rails ams, Nested models undefined method `' for nil:NilClass

问题

I have the following models:

class Appeal < ActiveRecord::Base
  belongs_to :applicant, :autosave => true
  belongs_to :appealer, :autosave => true
end

class Appealer < ActiveRecord::Base
  has_many :appeals, :autosave => true
end

class Applicant < ActiveRecord::Base
  has_many :appeals
end

What I want is for every appealer to hold a reference to the applicant of his last appeal

so I modified Appealer model to:

class Appealer < ActiveRecord::Base
  has_many :appeals, :autosave => true

  def last_applicant
    return self.appeals.last.applicant
  end
end

but I get the error:

undefined method `applicant' for nil:NilClass

what is strange that if I debug this (via RubyMine - Evaluate Expression) I can get the applicant.

If I try to get the last appeal:

class Appealer < ActiveRecord::Base
  has_many :appeals, :autosave => true

  def last_appeal
    return self.appeals.last
  end
end

everything works.

Im working with active-model-serializer, tried to do it also in the serializer (I actually need this value on a specific call - not allover the model) but it also didnt worked with the same errors.

the AMS code:

class AppealerTableSerializer < ActiveModel::Serializer
  attributes :id, :appealer_id, :first_name, :last_name, :city
  has_many :appeals, serializer: AppealMiniSerializer

  def city
    object.appeals.last.appealer.city
  end

end

MY QUESTION: How can I get the nested object attributes in my JSON? What am I doing wrong?

EDIT: My controller call:

class AppealersController < ApplicationController
  def index
    appealers = Appealer.all
    render json: appealers, each_serializer: AppealerTableSerializer, include: 'appeal,applicant'
  end
end

I've tried with and without the include, still not works

回答1:

Maybe I am missing something, because this looks like you have Appealer record that doesn't have any appeals yet.

In such case, this code

def last_appeal
  return self.appeals.last
end

Will return nil, which won't raise any errors. But if you call this

def last_applicant
  return self.appeals.last.applicant
end

return self.appeals.last is nil and you try to call applicant method on nil object instead of Appeal object.

To fix it just add check for nil

class Appealer < ActiveRecord::Base
  has_many :appeals, :autosave => true

  def last_applicant
    last = self.appeals.last

    if last.nil?
      return nil
    else
      return last.applicant
    end
  end
end

来源：https://stackoverflow.com/questions/39150731/rails-ams-nested-models-undefined-method-for-nilnilclass