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A proxy or collection wrapper is automatically initialized when any of its methods are invoked (except the identifier property getter, which may return the identifier value without fetching the underlying persistent object). But it s only possible to initialize a proxy or collection wrapper if it s currently associated with an open ISession. If you close the session and try to access an uninitialized proxy or collection, NHibernate throws a LazyInitializationException. Because of this behavior, it s sometimes useful to explicitly initialize an object before closing the session. This approach isn t as flexible as retrieving the complete required object subgraph with an HQL query, using arbitrary fetching strategies at runtime. You use the static method NHibernateUtil.Initialize() for manual initialization:
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using( ISession session = sessionFactory.OpenSession() ) using( session.BeginTransaction() ) { Category cat = session.Get<Category>(id); NHibernateUtil.Initialize( cat.Items ); session.Transaction.Commit(); } foreach(Item item in cat.Items) //...
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NHibernateUtil.Initialize() may be passed a collection wrapper, as in this exam-
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ple, or a proxy. You may also, in similar rare cases, check the current state of a property by calling NHibernateUtil.IsInitialized(). (Note that Initialize() doesn t cascade to any associated objects.) Another solution for this problem is to keep the session open until the application thread finishes, so you can navigate the object graph whenever you like and have NHibernate automatically initialize all lazy references. This is a problem of application design and transaction demarcation; we discuss it again in section 9.1. But your first choice should be to fetch the complete required graph, using HQL or criteria queries,
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Working with persistent objects
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with a sensible and optimized default fetching strategy in the mapping metadata for all other cases. NHibernate allows you to look at the underlying SQL that it sends to the database, so it s possible to tune object retrieval if performance problems are observed. This is discussed in the next section.
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Tuning object retrieval
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In most cases, your NHibernate applications will perform well when it comes to fetching data from the database. But occasionally, you may notice that some areas of your application aren t performing as well as they should. There can be many reasons for this; you need to understand how to analyze and tune your NHibernate applications so they work efficiently with the database. Let s look at the steps involved when you re tuning the object-retrieval operations in your application. Enable the NHibernate SQL log, as described in chapter 3. You should also be prepared to read, understand, and evaluate SQL queries and their performance characteristics for your specific relational model: will a single join operation be faster than two selects Are all the indexes used properly, and what is the cache-hit ratio inside the database Get your DBA to help you with the performance evaluation; only she will have the knowledge to decide which SQL execution plan is the best. Step through your application use case by use case, and note how many and what SQL statements NHibernate executes. A use case can be a single screen in your web application or a sequence of user dialogs. This step also involves collecting the object-retrieval methods you use in each use case: walking the graph, retrieval by identifier, HQL, and criteria queries. Your goal is to bring down the number (and complexity) of SQL queries for each use case by tuning the default fetching strategies in metadata. You may encounter two common issues:
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If the SQL statements use join operations that are too complex and slow, set outer-join to false for <many-to-one> associations (this is enabled by default). Also try to tune with the global hibernate.max_fetch_depth configuration option, but keep in mind that this is best left at a value between 1 and 4. If too many SQL statements are executed, use lazy="true" for all collection mappings; by default, NHibernate will execute an immediate additional fetch for the collection elements (which, if they re entities, can cascade further into the graph). In rare cases, if you re sure, enable outer-join="true" and disable lazy loading for particular collections. Keep in mind that only one collection property per persistent class may be fetched eagerly. Use batch fetching with values between 3 and 15 to further optimize collection fetching if the given unit of work involves several of the same collections or if you re accessing a tree of parent and child objects.
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After you set a new fetching strategy, rerun the use case and check the generated SQL again. Note the SQL statements, and go to the next use case.
Summary
After you optimize all use cases, check every use case again and see if any optimizations had side effects for others. With some experience, you ll be able to avoid negative effects and get it right the first time. This optimization technique isn t practical for more than the default fetching strategies; you can also use it to tune HQL and criteria queries, which can ignore and override the default fetching for specific use cases and units of work. We discuss runtime fetching in chapter 8. In this section, you ve started to think about performance issues, especially issues related to association fetching. The quickest way to fetch a graph of objects is to fetch it from the cache in memory, as we show in the next chapter.
Summary
The dynamic aspects of the object/relational mismatch are just as important as the better-known and better-understood structural mismatch problems. In this chapter, we were primarily concerned with the lifecycle of objects with respect to the persistence mechanism. We discussed the three object states defined by NHibernate: persistent, detached, and transient. Objects transition between these states when you invoke methods of the ISession interface, or when you create and remove references from a graph of already persistent instances. This latter behavior is governed by the configurable cascade styles available in NHibernate s model for transitive persistence. This model lets you declare the cascading of operations (such as saving or deletion) on a per-association basis, which is more powerful and flexible than the traditional persistence by reachability model. Your goal is to find the best cascading style for each association and therefore minimize the number of persistence manager calls you have to make when storing objects. Retrieving objects from the database is equally important: you can walk the graph of domain objects by accessing properties and let NHibernate transparently fetch objects. You can also load objects by identifier, write arbitrary queries in the HQL, or create an object-oriented representation of your query using the query by criteria API. In addition, you can use native SQL queries in special cases. Most of these object-retrieval methods use the default fetching strategies we defined in mapping metadata (HQL ignores them; criteria queries can override them). The correct fetching strategy minimizes the number of SQL statements that have to be executed by lazily, eagerly, or batch-fetching objects. You optimize your NHibernate application by analyzing the SQL executed in each use case and tuning the default and runtime fetching strategies. Next, we explore the closely related topics of transactions and caching.
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