SCKAN-373 fix: Add missing service file

backend/composer/services/layers_service.py

@@ -0,0 +1,28 @@
+from django.db import transaction
+
+
+
+def update_from_entities_on_deletion(connectivity_statement, deleted_entity_id):
+    """
+    Updates the 'from_entities' for Layers that used the deleted entity.
+    Scoped to the given ConnectivityStatement.
+    """
+    from composer.models import Destination, Via
+
+    # Filter affected layers within the same ConnectivityStatement
+    affected_via_layers = Via.objects.filter(
+        connectivity_statement=connectivity_statement,
+        from_entities__id=deleted_entity_id
+    )
+
+    affected_destination_layers = Destination.objects.filter(
+        connectivity_statement=connectivity_statement,
+        from_entities__id=deleted_entity_id
+    )
+
+    # Update 'from_entities' by removing the deleted entity
+    for via in affected_via_layers:
+        via.from_entities.remove(deleted_entity_id)
+
+    for destination in affected_destination_layers:
+        destination.from_entities.remove(deleted_entity_id)