Can you use ORM without a domain model in C#

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Can you use ORM without a domain model
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We stress that object persistence with full ORM is most suitable for applications based on a rich domain model. If your application doesn t implement complex business rules or complex interactions between entities (or if you have few entities), you may not need a domain model. Many simple and some not-so-simple problems are perfectly suited to table-oriented solutions, where the application is designed around the database data model instead of around an object-oriented domain model, often with logic executed in the database (stored procedures). But the more complex and expressive your domain model, the more you ll benefit from using NHibernate; it shines when dealing with the full complexity of object/relational persistence.
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Implementing the domain model
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Several issues typically must be addressed when you implement a domain model. For instance, how do you separate the business concerns from the cross-cutting concerns (such as transactions and even persistence) What kind of persistence is needed: automated or transparent Do you have to use a specific programming model to achieve this In this section, we examine these types of issues and how to address them in a typical NHibernate application. Let s start with an issue that any implementation must deal with: the separation of concerns. The domain-model implementation is usually a central, organizing component; it s reused heavily whenever you implement new application functionality. For this reason, you should be prepared to go to some lengths to ensure that concerns other than business aspects don t leak into the domain model implementation.
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Addressing leakage of concerns
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The domain-model implementation is such an important piece of code that it shouldn t depend on other .NET APIs. For example, code in the domain model shouldn t perform input/output operations or call the database via the ADO.NET API. This allows you to reuse the domain model implementation virtually anywhere. Most important, it makes it easy to unit-test the domain model (in NUnit, for example) outside of any application server or other managed environment. We say that the domain model should be concerned only with modeling the business domain. But there are other concerns, such as persistence, transaction management, and authorization. You shouldn t put code that addresses these cross-cutting concerns in the classes that implement the domain model. When these concerns start to appear in the domain model classes, we call this an example of leakage of concerns. The DataSet doesn t address this problem. It can t be regarded as a domain model mainly because it isn t designed to include business rules. Much discussion has gone into the topic of persistence, and both NHibernate and DataSets take care of that concern. But NHibernate offers something that DataSets don t: transparent persistence.
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Transparent and automated persistence
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A DataSet allows you to extract the changes performed on it in order to persist them. NHibernate provides a different feature, which is sophisticated and powerful: it can automatically persist your changes in a way that is transparent to your domain model. We use transparent to mean a complete separation of concerns between the persistent classes of the domain model and the persistence logic itself, where the persistent classes are unaware of and have no dependency on the persistence mechanism. The Item class, for example, won t have any code-level dependency to any NHibernate API. Furthermore:
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NHibernate doesn t require that any special base classes or interfaces be inher-
ited or implemented by persistent classes. Nor are any special classes used to implement properties or associations. Thus, transparent persistence improves code readability, as you ll soon see. Persistent classes may be reused outside the context of persistence in unit tests or in the user interface (UI) tier, for example. Testability is a basic requirement for applications with rich domain models. In a system with transparent persistence, objects aren t aware of the underlying data store; they need not even be aware that they re being persisted or retrieved. Persistence concerns are externalized to a generic persistence manager interface in the case of NHibernate, the ISession and IQuery interfaces.
Transparent persistence fosters a degree of portability; without special interfaces, the persistent classes are decoupled from any particular persistence solution. Your business logic is fully reusable in any other application context. You could easily change to another transparent persistence mechanism. By this definition of transparent persistence, certain non-automated persistence layers are transparent (for example, the DAO pattern) because they decouple the persistence-related code with abstract programming interfaces. Only plain .NET classes without dependencies are exposed to the business logic. Conversely, some automated persistence layers (like many ORM solutions) are non-transparent, because they require special interfaces or intrusive programming models. We regard transparency as required. Transparent persistence should be one of the primary goals of any ORM solution. But no automated persistence solution is completely transparent: every automated persistence layer, including NHibernate, imposes some requirements on the persistent classes. For example, NHibernate requires that collection-valued properties be typed to an interface such as IList or IDictionary (or their .NET 2.0 generic versions) and not to an actual implementation such as ArrayList (this is a good practice anyway). (We discuss the reasons for this requirement in appendix B, Going forward. ) You now know why the persistence mechanism should have minimal impact on how you implement a domain model and that transparent and automated persistence are required. DataSet isn t suitable here, so what kind of programming model should you use Do you need a special programming model at all In theory, no; in practice, you should adopt a disciplined, consistent programming model that is well accepted by the .NET community. Let s discuss this programming model and see how it works with NHibernate.
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