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_ab.model_a_id AND model_ab.model_b_id

If you like to study more about this topic then I recommend to read:

It helps a lot.