Data transfer with Migration (マイグレーション中のデータ移行)

Model.reset_column_information

class ChangeColumnTypeInCompanies < ActiveRecord::Migration[6.0]
  def up
    add_column    :companies, :r_id, :string, comment: 'registration_id', after: :id

    # マイグレーション中にデータ移行するので、直前に足したカラム情報を読み込む
    # https://github.com/rails/rails/blob/v5.1.4/activerecord/lib/active_record/migration.rb#L409-L424
    Company.reset_column_information
    Company.find_each {|company|
      company.update(r_id: company.registration_number&.to_s)
    }
    remove_column :companies, :registration_number
  end

  def down
    add_column    :companies, :registration_number, :integer
    Company.reset_column_information
    Company.find_each {|company|
      company.update(registration_number: company.r_id&.to_s)
    }
    remove_column :companies, :r_id

  end
end