Creating a static model hierarchy / tree?

How do i create an empty static model hierarchy / tree in a simple manner without doing it manually?


at the moment iam doing it like this:

$model_a = new model_a;

$model_a->model_b = new model_b;

$model_a->model_c = new model_c;

$model_a->model_d = new model_d;

$model_a->model_d->model_d_1 = new model_d_1;

$model_a->model_d->model_d_2 = new model_d_2;

$model_a->model_d->model_d_3 = new model_d_3;

is there a way to automatically create the static model tree above in a single line? something like:

$model_a = model_a::model('all'); // i know this does not work ! but it would be great.

the reason i do this "model tree" is i need to create "auto-draft" or working copies of the above structure for data persistence and other reasons.