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attribute unsaved-value is null if the type is nullable; otherwise, it s the default value of the type (0 for numerical types); because you ve chosen long as the identifier property type, you can omit the unsaved-value attribute in your auction application classes. Technically, NHibernate tries to guess the unsaved-value by instantiating an empty object and retrieving default property values from it.
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This approach works nicely for synthetic identifiers, but it breaks down in the case of keys assigned by the application, including composite keys in legacy systems. We discuss this issue in section 10.2. Avoid application-assigned (and composite) keys in new applications if possible (this is important for non-versioned entities).
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You now have the knowledge to optimize your NHibernate application and reduce the number of calls to the persistence manager if you want to save and delete objects. Check the unsaved-value attributes of all your classes and experiment with detached objects to get a feel for the NHibernate transitive persistence model. Having focused on how to persist objects with NHibernate, we can now switch perspectives and focus on how you go about retrieving (or loading) them.
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Retrieving objects
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Retrieving persistent objects from the database is one of the most interesting (and complex) parts of working with NHibernate. NHibernate provides the following ways to get objects out of the database:
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Navigating the object graph, starting from an already loaded object, by accessing the associated objects through property accessor methods such as aUser.Address.City. NHibernate automatically loads (or preloads) nodes of the graph while you navigate the graph if the ISession is open. Retrieving by identifier, which is the most convenient and performant method when the unique identifier value of an object is known. Using Hibernate Query Language (HQL), which is a full object-oriented query language. Using the NHibernate ICriteria API, which provides a type-safe and object-oriented way to perform queries without the need for string manipulation. This facility includes queries based on an example object. Using native SQL queries and having NHibernate take care of mapping the ADO.NET result sets to graphs of persistent objects. Using LINQ for NHibernate is another option available. This lets you specify your NHibernate queries using LINQ. At the time of writing, LINQ for NHibernate looks very promising despite the fact it s still a work in progress. We don t cover it in this book, but feel free to investigate further by visiting the NHContrib project website.
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Working with persistent objects
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In your NHibernate applications, you ll use a combination of these techniques. Each retrieval method may use a different fetching strategy that is, a strategy that defines what part of the persistent object graph should be retrieved. The goal is to find the best retrieval method and fetching strategy for every use case in your application while at the same time minimizing the number of SQL queries for best performance. We don t discuss each retrieval method in detail in this section; instead, we focus on the basic fetching strategies and how to tune NHibernate mapping files for the best default fetching performance for all methods. Before we look at the fetching strategies, we provide an overview of the retrieval methods. Note that we mention the NHibernate caching system, but we fully explore it in the next chapter. Let s start with the simplest case: retrieving an object by giving its identifier value (navigating the object graph should be self-explanatory). You saw a simple retrieval by identifier earlier in this chapter, but there is more to know about it.
Retrieving objects by identifier
The following NHibernate code snippet retrieves a User object from the database:
User user = session.Get<User>(userID);
And here s the code without .NET 2.0 generics:
User user = (User) session.Get(typeof(User), userID);
The Get() method is special because the identifier uniquely identifies a single instance of a class. Hence it s common for applications to use the identifier as a convenient handle to a persistent object. Retrieval by identifier can use the cache when retrieving an object, avoiding a database hit if the object is already cached. NHibernate also provides a Load() method:
User user = session.Load<User>(userID);
The difference between these two methods is trivial. If Load() can t find the object in the cache or database, an exception is thrown. The Load() method never returns null. The Get() method returns null if the object can t be found. The Load() method may return a proxy instead of a real persistent instance (when lazy loading is enabled). A proxy is a placeholder that triggers the loading of the real object when it s accessed for the first time; we discuss proxies later in this section. It s important to understand that Load() will return a proxy even if there is no row with the specified identifier; and an exception will be thrown if (and only if) NHibernate tries to load it. On the other hand, Get() never returns a proxy because it must return null if the entity doesn t exist. Choosing between Get() and Load() is easy: if you re certain the persistent object exists, and nonexistence would be considered exceptional, Load() is a good option. If you aren t certain there is a persistent instance with the given identifier, use Get() and test the return value to see if it s null. What if this object is already in the session s cache as an un-initialized proxy In this case, Load() will return the proxy as is, but Get() will initialize it before returning it.
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