config.initializers.active_record_extensions.rb

class ActiveRecord::Base
  # Returns hash prepared to be converted to json
  #
  # Can take the same options as ActiveRecord::Base#to_json
  def prepare_json(options = {})
    if include_root_in_json
      { self.class.json_class_name => JsonSerializer.new(self, options).serializable_record }
    else
      JsonSerializer.new(self, options).serializable_record
    end
  end
end

controller

  def update
    @document = Document.find(params[:document_id])

    saved = @document.update_attributes(params[:document])
    respond_to do |format|
      if @saved
        format.html {redirect_to documents_path }
      else
        format.html {render :action => 'edit'}
      end
        format.js {render :nothing => true}
        format.json { render :json => {
          :document => @document.prepare_json(:methods => :errors) } 
        }
    end
  end

environment.rb

ActiveRecord::Base.include_root_in_json = false

gistfile4.js

//Submit form with validation
$.fn.submitFormWithValidation = function(options) {
  var options = options || {};
  var saved = options.saved || function (){};
  var not_saved = options.not_saved || function (){};
  var object_name = options.object_name || 'item';

  return $(this).each(function() {
    options = jQuery.extend({
      type: "PUT",
      requestHeader: 'application/json',
      dataType: 'json',
      success: function (data, status) {
        var errors = data[object_name].errors;
        if(errors && errors.length > 0) {
          for(i in errors) {
            var error = errors[i];
            
            var field = $('form #document_'+error[0]);
            if(!field.parent().is('span.field-with-errors')) {
              field.wrap($('<span class="field-with-errors">'));
            }
          }
          not_saved.call(this, data, status);
        } else {
          saved.call(this, data, status);
        }
      },
      data: $(this).formSerialize()
    }, options);

    $.rjsAjax($(this).attr('action'), options);
  });
};

//Użycie:
$('#some-form').livequery("submit", function() {
  $(this).submitFormWithValidation({
    object_name: 'document',
    saved: function(data, status) {
      //do something if form was saved
    },
    not_saved: function(data, status) {
      //do something if form was not saved
    }
  });
  return false;
};