def self.down
    # By default, we don't want to make any assumption about how to roll back a migration when your
    # model already existed. Please edit below which fields you would like to remove in this migration.
    raise ActiveRecord::IrreversibleMigration
end

`StandardError: An error has occurred, this and all later migrations canceled:

ActiveRecord::IrreversibleMigration`

ActiveRecord::Connection.base.execute('your ALTER query')