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Retrieving objects
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Using Load() has a further implication: the application may retrieve a valid reference (a proxy) to a persistent instance without hitting the database to retrieve its persistent state. Load() may not throw an exception when it doesn t find the persistent object in the cache or database; the exception may be thrown later, when the proxy is accessed. This behavior has an interesting application. Let s say lazy loading is enabled on the class Category, and analyze the following code:
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using( ISession session = sessionFactory.OpenSession() ) using( session.BeginTransaction() ) { Category parent = session.Load<Category>(anId); Console.WriteLine( parent.Id ); Category child = new Category("test"); child.ParentCategory = parent; session.Save(child); session.Transaction.Commit(); }
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You first load a category. NHibernate doesn t hit the database to do this: it returns a proxy. Accessing the identifier of this proxy doesn t cause its initialization (as long as the identifier is mapped with the access strategy "property" or "nosetter"). Then you link a new category to the proxy, and you save it. An INSERT statement is executed to save the row with the foreign key value of the proxy s identifier. No SELECT statement is executed! Now, let s explore arbitrary queries, which are far more flexible than retrieving objects by identifier.
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Introducing Hibernate Query Language
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Hibernate Query Language (HQL) is an object-oriented dialect of the familiar relational query language SQL. HQL bears close resemblances to ODMG OQL and EJB-QL (from Java); but unlike OQL, it s adapted for use with SQL databases, and it s much more powerful and elegant than EJB-QL. JPA QL is a subset of HQL. HQL is easy to learn with a basic knowledge of SQL. HQL isn t a data-manipulation language like SQL. It s used only for object retrieval, not for updating, inserting, or deleting data. Object-state synchronization is the job of the persistence manager, not the developer. Most of the time, you ll only need to retrieve objects of a particular class and restrict by the properties of that class. For example, the following query retrieves a user by first name:
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IQuery q = session.CreateQuery("from User u where u.Firstname = :fname"); q.SetString("fname", "Max"); IList<User> result = q.List<User>();
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After preparing query q, you bind the identifier value to a named parameter, fname. The result is returned as a generic IList of User objects. Note that, instead of obtaining this list, you can provide one using q.List (myEmptyList), and NHibernate will fill it. This is useful when you want to use a collection with additional functionalities (like advanced data binding).
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Working with persistent objects
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HQL is powerful, and even though you may not use the advanced features all the time, you ll need them for some difficult problems. For example, HQL supports the following:
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Applying restrictions to properties of associated objects related by reference or held in collections (to navigate the object graph using query language). Retrieving only properties of an entity or entities, without the overhead of loading the entity itself in a transactional scope. This is sometimes called a report query; it s more correctly called projection. Ordering the query s results. Paginating the results. Aggregating with group by, having, and aggregate functions like sum, min, and max. Performing outer joins when retrieving multiple objects per row. Calling user-defined SQL functions. Performing subqueries (nested queries).
We discuss all these features in chapter 8, together with the optional native SQL query mechanism. We now look at another approach to issuing queries with NHibernate: Query by Criteria.
Query by Criteria
The NHibernate Query by Criteria (QBC) API lets you build a query by manipulating criteria objects at runtime. This approach lets you specify constraints dynamically without direct string manipulations, but it doesn t lose much of the flexibility or power of HQL. On the other hand, queries expressed as criteria are often less readable than queries expressed in HQL. Retrieving a user by first name is easy using a Criteria object:
ICriteria criteria = session.CreateCriteria(typeof(User)); criteria.Add( Expression.Like("Firstname", "Pierre Henri") ); IList result = criteria.List();
An ICriteria is a tree of ICriterion instances. The Expression class provides static factory methods that return ICriterion instances. Once the desired criteria tree is built, it s executed against the database. Many developers prefer QBC, considering it a more object-oriented approach. They also like the fact that the query syntax may be parsed and validated at compile time, whereas HQL expressions aren t parsed until runtime. The nice thing about the NHibernate ICriteria API is the ICriterion framework. This framework allows extension by the user, which is difficult in the case of a query language like HQL.
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