Sometimes, the difference between the domain model and database schema is so great that using NHibernate features to patch the gaps becomes a painful and buggy process. In such a situation, it is best to use custom Data Transfer Objects (DTOs) that are designed to match the database schema. Adding custom DTOs creates an additional layer in your architecture, which comes with its own overheads, and, if not designed with care, may lead to code that is inefficient and difficult to maintain. There is no magic formula to work with DTOs so that your persistence layer becomes more bearable. Fortunately, there is some common wisdom shared by veterans in the field, and a few tricks that I have learned from my experience that should help. This section is divided into two parts.

In the first part, we will cover basic areas such as: At what level should we add DTOs in the context of onion architecture and how to accommodate DTOs into an existing domain and domain service layers? Use...