We extend entities by adding additional attributes to them. Referring back to the magical getter and setter methods mentioned in the context of MagentoFrameworkDataObject, the logical thinking might be: what's the big deal; can't we just add new database columns via UpgradeSchema and use magical getter and setter methods to go around it? The answer is both yes and no, but mainly leaning toward no – we will soon learn why.

To better explain this, let's take a look at MagentoSalesModelOrder, the entity model. This model implements the MagentoSalesApiDataOrderInterface interface, which further extends MagentoFrameworkApiExtensibleDataInterface. Here, we can see a constant defining a key for the extension attributes object. This is somewhat of a starting point for extending entities. Suffice to say, there is an extra abstraction layer on top of some of the models. This abstraction layer, called service contracts, is a set of PHP interfaces that ensure a well-defined, durable API that other modules and third-party extensions might implement.

This, however, is easier said than done. When you think about it, if we had a module that's already heavily in use, adding even a simple attribute to one of its entity models might break its functionality. This is where extension attributes come into the picture.