Validation fail if trying to remove admin rights to last admin

user.rb

class User < ActiveRecord::Base
  validates :admin, last_stays: true
end

class LastStaysValidator < ActiveModel::EachValidator
  def validate_each(record, attribute, value)
    value_was = record.send("#{attribute}_was")

    # If last record with attribute was equal to true
    # it can't change this attribute value to false
    if value == false && value_was == true && last?(record, attribute)

      error = I18n.t('error.validation.last_stays', model: record.class.name)
      record.errors[attribute] << error
    end
  end

  private

  def last?(record, attribute)
    record.class.where("#{attribute}" => true).count <= 1
  end
end