ruby on rails - has_one :through association incorrectly returns nil for a new_record -

i have following models. yes, design convoluted, because we're revamping our architecture in stages; nonetheless, think principles here should work:

class translation < activerecord::base   belongs_to :translation_service_option   belongs_to :service, inverse_of: :translation, class_name: "translationservice"   # have tried belongs_to :translation_service, inverse_of: :translation, foreign_key: :service_id end  class translationservice < service   # service < activerecord::base   has_one :translation, inverse_of: :service   # have tried inverse_of: :translation_service in conjunction above   has_one :translation_service_option, through: :translation end 

i trying build before_save callback on translationservice needs access translation_service_option, however, returning nil new record:

t = translation.last #=> #<translation id: 154, translation_service_option_id: 5, [...] t.service = translationservice.new #=> #<translationservice id: nil [...] t.translation_service_option #=> #<translationserviceoption id: 5 [...] t.service.translation_service_option #=> nil t.service.translation #=> #<translation id: 154 [...] t.service.translation.translation_service_option #=> #<translationserviceoption id: 5 

of course, once save object, works fine, issue need access in before_save (or before_create) callback:

t.service.save t.translation_service_option #=> #<translationserviceoption [...] 

so problem t.service returns nil translation_service_option, though t.service.translation has valid translation_service_option.

i t.service doesn't have id yet, does know it's translation object. why can't know it's translation_service_option object? use self.translation.try(:translation_service_option) in callback, starts messy callback uses methods used more generally, , 1 quirk find myself needing replace uses of translation_service_option translation.try(:translation_service_option), seems un-railsy , negates purpose of setting has_one :through association in first place.

is there way can set associations correctly assign t.service.translation_service_option on initialize?