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

Question

我正在关注这个 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，在添加带有指定代码的 list.html.erb 并尝试运行“rails server”后，打开时 http://localhost:3000 ，我收到此迁移错误:

当我运行“rails db:migrate RAILS_ENV=development”时，我在控制台中收到以下错误日志:

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在我开始创建项目时，它也为 Books 表创建了一对迁移文件，另一个为 Subjects 表创建了另一个):

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。当我现在只是创建数据库并将其用于整个教程时，我什至没有看到使用 self.up 和 self.down 函数的重要性。

我应该怎么做才能解决这个问题？我想我需要去掉 self.up 和 self.down 函数，然后再次运行 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来自以前的版本。

尝试删除它们并重新启动 Rails 服务器，看看是否可以解决问题。

如果不通过运行 rails db:reset 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，但我不认为对你来说应该是个问题)。