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The approach taken in this book and the associated framework is very conducive to code generation. Many people use code generators to automate the process of building common data access logic for their objects, thus achieving high levels of productivity while retaining the ability to create a behavioral object-oriented model.
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The point is that the framework will simplify object persistence so that all developers need to do is implement these four methods in order to retrieve or update data. This places no restrictions on the object s ability to work with data and provides a standardized persistence and mapping mechanism for all objects.
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Preserving Encapsulation
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As I noted at the beginning of the chapter, one of my key goals is to design this framework to provide powerful features while following the key object-oriented concepts, including encapsulation. Encapsulation is the idea that all of the logic and data pertaining to a given business entity is held within the object that represents that entity. Of course, there are various ways in which one can interpret the idea of encapsulation nothing is ever simple. One approach is to encapsulate business data and logic in the business object and then encapsulate data access and ORM behavior in some other object: a persistence object. This provides a nice separation between the business logic and data access and encapsulates both types of behavior, as shown in Figure 2-7.
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Figure 2-7. Separation of ORM logic into a persistence object Although there are certainly some advantages to this approach, there are drawbacks, too. The most notable of these is that it can be challenging to efficiently get the data from the persistence object into or out of the business object. For the persistence object to load data into the business object, it must be able to bypass business and validation processing in the business object and somehow load raw data into it directly. If the persistence object tries to load data into the object using the object s public properties, you ll run into a series of issues: The data already in the database is presumed valid, so a lot of processing time is wasted unnecessarily revalidating data. This can lead to a serious performance problem when loading a large group of objects. There s no way to load read-only property values. Objects often have read-only properties for things such as the primary key of the data, and such data obviously must be loaded into the object, but it can t be loaded via the normal interface (if that interface is properly designed). Sometimes properties are interdependent due to business rules, which means that some properties must be loaded before others or errors will result. The persistence object would need to know about all these conditions so that it could load the right properties first. The result is that the persistence object would become very complex, and changes to the business object could easily break the persistence object. On the other hand, having the persistence object load raw data into the business object breaks encapsulation in a big way because one object ends up directly tampering with the internal fields of
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CHAPTER 2 FRAMEWORK DE SIGN
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another. This could be implemented using reflection or by designing the business object to expose its private fields for manipulation. But the former is slow and the latter is just plain bad object design: it allows the UI developer (or any other code) to manipulate these fields, too. That s just asking for the abuse of the objects, which will invariably lead to code that s impossible to maintain. What s needed is a workable compromise, where the actual data access code is in one object while the code to load the business object s fields is in the business object itself. This can be accomplished by creating a separate data access layer (DAL) assembly that is invoked by the business object. The DAL defines an interface the business object can use to retrieve and store information, as shown in Figure 2-8.
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Figure 2-8. Business object interacting with a data access layer This is a nice compromise because it allows the business object to completely manage its own fields and yet keeps the code that communicates with the database cleanly separated into its own location. There are several ways to implement such a DAL, including the use of raw ADO.NET and the use of LINQ to SQL. The raw ADO.NET approach has the benefit of providing optimal performance. In this case the DAL simply returns a DataReader to the business object and the object can load its fields directly from the data stream. Creating the DAL using LINQ to SQL or the ADO.NET Entity Framework provides a higher level of abstraction and simplifies the DAL code. However, this approach is slower, because the data must pass through a set of data transfer objects (DTOs) or entity objects as it flows to and from the business object.
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