Nowadays it’s a quite common approach to have models that essentially just represent database tables, and may support saving the model instance right to the database. While the ActiveRecord pattern and things like Doctrine are useful, you may sometimes want to decouple the actual storage mechanism from the rest of the code.
This is where the Data Access Object pattern comes in. It’s a pattern which abstracts the details of the storage mechanism – be it a relational database, OO database, an XML file or whatever. The advantage of this is that you can easily implement different methods to persist objects without having to rewrite parts of your code.
I’m again going to use the programming language quiz game I wrote as an example. Since I initially wrote it to use Doctrine ORM directly, and both the old and new code are available, you can easily see how the code was improved.