迁移 DB Rails 时出错答案

【问题标题】：Error to Migrate DB Rails迁移 DB Rails 时出错
【发布时间】：2013-09-28 12:08:18
【问题描述】：

我正在尝试使用此迁移，但我不断收到错误，例如这些

这是我的迁移文件

class CreateEmployees < ActiveRecord::Migration
  def self.up
    create_table :employees do |t|
      t.string :name 
      t.date :hiredate 
      t.float :salary 
      t.boolean :fulltime , :default => true 
      t.integer :vacationdays 
      t.text :comments
    end 
  end

  def self.down
    drop_table :employees
  end
end

当我尝试执行 rake db:migrate 时出现此错误：

== CreateEmployees：迁移 =========================================== ====== -- create_table(:employees) rake 中止！发生错误，此迁移和所有后续迁移均已取消：

SQLite3::SQLException: 表“employees”已经存在：CREATE TABLE "employees" ("id" INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL, "name" varchar (255), "hiredate" date, "salary" float, "fulltime" boolean DEFAULT 't', "vacationdays" 整数, "cmets" 文本) /库/Ruby/Gems/2.0.0/gems/sql ite3-1.3.8/lib/sqlite3/database.rb:91:in initialize' /Library/Ruby/Gems/2.0.0/gems/sqlite3-1.3.8/lib/sqlite3/database.rb:91:innew' /Library/Ruby/Gems/2.0.0/gems/sqlite3-1.3.8/lib/sqlite3/database.rb:91:in prepare' /Library/Ruby/Gems/2.0.0/gems/sqlite3-1.3.8/lib/sqlite3/database.rb:134:in执行' /Library/Ruby/Gems/2.0.0/gems/activerecord-4.0.0/lib/active_record/connection_adapters/sqlite3_adapter.rb:328:in block in execute' /Library/Ruby/Gems/2.0.0/gems/activerecord-4.0.0/lib/active_record/connection_adapters/abstract_adapter.rb:425:inblock in log' /Library/Ruby/Gems/2.0.0/gems/activesupport-4.0.0/lib/active_support/notifications/instrumenter.rb:20:in instrument' /Library/Ruby/Gems/2.0.0/gems/activerecord-4.0.0/lib/active_record/connection_adapters/abstract_adapter.rb:420:inlog' /Library/Ruby/Gems/2.0.0/gems/activerecord-4.0.0/lib/active_record/connection_adapters/sqlite3_adapter.rb:328:in execute' /Library/Ruby/Gems/2.0.0/gems/activerecord-4.0.0/lib/active_record/connection_adapters/abstract/schema_statements.rb:190:increate_table' /Library/Ruby/Gems/2.0.0/gems/activerecord-4.0.0/lib/active_record/migration.rb:625:in block in method_missing' /Library/Ruby/Gems/2.0.0/gems/activerecord-4.0.0/lib/active_record/migration.rb:597:inblock in say_with_time' /Library/Ruby/Gems/2.0.0/gems/activerecord-4.0.0/lib/active_record/migration.rb:597:in say_with_time' /Library/Ruby/Gems/2.0.0/gems/activerecord-4.0.0/lib/active_record/migration.rb:617:inmethod_missing' /Library/Ruby/Gems/2.0.0/gems/activerecord-4.0.0/lib/active_record/migration.rb:383:in method_missing' /Users/YuriAguirre/Documents/Aptana Studio 3 Workspace/company/db/migrate/20130923140626_create_employees.rb:3:inup' /Library/Ruby/Gems/2.0.0/gems/activerecord-4.0.0/lib/active_record/migration.rb:534:in up' /Library/Ruby/Gems/2.0.0/gems/activerecord-4.0.0/lib/active_record/migration.rb:574:inexec_migration' /Library/Ruby/Gems/2.0.0/gems/activerecord-4.0.0/lib/active_record/migration.rb:555:in block (2 levels) in migrate' /Library/Ruby/Gems/2.0.0/gems/activerecord-4.0.0/lib/active_record/migration.rb:554:inblock in 迁移' /Library/Ruby/Gems/2.0.0/gems/activerecord-4.0.0/lib/active_record/connection_adapters/abstract/connection_pool.rb:294:in with_connection' /Library/Ruby/Gems/2.0.0/gems/activerecord-4.0.0/lib/active_record/migration.rb:553:inmigrate' /Library/Ruby/Gems/2.0.0/gems/activerecord-4.0.0/lib/active_record/migration.rb:709:in migrate' /Library/Ruby/Gems/2.0.0/gems/activerecord-4.0.0/lib/active_record/migration.rb:959:inblock 在 execute_migration_in_transaction' /Library/Ruby/Gems/2.0.0/gems/activerecord-4.0.0/lib/active_record/migration.rb:1005:in block in ddl_transaction' /Library/Ruby/Gems/2.0.0/gems/activerecord-4.0.0/lib/active_record/connection_adapters/abstract/database_statements.rb:202:inblock 在交易中' /Library/Ruby/Gems/2.0.0/gems/activerecord-4.0.0/lib/active_record/connection_adapters/abstract/database_statements.rb:210:in within_new_transact ion' /Library/Ruby/Gems/2.0.0/gems/activerecord-4.0.0/lib/active_record/connection_adapters/abstract/database_statements.rb:202:intransaction' /Library/Ruby/Gems/2.0.0/gems/activerecord-4.0.0/lib/active_record/transactions.rb:209:in transaction' /Library/Ruby/Gems/2.0.0/gems/activerecord-4.0.0/lib/active_record/migration.rb:1005:inddl_transaction' /Library/Ruby/Gems/2.0.0/gems/activerecord-4.0.0/lib/active_record/migration.rb:958:in execute_migration_in_transaction' /Library/Ruby/Gems/2.0.0/gems/activerecord-4.0.0/lib/active_record/migration.rb:920:inblock in 迁移' /Library/Ruby/Gems/2.0.0/gems/activerecord-4.0.0/lib/active_record/migration.rb:916:in each' /Library/Ruby/Gems/2.0.0/gems/activerecord-4.0.0/lib/active_record/migration.rb:916:inmigrate' /Library/Ruby/Gems/2.0.0/gems/activerecord-4.0.0/lib/active_record/migration.rb:764:in up' /Library/Ruby/Gems/2.0.0/gems/activerecord-4.0.0/lib/active_record/migration.rb:742:inmigrate' /Library/Ruby/Gems/2.0.0/gems/activerecord-4.0.0/lib/active_record/railties/databases.rake:42:in '任务中的`block（2个级别）：TOP => db：migrate（请参阅通过使用 --trace 运行任务来完整跟踪）

【问题讨论】：

您已收到答案：SQLite3::SQLException: table "employees" already exists 您没有提供足够的详细信息来判断创建此表的原因，可能是您手动创建的，也可能是由另一个迁移创建的，谁知道呢。
哦，现在我看到了，我是 Rails 的新手，如何重新创建或删除已经存在的表？
最好的方法是对 db 所做的所有更改都使用迁移，这样您以后就不会迷路了。我不知道您是想从头开始创建该表还是对其进行修改，后一种您应该使用add_column :table_name, :column_name, :column_type语法
哦，明白了，但我仍然不知道如何现在删除该表并再次创建该表，哈哈，因为我不知道是否使用正确的语句创建了该表。
您在提供的迁移中得到了答案；) 只需在 self.up 部分（或 change 而不是 self.up 和 self.down）中写入 drop_table :employees 或手动删除它从 sqlite 控制台。

标签： ruby-on-rails database dbmigrate

【解决方案1】：

您的员工表已在您的数据库中创建。您需要修复它，然后您可以通过两种方式进行修复：1. 删除数据库并再创建一个或 2. 运行向下迁移然后向上迁移

rake db:drop
rake db:create
rake db:migrate

【讨论】：

【解决方案2】：

正如我在评论中所写，您收到该错误的原因是：

SQLite3::SQLException: table "employees" already exists

这仅仅意味着您的数据库中已经有该列。您可以通过以下几种方式解决该问题：

将迁移更改为修改而不是创建：

class CreateEmployees < ActiveRecord::Migration
  def self.up
    add_column :employees, :name, :string 
    (...)
    end 
  end

  def self.down
    remove_column :employees, :name
  end
end

添加另一个迁移，这将删除整个表（显然，一个应该比修改一个具有更早的时间戳，因此表将首先被删除然后再次创建）：
```
 class RemoveEmployees < ActiveRecord::Migration
   def change
     drop_table :employees
   end
 end
```
从控制台手动删除表，但这不是一个很好的选择，因为您应该学习使用迁移来更改数据库，当然这是最快的（如果您知道如何使用控制台)，如果这只是一些学习项目，也不会造成任何伤害，但是，你应该尽可能地学习好习惯。

您可能还想使用更新的语法 (AFAIK) def change 而不是 self.up 和 self.down，减少输入字母和重复；)

【讨论】：

【解决方案3】：

您的迁移未完全运行。您应该回滚迁移并再次迁移。

rails db:rollback
rails db:migrate

【讨论】：

【解决方案4】：

您已经运行了该迁移，但其中有错误。所以迁移进行到一半，创建的表不完整。回滚该迁移，然后再次运行迁移。

【讨论】：