为什么friendly-id在tabled上有add slug列，但是找不到？

Question

我有一篇文章

class CreateArticles < ActiveRecord::Migration[5.1]
def change
  create_table :articles do |t|
    t.string :title
    t.text :body

    t.timestamps
  end
end

结束

遵循friendly_id自述文件

1)。rails generate friendly_id

2.)向Article模型添加扩展

 class Article < ApplicationRecord
   extend FriendlyId
   friendly_id :title, use: :slugged'
 end

3)。添加slug field to article table

class AddSlugToArticle < ActiveRecord::Migration[5.1]
  def change
    add_column :articles, :slug, :string
    add_index :articles, :slug, unique: true
  end
end

打开web浏览器http://localhost:3000/article/new可以看到rails terminal

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这是文章控制器中的问题所在：

def show
  @article = Article.friendly.find(params[:id])
  @comments = @article.comments.order("created_at desc").paginate(:page => params[:page], per_page: 4)

  respond_to do |format|
    format.html
    format.json { render json: [@article, @comments], except: [:created_at, :updated_at] }
  end
end

此终端错误报告：

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