使用attr_accessor返回导航字段值

Question

I have a belongs_to relation in my Patient model, then I need to create an attr_accessor to read the state_id from the city specified in the Patient object.

How can I do this?

If I just use attr_accessor :state_id, I'd have to set it in the class initialization and the value would be static. I need it to be read only and updated automatically according to the current value of City navigation property.

Answer 1

You don't need an attr_accessor you just need a method.

class Patient

  def state_id
    city.state_id
  end

end

If the city association is optional you may want to handle the possibility of city being nil...

class Patient

  def state_id
    city.try(:state_id)
  end

end

Answer 2

If you want your API for your Patient object to look something like @patient.state_id , then either SteveTurczyn's answer or delegation could work for you in order to read the state_id from the city specified in the Patient object:

class Patient
  # you can remove `allow_nil` if the patient will *always* have a city
  delegate :state_id, to: :city, allow_nil: true
end