request.rb

class Request < ActiveRecord::Base
  before_save :reduce_users_karma

  validate :validate_karma
  def validate_karma
    if user
      errors.add(:karma, "You don't have enough karma") unless user.karma_current > karma
    else
      errors.add(:user, "No user in request")
    end
  end

  def reduce_users_karma
    user.karma -= karma
    user.save
  end
end

requests_controller.rb

def create
    @req = Request.new(params[:request])

    if @req.save
      flash[:notice] = "Request created!"
      redirect_to requests_path
    else
      flash[:error] = "We couldn't create your request, sorry."
      render :action => 'new'
    end
  end