self.assertEqual(response.status_code, 200)
+        self.assertTemplateUsed(response, 'wagtailadmin/generic/confirm_delete.html')
 
     def test_cannot_delete_root_collection(self):
         response = self.get(collection_id=self.root_collection.id)
@@ -140,6 +142,15 @@ class TestDeleteCollection(TestCase, WagtailTestUtils):
         response = self.get(collection_id=100000)
         self.assertEqual(response.status_code, 404)
 
+    def test_get_nonempty_collection(self):
+        Document.objects.create(
+            title="Test document", collection=self.collection
+        )
+
+        response = self.get()
+        self.assertEqual(response.status_code, 200)
+        self.assertTemplateUsed(response, 'wagtailadmin/collections/delete_not_empty.html')
+
     def test_post(self):
         response = self.post()
 
@@ -149,3 +160,14 @@ class TestDeleteCollection(TestCase, WagtailTestUtils):
         # Check that the collection was deleted
         with self.assertRaises(Collection.DoesNotExist):
             Collection.objects.get(id=self.collection.id)
+
+    def test_post_nonempty_collection(self):
+        Document.objects.create(
+            title="Test document", collection=self.collection
+        )
+
+        response = self.post()
+        self.assertEqual(response.status_code, 403)
+
+        # Check that the collection was not deleted
+        self.assertTrue(Collection.objects.get(id=self.collection.id))

wagtail/wagtailadmin/views/collections.py

@@ -1,7 +1,11 @@
+from django.http import HttpResponseForbidden
+from django.shortcuts import redirect, get_object_or_404
 from django.utils.translation import ugettext_lazy as __
 
+from wagtail.wagtailcore import hooks
 from wagtail.wagtailcore.models import Collection
 from wagtail.wagtailcore.permissions import collection_permission_policy
+from wagtail.wagtailadmin import messages
 from wagtail.wagtailadmin.forms import CollectionForm
 from wagtail.wagtailadmin.views.generic import IndexView, CreateView, EditView, DeleteView
 
@@ -70,3 +74,36 @@ class Delete(DeleteView):
     def get_queryset(self):
         # Only return children of the root node, so that the root is not editable
         return Collection.get_first_root_node().get_children()
+
+    def get_collection_contents(self):
+        collection_contents = [
+            hook(self.instance)
+            for hook in hooks.get_hooks('describe_collection_contents')
+        ]
+        # filter out any hook responses that report that the collection is empty
+        # (by returning None, or a dict with 'count': 0)
+        is_nonempty = lambda item_type: item_type and item_type['count'] > 0
+        return list(filter(is_nonempty, collection_contents))
+
+    def get_context(self):
+        context = super(Delete, self).get_context()
+        collection_contents = self.get_collection_contents()
+
+        if collection_contents:
+            # collection is non-empty; render the 'not allowed to delete' response
+            self.template_name = 'wagtailadmin/collections/delete_not_empty.html'
+            context['collection_contents'] = collection_contents
+
+        return context
+
+    def post(self, request, instance_id):
+        self.instance = get_object_or_404(self.get_queryset(), id=instance_id)
+        collection_contents = self.get_collection_contents()
+
+        if collection_contents:
+            # collection is non-empty; refuse to delete it
+            return HttpResponseForbidden()
+
+        self.instance.delete()
+        messages.success(request, self.success_message.format(self.instance))
+        return redirect(self.index_url_name)

wagtail/wagtailadmin/views/generic.py

@@ -198,9 +198,7 @@ class DeleteView(PermissionCheckedMixin, View):
     def get_delete_url(self):
         return reverse(self.delete_url_name, args=(self.instance.id,))
 
-    def get(self, request, instance_id):
-        self.instance = get_object_or_404(self.get_queryset(), id=instance_id)
-
+    def get_context(self):
         context = {
             'view': self,
             'object': self.instance,
@@ -208,6 +206,13 @@ class DeleteView(PermissionCheckedMixin, View):
         if self.context_object_name:
             context[self.context_object_name] = self.instance
 
+        return context
+
+    def get(self, request, instance_id):
+        self.instance = get_object_or_404(self.get_queryset(), id=instance_id)
+
+        context = self.get_context()
+
         return render(request, self.template_name, context)
 
     def post(self, request, instance_id):

wagtail/wagtaildocs/wagtail_hooks.py

@@ -1,7 +1,7 @@
 from django.conf.urls import include, url
 from django.core import urlresolvers
 from django.utils.html import format_html, format_html_join
-from django.utils.translation import ugettext_lazy as _
+from django.utils.translation import ugettext_lazy as _, ungettext
 from django.contrib.staticfiles.templatetags.staticfiles import static
 
 from wagtail.wagtailcore import hooks
@@ -101,3 +101,19 @@ def register_documents_search_area():
 @hooks.register('register_group_permission_panel')
 def register_document_permissions_panel():
     return GroupDocumentPermissionFormSet
+
+
+@hooks.register('describe_collection_contents')
+def describe_collection_docs(collection):
+    docs_count = get_document_model().objects.filter(collection=collection).count()
+    if docs_count:
+        url = urlresolvers.reverse('wagtaildocs:index') + ('?collection_id=%d' % collection.id)
+        return {
+            'count': docs_count,
+            'count_text': ungettext(
+                "%(count)s document",
+                "%(count)s documents",
+                docs_count
+            ) % {'count': docs_count},
+            'url': url,
+        }

wagtail/wagtailimages/wagtail_hooks.py

@@ -1,7 +1,7 @@
 from django.conf.urls import include, url
 from django.core import urlresolvers
 from django.utils.html import format_html, format_html_join
-from django.utils.translation import ugettext_lazy as _
+from django.utils.translation import ugettext_lazy as _, ungettext
 from django.contrib.staticfiles.templatetags.staticfiles import static
 
 from wagtail.wagtailcore import hooks
@@ -110,3 +110,19 @@ def register_images_search_area():
 @hooks.register('register_group_permission_panel')
 def register_image_permissions_panel():
     return GroupImagePermissionFormSet
+
+
+@hooks.register('describe_collection_contents')
+def describe_collection_docs(collection):
+    images_count = get_image_model().objects.filter(collection=collection).count()
+    if images_count:
+        url = urlresolvers.reverse('wagtailimages:index') + ('?collection_id=%d' % collection.id)
+        return {
+            'count': images_count,
+            'count_text': ungettext(
+                "%(count)s image",
+                "%(count)s images",
+                images_count
+            ) % {'count': images_count},
+            'url': url,
+        }