Dec 29, 2010

What's wrong with this code (Part XII)

In the last post in this series ("what's wrong with this code") we discussed how db4o's reference system remembers which objects have been stored / retrieved.

In this post I'll show you another piece of code that doesn't always work as the developer expects:

class User
   public string Name;
   public User Manager;
   public IList Addresses;

   public User(string name, User manager)
      Name = name;
      Manager = manager;

class Address
   public string Street;

   public Address(string street)
      Street = street;

using (var db = Db4oEmbedded.OpenFile(databaseFileName))
   List<Address> addrs = new List();
   addrs.Add(new Address("Street1"));
   addrs.Add(new Address("Street2"));
   User fooManager = new User("Foo") { Addresses = addrs };

   User fooBarManager = new User("FooBar", fooManager);
   User barManager = new User("John Doe",  fooBarManager);
   User user = new User("Adriano",  barManager);

using (var db = Db4oEmbedded.OpenFile(databaseFileName))
   var query = db.Query();
   query.Constrain(typeof (User));
   User user = (User) query.Execute()[0];

Can you see the problem? No, I am not talking about Law of Demeter "violation" :) (By the way, if you are interested in this subject you can find a nice discussion here).

Again, if you have been working with db4o for some time I am sure you already spent some time debugging this kind of issue.

As always, the answer comes in a future post.

Have fun!