Provide the parent page of the round on the object before the save

opentech/apply/funds/models.py

@@ -143,7 +143,7 @@ class Round(AbstractStreamForm):
                 Q(end_date__gte=self.start_date)
             )
 
-        conflicting_rounds = Round.objects.sibling_of(self).filter(
+        conflicting_rounds = Round.objects.child_of(self.parent_page).filter(
             conflict_query
         ).exclude(id=self.id)
 

opentech/apply/funds/wagtail_hooks.py

+from wagtail.wagtailcore import hooks
 from wagtail.contrib.modeladmin.options import modeladmin_register
 
 from .admin import ApplyAdminGroup
+from .models import Round
 
 
 modeladmin_register(ApplyAdminGroup)
+
+
+@hooks.register('before_create_page')
+def before_create_page(request, parent_page, page_class):
+    if page_class == Round:
+        page_class.parent_page = parent_page
+    return page_class