private function read_author_mapping_file( $file ) {
		$author_mapping = array();

		foreach ( new \WP_CLI\Iterators\CSV( $file ) as $i => $author ) {
			if ( ! array_key_exists( 'old_user_login', $author ) || ! array_key_exists( 'new_user_login', $author ) ) {
				return new WP_Error( 'invalid-author-mapping', "Author mapping file isn't properly formatted." );
			}

			$author_mapping[] = $author;
		}

		return $author_mapping;
	}