Domain Modeling Made Functional

Aggregates

Let’s take a closer look at two data types that are especially relevant to our design: Order and OrderLine.

First, is Order an Entity or a Value Object? Obviously it’s an Entity—the details of the order may change over time, but it’s the same order.

What about an OrderLine, though? If we change the quantity of a particular order line, for example, is it still the same order line? In most designs, it would make sense to say yes, it is still the same order line, even though the quantity or price has changed over time. So OrderLine is an Entity too, with its own identifier.

But now here’s a question: if you change an order line, have you also changed the order that it belongs to?

In this case, it’s clear that the answer is yes: changing ...