Create new event for collection add w/o mutation

Fixed issue when using :paramref:`_orm.relationship.cascade_backrefs`
parameter set to ``False``, which per :ref:`change_5150` is set to become
the standard behavior in SQLAlchemy 2.0, where adding the item to a
collection that uniquifies, such as ``set`` or ``dict`` would fail to fire
a cascade event if the object were already associated in that collection
via the backref. This fix represents a fundamental change in the collection
mechanics by introducing a new event state which can fire off for a
collection mutation even if there is no net change on the collection; the
action is now suited using a new event hook
:meth:`_orm.AttributeEvents.append_wo_mutation`.

Fixes: #6471
Change-Id: Ic50413f7e62440dad33ab84838098ea62ff4e815

1 files changed, 6 insertions, 0 deletions

diff --git a/lib/sqlalchemy/orm/attributes.py b/lib/sqlalchemy/orm/attributes.py
index 05b12dda2..105a9cfd2 100644
--- a/lib/sqlalchemy/orm/attributes.py
+++ b/lib/sqlalchemy/orm/attributes.py
@@ -1389,6 +1389,12 @@ class CollectionAttributeImpl(AttributeImpl):
 
         return value
 
+    def fire_append_wo_mutation_event(self, state, dict_, value, initiator):
+        for fn in self.dispatch.append_wo_mutation:
+            value = fn(state, value, initiator or self._append_token)
+
+        return value
+
     def fire_pre_remove_event(self, state, dict_, initiator):
         """A special event used for pop() operations.