after_validation

class Person
  include ActiveModel::Validations
  include ActiveModel::Validations::Callbacks

  attr_accessor :name, :status

  validates_presence_of :name

  after_validation :set_status

  private

  def set_status
    self.status = errors.empty?
  end
end

person = Person.new
person.name = ''
person.valid? # => false
person.status # => false
person.name = 'bob'
person.valid? # => true
person.status # => true