Foundations of Programming - Karl Seguin

Chapter 6 - Object Relational Mappers
supports OO principles like inheritance and polymorphism. Either query methods are abstractions on
top of SQL, which means you get total portability – all you need to do to target a different database is
change your dialect configuration.
HQL works off of the IQuery interface, which is created by calling CreateQuery on your session. With
IQuery you can return individual entities, collections, substitute parameters and more. Here are some
examples:
string lastName = "allen";
ISession session = _sessionFactory.OpenSession();
//retrieve a salesperson by last name
IQuery query = s.CreateQuery("from SalesPerson p where p.LastName =
'allen'");
SalesPerson p = query.UniqueResult();
//same as above but in 1 line, and with the last name as a variable
SalesPerson p = session.CreateQuery("from SalesPerson p where p.LastName =
?").SetString(0, lastName).UniqueResult();
//people with few sales
IList slackers = session.CreateQuery("from SalesPerson person
where size(person.Sales) < 5").List();
This is just a subset of what can be accomplished with HQL (the downloadable sample has slightly more
complicated examples).
Lazy Loading
When we load a salesperson, say by doing: SalesPerson person =
session.Get(1); the Sales collection won’t be loaded. That’s because, by default,
collections are lazily loaded. That is, we won’t hit the database until the information is specifically
requested (i.e., we access the Sales property). We can override the behavior by setting
lazy=”false” on the bag element.
The other, more interesting, lazy load strategy implemented by NHibernate is on entities themselves.
You’ll often want to add a reference to an object without having to load the actual object from the
database. For example, when we add a Sales to a SalesPerson, we need to specify the Model, but
don’t want to load every property – all we really want to do is get the Id so we can store it in the
ModelId column of the Sales table. When you use session.Load(id) NHibernate will load a
proxy of the actual object (unless you specify lazy=”false” in the class element). As far as you’re
concerned, the proxy behaves exactly like the actual object, but none of the data will be retrieved from
the database until the first time you ask for it. This makes it possible to write the following code:
Sale sale = new Sale(session.Load(1), DateTime.Now, 46000.00);
salesPerson.AddSales(sale);
Foundations of Programming Copyright © Karl Seguin www.codebetter.com
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