i am using forem gem and i have post model decorator. i would like to make the post taggable and using the act-as-taggable-on gem. but it giving me error that tag_list method is undefine. when i do post = Forem.Post.new and post.tag_list = 'tag1 tag2' in

_form.html.erb

<%= f.input :text %>
<%= f.input :tag_list %>

<% if params[:reply_to_id] %>
  <%= f.hidden_field :reply_to_id, :value => params[:reply_to_id] %>
<% end %>

new.html.erb

<h2><%= t("forem.topic.new") %></h2>

<%= render "form" %>

post_decorator.rb

Forem::Post.class_eval do
  acts_as_taggable_on :tags
  attr_accessible :tag_list
end

topics_controller.rb

module Forem
  class TopicsController < Forem::ApplicationController
    helper 'forem/posts'
    before_filter :authenticate_forem_user, :except => [:show]
    before_filter :find_forum
    before_filter :block_spammers, :only => [:new, :create]

    def new
      authorize! :create_topic, @forum
      @topic = @forum.topics.build
      @topic.posts.build
    end

    def create
      authorize! :create_topic, @forum
      @topic = @forum.topics.build(params[:topic])
      @topic.user = forem_user
      if @topic.save
        flash[:notice] = t("forem.topic.created")
        redirect_to [@forum, @topic]
      else
        flash.now.alert = t("forem.topic.not_created")
        render :action => "new"
      end
    end

    

    private
    def find_forum
      @forum = Forem::Forum.find(params[:forum_id])
      authorize! :read, @forum
    end

    def find_topic
      begin
        scope = forem_admin_or_moderator?(@forum) ? @forum.topics : @forum.topics.visible.approved_or_pending_review_for(forem_user)
        @topic = scope.find(params[:id])
        authorize! :read, @topic
      rescue ActiveRecord::RecordNotFound
        flash.alert = t("forem.topic.not_found")
        redirect_to @forum and return
      end
    end

    def register_view
      @topic.register_view_by(forem_user)
    end

    def block_spammers
      if forem_user.forem_state == "spam"
        flash[:alert] = t('forem.general.flagged_for_spam') + ' ' + t('forem.general.cannot_create_topic')
        redirect_to :back
      end
    end
  end
end