Add an error message when deleting a page with protected foreign keys

Resolves MS-691 #complete

server/core/wagtail_hooks.py

@@ -1,8 +1,12 @@
+from django.db.models import ProtectedError
+from django.shortcuts import redirect
+from wagtail.admin import messages
 import wagtail.admin.rich_text.editors.draftail.features as draftail_features
 from wagtail.admin.rich_text.converters.html_to_contentstate import (
     InlineStyleElementHandler,
 )
 from wagtail import hooks
+from wagtail.admin.utils import get_valid_next_url_from_request
 
 from basicknowledge.models import BasicKnowledge
 from books.models import ContentBlockSnapshot
@@ -142,3 +146,55 @@ def after_create_hook(request, page):
     if type(page.specific) == ContentBlock:
         content_block = page.specific
         content_block.reassign_entities()
+
+
+@hooks.register("before_delete_page")
+def on_page_delete(request, page):
+    if request.method != "POST":
+        return
+
+    try:
+        next_url = get_valid_next_url_from_request(request)
+        parent_id = page.get_parent().id
+        page.delete()
+
+        messages.success(
+            request, ("Page '{0}' deleted.").format(page.get_admin_display_title())
+        )
+
+        for fn in hooks.get_hooks("after_delete_page"):
+            result = fn(request, page)
+            if hasattr(result, "status_code"):
+                return result
+
+        if next_url:
+            return redirect(next_url)
+        return redirect("wagtailadmin_explore", parent_id)
+
+    except ProtectedError as exc:
+        protected_objects = {}
+        for obj in exc.protected_objects:
+            model_name = obj._meta.verbose_name_plural
+            if model_name in protected_objects:
+                protected_objects[model_name].append(str(obj))
+            else:
+                protected_objects[model_name] = [str(obj)]
+
+        dependency_summary = []
+        for model_name, items in protected_objects.items():
+            if len(items) == 1:
+                items_summary = ("{0} ({1})").format(model_name, items[0])
+            else:
+                items_summary = ("{0} ({1} and {2} more...)").format(
+                    model_name, items[0], len(items) - 1
+                )
+
+            dependency_summary.append(items_summary)
+
+        messages.error(
+            request,
+            ("Page '{0}' cannot be deleted while it's used by {1}").format(
+                page.get_admin_display_title(), ", ".join(dependency_summary)
+            ),
+        )
+        return redirect("wagtailadmin_pages:delete", page.id)