I was finally able to eager load everything, using:
data = current_country.manufacturers.to_json :include => [{:cars => {:methods => [:attr_accessor_1, :attr_accessor_1]}}, :comfort_levels]
The above call did not nest comfort level
records inside car
records, however, the following did:
data = current_country.manufacturers.to_json :include => {:cars => {:methods => [:attr_accessor_1, :attr_accessor_1, :comfort_levels]}}
This link helped me clear some concepts around the syntax for eager loading.