topic.rb

# @topic = Topic.new(params[:topic])
# @topic.this_user = current_user

class Topic < ActiveRecord::Base
  belongs_to :user
  attr_accessor :this_user

  def validate_on_update
    # Make sure that this_user is an instance of User, otherwise just use the id
    errors.add(:user, "Only the topic creator can update the topic") if user_id != this_user.id;
  end
end