Rubyはレールの移行エラー

Question

に私は宝石acts-as-taggable-onをインストールし、それを出力し、このう：

C:\Sites\novosti>rails db:migrate 
rails aborted! 
StandardError: An error has occurred, this and all later migrations canceled: 

Directly inheriting from ActiveRecord::Migration is not supported. Please specify the Rails release the migration was written for: 

    class ActsAsTaggableOnMigration < ActiveRecord::Migration[4.2] 
C:/Sites/novosti/db/migrate/20170708123900_acts_as_taggable_on_migration.acts_as_taggable_on_engine.rb:2:in `<top (required)>' 
bin/rails:4:in `require' 
bin/rails:4:in `<main>' 
StandardError: Directly inheriting from ActiveRecord::Migration is not supported. Please specify the Rails release the migration was written for: 

    class ActsAsTaggableOnMigration < ActiveRecord::Migration[4.2] 
C:/Sites/novosti/db/migrate/20170708123900_acts_as_taggable_on_migration.acts_as_taggable_on_engine.rb:2:in `<top (required)>' 
bin/rails:4:in `require' 
bin/rails:4:in `<main>' 
Tasks: TOP => db:migrate 
(See full trace by running task with --trace) 

だから、僕は[5.1]を追加し、それが問題を修正しましたが、別の1が

C:\Sites\novosti>rails db:migrate 
== 20170708123900 ActsAsTaggableOnMigration: migrating ======================== 
-- create_table(:tags) 
    -> 0.0028s 
-- create_table(:taggings) 
    -> 0.0033s 
-- add_index(:taggings, :tag_id) 
rails aborted! 
StandardError: An error has occurred, this and all later migrations canceled: 

Index name 'index_taggings_on_tag_id' on table 'taggings' already exists 
C:/Sites/novosti/db/migrate/20170708123900_acts_as_taggable_on_migration.acts_as_taggable_on_engine.rb:23:in `up' 
bin/rails:4:in `require' 
bin/rails:4:in `<main>' 
ArgumentError: Index name 'index_taggings_on_tag_id' on table 'taggings' already exists 
C:/Sites/novosti/db/migrate/20170708123900_acts_as_taggable_on_migration.acts_as_taggable_on_engine.rb:23:in `up' 
bin/rails:4:in `require' 
bin/rails:4:in `<main>' 
Tasks: TOP => db:migrate 
(See full trace by running task with --trace) 

が登場

Answer 1

テーブルのインデックス名は一意である必要があります。これはインデックスが既に存在するため、dbからのエラーです。移行前にデータベースのテーブルにインデックスを落とすか、インデックスがテーブル内で既にOKであればadd_indexを削除することができます