Foundations of Programming - Karl Seguin

Chapter 9 - Back to Basics: Proxy This and Proxy That
Unfortunately, we've had to go to the database twice to load the appropriate SalesPerson and Model
- even though we aren't really using them. The truth is all we need is their ID (since that's what gets
inserted into our database), which we already have.
By creating a proxy, NHibernate lets us fully lazy-load an object for just this type of circumstance. The
first thing to do is change our mapping and enable lazy loading of both Models and SalesPeoples:
...
...
The proxy attribute tells NHibernate what type should be proxied. This will either be the actual class
you are mapping to, or an interface implemented by the class. Since we are using the actual class as our
proxy interface, we need to make sure all members are virtual - if we miss any, NHibernate will throw a
helpful exception with a list of non-virtual methods. Now we're good to go:
Sale sale = new Sale();
sale.SalesPerson = session.Load (1);
sale.Model = session.Load(2);
sale.Price = 25000;
session.Save(sale);
Notice that we're using Load instead of Get. The difference between the two is that if you're retrieving
a class that supports lazy loading, Load will get the proxy, while Get will get the actual object. With this
code in place we're no longer hitting the database just to load IDs. Instead, calling
Session.Load(2) returns a proxy - dynamically generated by NHibernate. The proxy will
have an id of 2, since we supplied it the value, and all other properties will be uninitialized. Any call to
another member of our proxy, such as sale.Model.Name will be transparently intercepted and the
object will be just-in-time loaded from the database.
Just a note, NHibernate's lazy-load behavior can be hard to spot when debugging code in Visual Studio.
That's because VS.NET's watch/local/tooltip actually inspects the object, causing the load to happen
right away. The best way to examine what's going on is to add a couple breakpoints around your code
and check out the database activity either through NHibernate's log, or SQL profiler.
Hopefully you can imagine how proxies are used by RhinoMocks for recording, replaying and verifying
interactions. When you create a partial you're really creating a proxy to your actual object. This proxy
intercepts all calls, and depending on which state you are, does its own thing. Of course, for this to work,
you must either mock an interface, or a virtual members of a class.
Foundations of Programming Copyright © Karl Seguin www.codebetter.com
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