Finding rows not connected through :though

For example, you can use find_by_sql method (if you have a
corresponding tables model_a, model_b, model_ab):
...
a_without_b = ModelA.find_by_sql "SELECT model_a.* FROM model_a,
model_ab WHERE model_a.id = model_ab.model_a_id AND model_ab.model_b_id
= NULL"
...

If you like to study more about this topic then I recommend to read:
http://blog.hasmanythrough.com/articles/category/associations

It helps a lot.

Priit