'Fetches the BlobMigrationRecord for the given blob key.
Args:
old_blob_key: The blob key used in the previous app.
Returns:
A instance of blobstore.BlobMigrationRecord or None'

@classmethod
def get_by_blob_key(cls, old_blob_key):

    return cls.get_by_key_name(str(old_blob_key))
