title: Datamapper "Update" does not update record
body: <p>I am having trouble with datamapper not updating a model. I can create and save models without issue. I have enabled <code>raise_on_save_failure</code> and checked the return value of <code>update</code> but see no errors. </p>
  
  <p>Here is the model:</p>
  
  <pre><code>class UserProfile   
    include DataMapper::Resource  
    attr_accessor :id, :wants_hints, :is_beta_user  
    property :id, Serial #auto-increment integer key  
    property :is_beta_user, Boolean  
    property :wants_hints, Boolean  
    has 1, :user, :through =&gt; Resource 
  end 
  </code></pre>
  
  <p>And here is where it is updated in the controller:</p>
  
  <pre><code>if user = User.get(request.session[:user])      
    if request.params[:user_profile]        
      beta = request.params[:user_profile].has_key?('is_beta_user')        
      hints = request.params[:user_profile].has_key?('wants_hints')        
      user.user_profile.update({:is_beta_user =&gt; beta, :wants_hints =&gt; hints}) # returns true
      Log.puts user.user_profile.errors.each {|e| Log.puts e.to_s} # returns empty list []
    end    
  end
  </code></pre>
  
  <p>When the controller is called <code>update</code> always returns true, and there are never errors in the error list. The datamapper log, which is set to <code>:debug</code>, only shows the <code>SELECT</code> queries for retrieving the <code>user</code> and <code>user_profile</code> and that is all. Why would I be able to <code>save</code> a newly created model, but not be allowed to <code>update</code> that same model?</p>