ruby-on-rails - 未定义方法 `email' 为 nil :NilClass link. user.email

Question

在我的 index.html.erb 文件中，我得到了一个未定义的电子邮件方法。我进入 Rails 控制台并尝试运行 User.first.email 并且它运行良好。我尝试取出电子邮件并运行 link.user，这也奏效了。我不确定为什么当用户绑定(bind)到链接并且用户有电子邮件时，电子邮件会显示为未定义。我正在寻找一些关于可能出现问题的建议。

Index.html.erb

<%= link.user.email %>

已经运行迁移的架构

create_table "users", force: :cascade do |t|
    t.string   "email",                  default: "", null: false

模型:

class Link < ActiveRecord::Base
    belongs_to :user
    validates :user, presence: true
    acts_as_votable
    attr_accessor :avatar
    mount_uploader :avatar, AvatarUploader
end

class User < ActiveRecord::Base
    has_many :links
    acts_as_voter
    devise :database_authenticatable, :registerable,
         :recoverable, :rememberable, :trackable, :validatable
end

链接 Controller :

class LinksController < ApplicationController
  before_filter :authenticate_user!, except: [:index, :show]

  def index
    @links = Link.all
  end

  def show
    @link = Link.find(params[:id])
  end

  def new
    @link = Link.new
  end

  def edit
  end

  def create
    @link = Link.new(link_params)

    if @link.save
      redirect_to root_path
    else
      render 'new'
    end
  end

    private
    def link_params
      params.require(:link).permit(:title, :url, :avatar, :user_id)
    end
end

错误信息:

User Load (0.1ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = ?  ORDER BY "users"."id" ASC LIMIT 1  [["id", 1]]
   (0.1ms)  begin transaction
   (0.1ms)  rollback transaction
  Rendered links/_form.html.erb (2.7ms)
  Rendered links/new.html.erb within layouts/application (3.8ms)
Completed 200 OK in 505ms (Views: 143.6ms | ActiveRecord: 0.4ms)

Answer 1

undefined method `email' for nil:NilClass link.user.email

您收到此错误是因为某些链接的 link.user 为 nil，因此它调用 nil.email 并因上述问题而失败错误信息。

您可以使用 try像这样摆脱这个问题:

<%= link.user.try(:email) %>

这样即使 link.user 在某些情况下为 nil，您的程序也不会崩溃。