Question

我正在关注此Ruby on Rails tutorial from TutorialsPoint.com。我对Ruby on Rails开发完全陌生，并且已经遇到了一些小问题。

我正在使用Ruby 2.3.3p222（2016-11-21修订版56859）[i386-mingw32]和Ruby on Rails 5.1.5。

出于某种原因，在使用指定的代码添加list.html.erb并尝试运行＆＃34; rails server＆＃34;后，在打开http://localhost:3000时，我收到此迁移错误：

当我运行＆＃34; rails db：migrate RAILS_ENV = development＆＃34;时，我在控制台中收到了这个错误日志：

C:\Users\gregp\Documents\<secret directory>\Programming Experiments\TutorialsPointRoR\demo2>rails db:migrate RAILS_ENV=development
== 20180221025323 Books: migrating ============================================
-- create_table(:books)
rails aborted!
StandardError: An error has occurred, this and all later migrations canceled:

PG::DuplicateTable: ERROR:  relation "books" already exists
: CREATE TABLE "books" ("id" bigserial primary key, "title" character varying(32) NOT NULL, "price" float, "subject_id" integer, "description" text, "created_at" timestamp)
C:/Users/gregp/Documents/<secret directory>/Programming Experiments/TutorialsPointRoR/demo2/db/migrate/20180221025323_books.rb:3:in `up'
bin/rails:4:in `require'
bin/rails:4:in `<main>'

Caused by:
ActiveRecord::StatementInvalid: PG::DuplicateTable: ERROR:  relation "books" already exists
: CREATE TABLE "books" ("id" bigserial primary key, "title" character varying(32) NOT NULL, "price" float, "subject_id" integer, "description" text, "created_at" timestamp)
C:/Users/gregp/Documents/<secret directory>/Programming Experiments/TutorialsPointRoR/demo2/db/migrate/20180221025323_books.rb:3:in `up'
bin/rails:4:in `require'
bin/rails:4:in `<main>'

Caused by:
PG::DuplicateTable: ERROR:  relation "books" already exists
C:/Users/gregp/Documents/<secret directory>/Programming Experiments/TutorialsPointRoR/demo2/db/migrate/20180221025323_books.rb:3:in `up'
bin/rails:4:in `require'
bin/rails:4:in `<main>'
Tasks: TOP => db:migrate
(See full trace by running task with --trace)

我在db / migrate目录中有这些文件（我不知道为什么即使我开始创建项目，它为Books表创建了一对迁移文件，为Subjects表创建了另一个）： / p>

20180221024754_create_books.rb：

class CreateBooks < ActiveRecord::Migration[5.1]
  def change
    create_table :books do |t|
       t.column :title, :string, :limit => 32, :null => false
       t.column :price, :float
       t.column :subject_id, :integer
       t.column :description, :text
       t.column :created_at, :timestamp
    end
  end
end

20180221024922_create_subjects.rb：

class CreateSubjects < ActiveRecord::Migration[5.1]
  def change
    create_table :subjects do |t|
       t.column :name, :string
    end

    Subject.create :name => "Physics"
    Subject.create :name => "Mathematics"
    Subject.create :name => "Chemistry"
    Subject.create :name => "Psychology"
    Subject.create :name => "Geography"
  end
end

20180221025323_books.rb：

class Books < ActiveRecord::Migration[5.1]
  def self.up
      create_table :books do |t|
         t.column :title, :string, :limit => 32, :null => false
         t.column :price, :float
         t.column :subject_id, :integer
         t.column :description, :text
         t.column :created_at, :timestamp
      end
   end

   def self.down
      drop_table :books
  end
end

20180221025434_subjects.rb：

class Subjects < ActiveRecord::Migration[5.1]
  def self.up

     create_table :subjects do |t|
        t.column :name, :string
     end

     Subject.create :name => "Physics"
     Subject.create :name => "Mathematics"
     Subject.create :name => "Chemistry"
     Subject.create :name => "Psychology"
     Subject.create :name => "Geography"
  end

  def self.down
     drop_table :subjects
  end
end

这真的很奇怪，因为我不认为我应该在迁移文件之间有重复的代码。当我现在只创建数据库并将其用作整个教程时，我甚至看不到使用self.up和self.down函数的重要性。

我该怎么做才能解决这个问题？我想我需要摆脱self.up和self.down函数，再次运行rake db：migrate，但我不知道是否需要额外的步骤或其他方式。

Answer 1

我不确定教程的来源，但最后两个迁移文件看起来可能与以前的版本相同。

尝试删除它们并重新启动Rails服务器，看看是否修复了它。

如果没有通过运行<system.serviceModel> <services> <service name="WcfService1.Service1" behaviorConfiguration="ServiceBehaviour"> <endpoint address="" behaviorConfiguration="web" binding="webHttpBinding" contract="WcfService1.IService1"/> <endpoint contract="IMetadataExchange" binding="mexHttpBinding" address="mex"/> </service> </services> <behaviors> <endpointBehaviors> <behavior name="web"> <webHttp helpEnabled="true" automaticFormatSelectionEnabled="false" /> </behavior> </endpointBehaviors> <serviceBehaviors> <behavior name="ServiceBehaviour"> <serviceMetadata httpGetEnabled="true" httpsGetEnabled="true"/> <serviceDebug includeExceptionDetailInFaults="false"/> </behavior> </serviceBehaviors> </behaviors>  <serviceHostingEnvironment aspNetCompatibilityEnabled="true" multipleSiteBindingsEnabled="true" /> </system.serviceModel>重置数据库（这相当于删除数据库然后重新创建它。数据库中的任何数据都将丢失，但我不认为这应该是你的问题）。

Answer 2

似乎您的/db/migrate文件夹中有相同的迁移文件。我的意思是schema.rb文件中存在的用于创建表的相同文件。因此，您只需从/db/migrate文件夹中删除这些文件，然后尝试再次运行rake db:migrate命令。

Answer 3

我遇到了这个问题。我使用了“ .Env”文件来存储我的PG凭据，如果是这种情况，请确保您的Gem文件中包含该gem。

gem 'dotenv-rails'

Ruby on Rails 5.1.5 - 以下TutorialsPoint教程中的迁移错误（已存在关系）

3 个答案: