title: How to update attributes without validation
body: <p>I've got a model with its validations, and I found out that I can't update an attribute without validating the object before.</p>
  
  <p>I already tried to add <code>on =&gt; :create</code> syntax at the end of each validation line, but I got the same results.</p>
  
  <p>My announcement model have the following validations:</p>
  
  <pre><code>  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 =&gt; 6..255, :on =&gt; :save
    validates_length_of :subtitle, :in =&gt; 0..255, :on =&gt; :save
    validates_length_of :subtitle, :in =&gt; 0..255, :on =&gt; :save
    validates_length_of :place, :in =&gt; 0..50, :on =&gt; :save
  
    validates_numericality_of :vacants, :greater_than_or_equal_to =&gt; 0,  :only_integer =&gt; true
    validates_numericality_of :price, :greater_than_or_equal_to =&gt; 0,  :only_integer =&gt; true
  </code></pre>
  
  <p>My rake task does the following:</p>
  
  <pre><code>  task :announcements_expiration =&gt; :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 =&gt; a.state)
          puts 'state changed to deactivated'
        else
          a.errors.each do |e|
            puts e
          end
  
        end
      end
  </code></pre>
  
  <p>This throws all the validation exceptions for that model, in the output.</p>
  
  <p>Does anybody how to update an attribute without validating the model?</p>