PDO doesn't handle mapping of nested relationships, though which is where it falls down when compared to ActiveRecord implementations. For example I want to fetch an author and all of the author's books in a single query then loop over and print out author name, and each book. A standard ActiveRecord ORM will give you an array of authors where each author contains an array of books, where as with PDO you're still left to manually detect a change in author by a change in the primary key which is more complex than just writing a nested for loop.