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CHAPTER 1 HAND-CODED ENTITIES
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Object/relational persistence in .NET
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Returning to the example of an auction application, consider the entities: User, Item, and Bid. In addition to the data they contain, you expect relationships to exist between them. For example, an Item has a collection of bids, and a Bid refers to an Item; in C# classes, this might be expressed using a collection like item.Bids and a property like bid.Item. The object-oriented view is different that the relational view: instead of having primary and foreign keys, you have associations. Object-oriented design also provides other powerful modeling concepts, such as inheritance and polymorphism. Hand-coded entities are free from any constraints; they re even free from the way they re persisted in the database. They can evolve (almost) independently and be shared by different applications; this is an important advantage when you re working in a complex environment. But they re difficult and tedious to code. Think about the manual work required to support the persistence of entities inheriting from other entities. It s common to use code generation or base classes (like DataSet) to add features with a minimal effort. These features may be related to the persistence, transfer, or presentation of information. But without a helpful framework, these features can be time consuming to implement.
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ENTITIES IN A DATASET
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A DataSet represents a collection of database tables, and in turn these tables contain the data of the entities. A DataSet stores data about business objects in a fashion similar to a database. You can use a generated-typed DataSet to ease the manipulation of data, and it s also possible to insert business logic and rules. As long as you want to manipulate data, .NET and IDEs provide most features required to work with a DataSet. But as soon as you think about business objects as objects in the sense of object-oriented design, you can hardly be satisfied by a DataSet (typed or not). After all, business objects represent real-world elements, and these elements have data and behavior. They may be linked by advanced relationships like inheritance, but this isn t possible with DataSets. This level of freedom in the design of entities can be achieved only by hand coding them.
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ENTITIES AND NHIBERNATE
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NHibernate is non-intrusive. It s common to use it with hand-coded (or generated)
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entities. You must provide mapping information indicating how these entities should be loaded and saved. Once this is done, NHibernate can take care of moving your object-oriented entities to and from a relational database. There are many fundamental differences between objects and relational data. Trying to use them together reveals the paradigm mismatch (also called the object/relational impedance mismatch). We explore this mismatch in section 1.3. By the end of this chapter, you ll have a clear idea of the problems caused by the paradigm mismatch and how NHibernate solves these problems.
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Approaches to persistence in .NET
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ENTITIES AND LINQ TO SQL
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The LINQ to SQL approach looks a lot like the NHibernate way of doing ORM. LINQ to SQL uses POCO objects to represent your application data (the entities). The mapping of those objects to database tables is described either in declarative attributes in code or in an XML document. After the mapping and the classes are complete, the LINQ to SQL framework takes care of generating SQL for database operations. Once the entities are implemented, you must think about how they will be presented to the end user.
Displaying entities in the user interface
Using NHibernate implies using entities, and using entities has consequences for the way the user interface (UI) is written. For the end user, the UI is one of the most important elements. Whether it s a web application (using ASP.NET) or a Windows application, it must satisfy the needs of the user. A deep discussion of implementing a UI isn t in the scope of this book; but the way the persistence layer is implemented has a direct effect on the way the UI will be implemented. In this book, we refer to the UI as the presentation layer. .NET provides controls to display information. The simplicity of this operation depends on how the information is stored. It s worth noting that we expect .NET entity data binding to change soon. Microsoft is beginning to actively push the use of entities in .NET applications as the company promotes the ADO.NET Entity Framework and LINQ to SQL. For this reason, we won t discuss those technologies in this section.
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