Models in Model, Composite Model,design problem, how to do?

[eluser]Imkow@CN[/eluser]
As my project is getting big and messy, each single model can't accomplish the job. A complex model seems needed.
For example: my Article model might need to contain Tag model (handling tags), File model(handling uploaded files) and Categroy Model(handling categroy infomation). Together, they form an article object handling post,view,updates of each article.

But in CI, how do i write the model object that makes use of many sub-objects?（$this->load->model is way not capable enough） or CI has some special way to deal in such scenario?