rails ams，nil:NilClass的嵌套模型未定义方法'

问题描述：

我有以下型号:

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

我想要的是每个上诉人都对他的最后上诉的申请人提起参考文件

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

所以我将Appealer模型修改为:

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

但是我得到了错误:

undefined method `applicant' for nil:NilClass

奇怪的是，如果我调试它(通过RubyMine-评估表达式)，我可以找到申请人.

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

如果我想上诉到最后，

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

  def last_appeal
    return self.appeals.last
  end
end

一切正常.

我正在使用active-model-serializer，尝试在序列化器中也这样做(我实际上在特定的调用中需要此值-并非遍及整个模型)，但是它也没有出现相同的错误.

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.

AMS代码:

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

我的问题: 如何在JSON中获取嵌套对象属性? 我在做什么错了?

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

我的控制器呼叫:

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

无论是否有包含，我都尝试过，仍然无法正常工作

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

答

也许我遗漏了一些东西，因为这看起来像您具有上诉人"记录，但还没有任何上诉.

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

在这种情况下，此代码

def last_appeal
  return self.appeals.last
end

将返回nil，这不会引发任何错误.但是如果你叫这个

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为nil，您尝试在nil对象而不是Appeal对象上调用applicant方法.

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

要修复此问题，只需添加nil检查即可

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

rails ams，nil:NilClass的嵌套模型未定义方法'

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