Railsプラグインacts-as-taggable-onでタグ機能を実装

# add gemfile
gem 'acts-as-taggable-on', '~> 6.0'

# commands
bundle install
rake acts_as_taggable_on_engine:install:migrations
rake db:migrate

class User < ActiveRecord::Base
  acts_as_taggable # Alias for acts_as_taggable_on :tags
  acts_as_taggable_on :skills, :interests
end

class UsersController < ApplicationController
  def user_params
    params.require(:user).permit(:name, :tag_list) ## Rails 4 strong params usage
  end
end

@user = User.new(:name => "Bobby")
@user.tag_list.add("awesome")   # add a single tag. alias for <<
@user.tag_list.remove("awesome") # remove a single tag
@user.save # save to persist tag_list

@hashmodel = Hashmodel.find(1)
@hashmodel.tag_list = "hoge,fuga"
@hashmodel.save
Hashmodel.find(1).tag_list #=> ["hoge", "fuga"]
Hashmodel.find(6).tag_list #=> ["foo", "fuga"]

Hashmodel.tagged_with("fuga").by_join_date.ids

# Userモデルのタグ"awesome"を含んだオブジェクトを返す
User.tagged_with("awesome").by_join_date

# e.g. Hashmodel.tagged_with("fuga")[0].id
# => 1
# Hashmodel.tagged_with("fuga,hoge")[0].id
# 配列ではなくコンマ区切り文字列でもOK

# 全てのタグを含むオブジェクトを返す(AND検索)
User.tagged_with(["awesome", "cool"], :match_all => true)

# 1つでもマッチすれば(OR検索)
User.tagged_with(["awesome", "cool"], :any => true)

# 除外；タグを含まない
User.tagged_with(["awesome", "cool"], :exclude => true)

rails g controller tags index show

class TagsController < ApplicationController
  def index
    @tags = ActsAsTaggableOn::Tag.all
  end

  def show
    @tag =  ActsAsTaggableOn::Tag.find(params[:id])
    @posts = Hashmodel.tagged_with(@tag.name)
  end
end

<h1>Tags List</h1>
<table>
  <thead>
    <tr>
      <th>Tag Name</th>
      <th>Tag count</th>
      <!-- <th colspan="3"></th> -->
    </tr>
  </thead>

  <tbody>
    <% @tags.each do |tag| %>
      <tr>
        <td><%= tag.name %></td>
        <td><%= tag.taggings_count %></td>
      </tr>
    <% end %>
  </tbody>
</table>

<h1>Tag::<%= @tag.name %></h1>
<ul>
<% @posts.each do |post| %>
  <li><%= post.title %></li>
<% end %>
</ul>