How do you handle constraints and validation inside your application? Most developers put this logic somewhere close to their application's boundary. Actually, this prevents them from having a rich domain model that could ensure consistency.
Developers tend to get confused when they need to find a good place for their business logic. I suspect that most probably the reasons are related to all those bad examples circulating in the documentations of popular frameworks and bad habits from coding in old-fashioned enterprise platforms like J2EE. Most of the time, they're afraid to keep these vital pieces of information in the language elements most relevant to the target domain: I'm talking about the simple classes reflecting the "nouns" of the business logic. Often, you see something like the example below: