Here goes pseudo code of bulk update/delete entities of different kind in single transaction. Note that Album and Song entities have AlbumGroup as root entity.

class AlbumGroup:
  pass
class Album:
  group = db.ReferenceProperty(reference_class=AlbumGroup,collection_name="albums")
class Song:
  album = db.ReferenceProperty(reference_class=Album,collection_name="songs")

def bulk_update_album_group(album_group):
  updated = [album_group]
  deleted = []
  for album in album_group.albums:
    updated.append(album)
    for song in album.songs:
      if song.is_updated:
        updated.append(song)        
      if song.is_deleted:
        deleted.append(song)
  db.put(updated)
  db.delete(deleted)

a = AlbumGroup.all().filter("...").get()

# bulk update/delete album group. for simplicity, album cannot be deleted.

db.run_in_transaction(bulk_update_album_group,a)

But I met a famous "Only Ancestor Queries in Transactions" error at the iterating reference properties like album.songs or album_group.albums. I guess ancestor() filter does not help because those entities are modified in memory.

• Should I not to iterate reference property in transaction function and always provide them as function parameters like def bulk_update_album_group(updated,deleted): ???
• Is there any good coding pattern for this situation?