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:
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);
db.Store(user);
}
using (var db = Db4oEmbedded.OpenFile(databaseFileName))
{
var query = db.Query();
query.Constrain(typeof (User));
query.Descend("Name").Constrain("Adriano");
User user = (User) query.Execute()[0];
Console.WriteLine(user.Manager.Manager.Manager.Addresses[0].Street);
}
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.
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!
Adriano
