A 3-layer `relation` in D7?
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Using the `relations` module, I've been trying to create a relationship as follows: car belongs to model, model belongs to manufacturer.  As you can see, there are three layers, from the top down they are: `manufacturer`, `model`, and `car`.

The relations module allows you to add new relations.  The example by NodeOne at DrupalCity showed a friend-to-friend relationship: it's a non-directional relationship (friend a is friend to friend b; therefore, friend b is friend to friend a).  It's always `a->b & b->a`.  This kind of relationship only requires one machine name "is friend to", so that it can be put in human-readable form with the entities that are related to one another.  Also, the "arity" of a friend-friend relationship is 2 (min 2, max 2)....

With the manufacturer->model->car relationship, the two major difference are directional.  When `a contains b` works, `b contains a` does not work.  There need to be two machine-readable names per relationship.  Compounding this, machine names all need to be unique in Drupal, making 'contains' superfluous in the most unwelcoming way.

I can see possible solutions, but many of these seem ambiguous (for example, `manufacturer contains models and cars` can be arranged, but a manufacturer isn't supposed to directly contain a car--"`manufacturers contain cars through models`" seems more accurate, on-site....).

I'm not sure how to arrange this.  What should I do? 