Loading objects in model, how do I do this?

[eluser]rvbd[/eluser]
Hi All,

I have a design question for the more experienced of you. I'm about to build a software using codeigniter and wanting use full object oriented way of doing it. I previously used codeigniter's 1.9.x MVC framework to some extend, pretty simple just using controller as the logic and models to persist into database.

What I can't find in the forums and Google is a clear explanation of how to load objects in the models. If I have a Car model for argument sake, and it should have multiple Tire objects which I may store in an array, what's the best way of doing so?

Should I include the Tire object directly from the Car model?
I can also do cross loading with the Tire model if I create one, but it won't help with generating multiple instances of Tire.

Forgive me if this has been asked over and over again, I just need a bit of pointer with this.

Thanks in advance.