post.rb

  def tag_list=(tags_string, current_user)
    self.taggings.destroy_all

    tag_names = tags_string.split(",").collect{|s| s.strip.downcase}.uniq
 
    tag_names.each do |tag_name|
      tag = current_user.tags.find_or_create_by_name(tag_name)
      tagging = self.taggings.new # <= code smell
      tagging.tag_id = tag.id     # <= code smell
    end
  end

# OR, if the post belongs to the user, then

  def tag_list=(tags_string)
    self.taggings.destroy_all

    tag_names = tags_string.split(",").collect{|s| s.strip.downcase}.uniq
 
    tag_names.each do |tag_name|
      tag = self.user.tags.find_or_create_by_name(tag_name)
      tagging = self.taggings.new # <= code smell
      tagging.tag_id = tag.id     # <= code smell
    end
  end

but, seriously @Dolinski, what's the deal with line 8 and 9?