Rails 5：attr_accessor抛出NoMethodError（nil：NilClass的未定义方法“键”）：

Question

I have 2 non database attributes in my model. If one of them has a value, I need to return the other one in the json response:

class Car < ApplicationRecord

  attr_accessor :max_speed_on_track
  attr_accessor :track

  def attributes
    if !self.track.nil? 
      super.merge('max_speed_on_track' => self.max_speed_on_track)
    end
  end
end

The problem is that the line 'if !self.track.nil?' throws an error when the controller tries to return the json

Perhaps there is a better way as I read that using attr_accessor is a code smell.

What I am trying to do is if the user passes me a track value as a query parameter, then I pass that value to the model and it uses it to calculate the max_speed_on_track , and return that value.

Obviously if no track is provided by the user then I don't want to return max_speed_on_track in the json.

The controller method is very basic for now (I still need to add the code that checks for the track param). The code throws the error on the save line.

  def create
    @car = Car.new(car_params)

    if @car.save
      render json: @car, status: :created
    else
      render json: @car.errors, status: :unprocessable_entity
    end
  end

Answer 1

Try this out:

class Car < ApplicationRecord

  attr_accessor :max_speed_on_track
  attr_accessor :track

  def as_json(options = {})
    if track.present?
      options.merge!(include: [:max_speed_on_track])
    end
    super(options)
  end
end

Since Rails uses the attributes method, and you're only needing this for json output, you can override the as_json method just like in this article . This will allow you to include your max_speed_on_track method in your json output when the track is present (not nil).