【Rails】テーブル削除(drop_table)の注意点！エラー対処法：ActiveRecord::IrreversibleMigration: To avoid mistakes, drop_table is only reversible if given options or a block (can be empty).

class DropTestCamel < ActiveRecord::Migration[6.1]
  def change
    drop_table :test_camels
  end
end

rails g migration Drop<テーブル名>

# rails g migration DropTests
Running via Spring preloader in process 85
      invoke  active_record
      create    db/migrate/20210804002221_drop_tests.rb

class DropTests < ActiveRecord::Migration[6.1]
  def change
  end
end

class <クラス名> < ActiveRecord::Migration[6.1]
  def change
    drop_table :<テーブル名> do |t|
      t.<型1> :<カラム名1>, <オプション: 値>,,,
      t.<型2> :<カラム名2>, <オプション: 値>,,,
      ,,,
    end
  end
end

class <クラス名> < ActiveRecord::Migration[6.1]
  def change
    drop_table :<テーブル名> do
    end
  end
end

class DropTestCamel < ActiveRecord::Migration[6.1]
  def change
    drop_table :test_camels
  end
end

class DropTestCamel < ActiveRecord::Migration[6.1]
  def change
    drop_table :test_camels do |t|
      t.string :name, null: false
    end
  end
end

# rails db:migrate:down VERSION=20210724075541
== 20210724075541 DropTestCamel: reverting ====================================
-- create_table(:test_camels)
   -> 0.2541s
== 20210724075541 DropTestCamel: reverted (0.2880s) ===========================

class <クラス名> < ActiveRecord::Migration[6.1]
  def up
    drop_table :<テーブル名>
  end

  def down
    drop_table :<テーブル名> do |t|
      t.<型1> :<カラム名1>, <オプション: 値>,,,
      t.<型2> :<カラム名2>, <オプション: 値>,,,
      ,,,
    end
  end
end

class DropTestCamel < ActiveRecord::Migration[6.1]
  def change
    drop_table :test_camels
  end
end

class DropTestCamel < ActiveRecord::Migration[6.1]
  def up
    drop_table :test_camels
  end

  def down
    create_table :test_camels do |t|
      t.string :name, null: false
    end
  end
end

class <クラス名> < ActiveRecord::Migration[6.1]
  def up
    drop_table :<テーブル名>
  end

  def down
    raise ActiveRecord::IrreversibleMigration    
  end
end

class DropTestCamel < ActiveRecord::Migration[6.1]
  def change
    drop_table :test_camels
  end
end

class DropTestCamel < ActiveRecord::Migration[6.1]
  def up
    drop_table :test_camels
  end

  def down
    fail ActiveRecord::IrreversibleMigration, "ロールバックを許可していません"
  end
end

# rails db:migrate:down VERSION=20210724075541
== 20210724075541 DropTestCamel: reverting ====================================
rails aborted!
StandardError: An error has occurred, this and all later migrations canceled:



ロールバックを許可していません

/rails-vue/db/migrate/20210724075541_drop_test_camel.rb:7:in `down'
/rails-vue/bin/rails:5:in `<top (required)>'
/rails-vue/bin/spring:10:in `block in <top (required)>'
/rails-vue/bin/spring:7:in `tap'
/rails-vue/bin/spring:7:in `<top (required)>'

Caused by:
ActiveRecord::IrreversibleMigration:

ロールバックを許可していません

/rails-vue/db/migrate/20210724075541_drop_test_camel.rb:7:in `down'
/rails-vue/bin/rails:5:in `<top (required)>'
/rails-vue/bin/spring:10:in `block in <top (required)>'
/rails-vue/bin/spring:7:in `tap'
/rails-vue/bin/spring:7:in `<top (required)>'
Tasks: TOP => db:migrate:down
(See full trace by running task with --trace)

class ChangeProductsPrice < ActiveRecord::Migration[5.0]
  def up
    change_table :products do |t|
      t.change :price, :string
    end
  end

  def down
    change_table :products do |t|
      t.change :price, :integer
    end
  end
end

class ChangeProductsPrice < ActiveRecord::Migration[5.0]
  def change
    reversible do |dir|
      change_table :products do |t|
        dir.up   { t.change :price, :string }
        dir.down { t.change :price, :integer }
      end
    end
  end
end

class <クラス名> < ActiveRecord::Migration[5.0]
  def change
    reversible do |dir|
        dir.up   { マイグレーション時の処理 }
        dir.down { ロールバック時の処理 }
      end
    end
  end
end

class ExampleMigration < ActiveRecord::Migration[5.0]
  def change
    create_table :distributors do |t|
      t.string :zipcode
    end

    reversible do |dir|
      dir.up do
        # CHECK制約を追加
        execute <<-SQL
          ALTER TABLE distributors
            ADD CONSTRAINT zipchk
              CHECK (char_length(zipcode) = 5) NO INHERIT;
        SQL
      end
      dir.down do
        execute <<-SQL
          ALTER TABLE distributors
            DROP CONSTRAINT zipchk
        SQL
      end
    end

    add_column :users, :home_page_url, :string
    rename_column :users, :email, :email_address
  end
end