Rails6でインスタやTwitterのようなハッシュタグ検索を実装したい

初心者で、初めてオリジナルのアプリを作成しています。
github

ハッシュタグ検索→関連トピックの一覧表示を実装したい

Rails6で、ハッシュタグの便利な機能を実装したいと考えています。

見よう見まねで実装中に以下のエラーメッセージが発生しました。

発生している問題・エラーメッセージ①

詳細ページに遷移しようとした時

NoMethodError in Topics#show
undefined method `gsub' for #<Tag::ActiveRecord_Associations_CollectionProxy:0x00007fae28d01a50>
Extracted source (around line #3):

発生している問題・エラーメッセージ②

http://localhost:3000/topics/tagと直接打ち込んだ時

Couldn't find Topic with 'id'=tag

発生している問題・エラーメッセージ③

http://localhost:3000/topics/tag/testと直接打ち込んだ時

Routing Error
No route matches [GET] "/topics/tag/test"

該当のソースコード

Ruby
1Rails.application.routes.draw do
2  devise_for :users, controllers: {
3    invitations: 'users/invitations'
4  }
5  root to: 'topics#index'
6  resources :users, only: %i[edit update show]
7  resources :topics do
8    collection do
9      get 'all'
10      get 'about'
11      get 'search'
12      get 'tagsearch'
13      get '/topic/tag/:name', to: "topics#tag"
14    end
15    resources :comments, only: %i[create destroy]
16  end
17end

Ruby
1module TopicsHelper
2  def render_with_tags(tagname)
3    tagname.gsub(/[#＃][\w\p{Han}ぁ-ヶｦ-ﾟー]+/){|word| link_to word, "/topic/tag/#{word.delete("#")}"}.html_safe
4  end 
5end

Ruby
1class TopicsController < ApplicationController
2  before_action :set_topic, only: %i[show edit update destroy]
3  before_action :contributor_confirmation, only: %i[edit update destroy]
4
5  def index
6    @topics = Topic.limit(5).order(created_at: :desc)
7  end
8
9  def all
10    @topics = Topic.all.order(created_at: :desc)
11  end
12
13  def new
14    @topic = TopicsTag.new
15  end
16
17  def create
18    @topic = TopicsTag.new(topics_tag_params)
19    if @topic.valid?
20      @topic.save
21      redirect_to root_path
22    else
23      render 'new'
24    end
25  end
26
27  def show
28    @comment = Comment.new
29    @comments = @topic.comments.includes(:user)
30  end
31
32  def edit; end
33
34  def update
35    if @topic.update_attributes(topic_params)
36      redirect_to topic_path(@topic)
37    else
38      render :edit
39    end
40  end
41
42  def destroy
43    if @topic.destroy
44      redirect_to root_path
45    else
46      redirect_to topic_path(@topic)
47    end
48  end
49
50  def about; end
51
52  def search
53    @topics = Topic.search(params[:keyword])
54  end
55
56  def tagsearch
57    return nil if params[:keyword] == ''
58
59    tag = Tag.where(['tagname LIKE ?', "%#{params[:keyword]}%"])
60    render json: { keyword: tag }
61  end
62
63  def tag
64    @user = current_user
65    @tag = Tag.find_by(tagname: params[:name])
66    @topics = @tag.topics
67  end
68
69  private
70
71  def topics_tag_params
72    params.require(:topics_tag).permit(:title, :text, :image, :tagname).merge(user_id: current_user.id)
73  end
74
75  def topic_params
76    params.require(:topic).permit(:title, :text, :image).merge(user_id: current_user.id)
77  end
78
79  def set_topic
80    @topic = Topic.find(params[:id])
81  end
82
83  def contributor_confirmation
84    redirect_to root_path unless current_user == @topic.user
85  end
86end

Ruby
1<%= render "topics/header" %>
2<div class="card border-light mb-8" style="mx-auto;">
3  <div class="card-body">
4    <div class="card-header">
5      <h5><%= @topic.title %></h5>
6    </div>
7    <%= image_tag @topic.image.variant(resize: '400'), class: 'd-block user-select-none' if @topic.image.attached?%>
8    <p class="card-text"><%= @topic.text %></p>
9      <% @topic.tags.each do |tag| %>
10        <%= tag.tagname%>
11      <% end %>
12      <%= render_with_tags(@topic.tags) %>
13    <h6 class="text-muted">by <%= link_to @topic.user.name, user_path(@topic.user) %>
14    <%= @topic.created_at %></h6>
15    <% if current_user == @topic.user%>
16      <%= link_to "編集する", edit_topic_path(@topic), class: "badge badge-warning" %>
17      <%= link_to "削除する", topic_path(@topic), method: :delete, class: "badge badge-warning" %>
18    <% end %>
19  </div>
20</div>

Ruby
1class Topic < ApplicationRecord
2  belongs_to :user
3  has_one_attached :image
4  has_many :topic_tag_relations, dependent: :destroy
5  has_many :tags, through: :topic_tag_relations
6  has_many :comments, dependent: :destroy
7  accepts_nested_attributes_for :topic_tag_relations
8
9  validate :text, unless: :was_attached?
10
11  def was_attached?
12    image.attached?
13  end
14
15  def self.search(search)
16    if search != ''
17      Topic.where('text LIKE(?)', "%#{search}%")
18    else
19      Topic.all
20    end
21  end
22
23  after_create do
24    topic = Topic.find_by(id: self.id)
25    tags  = self.tagname.scan(/[#＃][\w\p{Han}ぁ-ヶｦ-ﾟー]+/)
26    tags.uniq.map do |tag|
27      tag = Tag.find_or_create_by(tagname: tag.downcase.delete('#'))
28      topic.tags << tag
29    end
30  end
31
32  before_update do 
33    topic = Topic.find_by(id: self.id)
34    topic.tags.clear
35    tags = self.tagname.scan(/[#＃][\w\p{Han}ぁ-ヶｦ-ﾟー]+/)
36    tags.uniq.map do |tag|
37      tag = Tag.find_or_create_by(tagname: tag.downcase.delete('#'))
38      topic.tags << tag
39    end
40  end
41end