Ruby-on-rails – How to update attributes without validation

I've got a model with its validations, and I found out that I can't update an attribute without validating the object before.

I already tried to add on => :create syntax at the end of each validation line, but I got the same results.

My announcement model have the following validations:

  validates_presence_of :title
  validates_presence_of :description
  validates_presence_of :announcement_type_id

  validate :validates_publication_date
  validate :validates_start_date
  validate :validates_start_end_dates
  validate :validates_category
  validate :validates_province

  validates_length_of :title, :in => 6..255, :on => :save
  validates_length_of :subtitle, :in => 0..255, :on => :save
  validates_length_of :subtitle, :in => 0..255, :on => :save
  validates_length_of :place, :in => 0..50, :on => :save

  validates_numericality_of :vacants, :greater_than_or_equal_to => 0,  :only_integer => true
  validates_numericality_of :price, :greater_than_or_equal_to => 0,  :only_integer => true

My rake task does the following:

  task :announcements_expiration => :environment do
    announcements = Announcement.expired

    announcements.each do |a|
      #Gets the user that owns the announcement
      user = User.find(a.user_id)
      puts a.title + '...'

      a.state = 'deactivated'

      if a.update_attributes(:state => a.state)
        puts 'state changed to deactivated'
      else
        a.errors.each do |e|
          puts e
        end

      end
    end

This throws all the validation exceptions for that model, in the output.

Does anybody how to update an attribute without validating the model?