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THE UNIT OF WORK PATTERN
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When you re working with a relational database, you may tend to think of commands: saving or loading. But an application can perform operations involving many entities. When these entities are loaded or saved depends on the context. For example, if you want to load the last item created by a user, you must first save the user (and the user s collection of items); then you can run a query retrieving the item. If you forget to save the user, you ll start getting hard-to-detect bugs.
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The Identity Map pattern
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NHibernate uses the Identity Map pattern to make sure an item s user is the same object as the user you had before loading the item (as long as you re working in the same transaction). You ll learn more about the concept of identity in section 3.5.
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Now imagine that you re involved in a complex conversation involving many updates and deletes. If you have to manually track which entities to save or delete, while making sure you load each entity only once, things can quickly become very difficult. NHibernate follows the Unit of Work pattern to solve this problem and ease the implementation of conversations. (We cover conversations in chapter 5 and implement them in chapter 10.) You can create entities and associate them with NHibernate; then, NHibernate keeps track of all loading and saving of changes only when required. At the end of the transaction, NHibernate figures out and applies all changes in their correct order.
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Because NHibernate keeps track of all entities, it can greatly simplify your application and increase the application s performance. Here are two simple examples. When working on an item in the auction application, users can add, modify, or delete their bids. It would be painful to manually track these changes one by one.
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Object/relational persistence in .NET
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Instead, you can use NHibernate s transparent persistence feature: you ask NHibernate to save all changes in the collection of bids when the item is persisted. It automatically figures out which CRUD operations must be executed. Now, if you want to modify a User, you load, change, and persist it. But what about the collection of items this user has Should you load these items or leave the collection un-initialized Loading the items would be inefficient, but leaving the collection un-initialized will limit your ability to manipulate the user. NHibernate support a feature called lazy loading to solve this problem. When loading the user, you can decide between loading the items or not. If you choose not to do so, the collection is transparently initialized when you need it. Using these features has many implications; we progressively cover them in this book.
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Tracking entities implies keeping their references somewhere. NHibernate uses a cache. This cache is indispensable for implementing the Unit of Work pattern, and it can also make applications more efficient. We cover caching in depth in section 5.3. NHibernate s identity map uses a cache to avoid loading an entity many times. This cache can be shared by transactions and applications. Suppose you build a website for the auction application. Visitors may be interested in some items. Without a cache, these items will be loaded from the database each time a visitor wants to see them. With a few lines of code, you can ask NHibernate to cache these items, and then enjoy the performance gain.
Complex queries and the ADO.NET Entity Framework
This is the last (but not least) feature related to persistence. In section 1.2.5, we talked about CRUD operations. You ve learned about features related to CRUD (all having to do with the Unit of Work pattern). Now we talk about retrieve operations: searching for and loading information. You can easily generate code to load an entity using its identifier (its primary key, in the context of a relational database). But in real-world applications, users rarely deal with identifiers; instead, they use criteria to run a search and then pick the information they want.
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