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The example in this chapter isn t focused on automated testing, so many of the necessary automated tests are omitted for the sake of brevity.
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Now that we understand how the application is structured at a high level, we ll explore each layer bit by bit. We ll begin with the domain model.
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23.3 Domain model the application core
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The domain model is the most important part of the application. Without the domain model, all of the pertinent concepts would be represented only in the UI. Our particular domain model contains a single aggregate made up of a single entity, the Visitor. The code for the Visitor class is shown in listing 23.1.
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Figure 23.4 No project references Infrastructure. This arrangement is important for decoupling.
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Data access with NHibernate
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Listing 23.1 The Visitor class, the domain model for this example
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using System; namespace Core { public class Visitor { public virtual Guid Id { get; set; } public virtual string PathAndQuerystring { get; set; } public virtual string LoginName { get; set; } public virtual string Browser { get; set; } public virtual DateTime VisitDate { get; set; } public virtual string IpAddress { get; set; } } }
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We have no business logic here, and at first glance it looks just like a data structure. All other concerns have been left out in an effort to include only abstractions and logic that are necessary for leveraging NHibernate in a loosely coupled way. The Visitor class contains properties for all the pieces of information that we want to record. The Id property exists as an identifier for the particular visit. We could certainly use Int32 as the ID, but in a data persistence environment, that forces a dependency on the data store for the generation of a unique Int32 value. Sometimes this is appropriate, but in DDD, the developer errs on the side of giving responsibility to the domain model, not the data store. In line with that, the Id is a Guid, and the application will generate a Guid before attempting to save to the database. The mechanism for persisting or retrieving a Visitor is called a repository. The repository will save our entity as well as retrieve it. It can also represent filtering operations. In our domain model, we have an IVisitorRepository. This interface is seen in listing 23.2.
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Listing 23.2 The repository that defines the persistence operations
namespace Core { public interface IVisitorRepository { void Save(Visitor visitor); Visitor[] GetRecentVisitors(int numberOfVisitors); } }
With our repository, we re able to save a Visitor as well as get a specific number of the most recent visitors. In figure 23.4, you see that the Core project doesn t contain any class that implements IVisitorRepository. This is important because the class that does the work represented by the interface will be responsible for the persistence, which isn t a domain model concern. Persistence is infrastructure. This functionality would work equally well if we persisted the data to a file instead of the database. The mechanism of persistence isn t a concern for the domain model, so the class responsible for it isn t in the Core project.
NHibernate configuration infrastructure of the application
The concern that s in the Core project is an abstract factory capable of locating or creating an instance of IVisitorRepository. The VisitorRepositoryFactory is responsible for returning an instance of our repository. Listing 23.3 shows that the knowledge for creating the repository doesn t reside with the factory. This factory merely represents the capability to return the repository.
Listing 23.3 The factory that provides the repository
using System; namespace Core { public class VisitorRepositoryFactory { public static Func<IVisitorRepository> RepositoryBuilder = CreateDefaultRepositoryBuilder;
Initializes at application startup
private static IVisitorRepository CreateDefaultRepositoryBuilder() { Throws if factory throw new Exception( not initialized "No repository builder specified."); } public IVisitorRepository BuildRepository() { Uses delegate to IVisitorRepository repository = build repository RepositoryBuilder(); return repository; } } }
To even the inexperienced eye, this class doesn t seem useful alone. When BuildFactory() is called, an exception will be thrown. Out of the box, the domain model doesn t know the implementation of IVisitorRepository that will be used, so there s no way to embed this knowledge into compiled code. The public static RepositoryBuilder property will have to be set to something useful before the factory will work properly. We ll see how this is accomplished after all the pieces have been introduced. This explicit factory isn t necessary if you re using an IoC container, which has been left out for the sake of simplicity. This domain model is intentionally simple. The next step is to understand how we configure NHibernate to automatically persist our entity to the database.
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