Tagging from Scratch in Rails 5

In this article, you will learn how to tag just one model in your Rails 5 app from scratch. This is based on railscast episode number 382. Create a new Rails 5 app.

rails new tug

Create an article model.

rails g model article name published_on:date content:text

Create sample data in seeds.rb:

batman = Article.create! name: "Batman", content: <<-ARTICLE
Batman is a fictional character created by the artist Bob Kane and writer Bill Finger. A comic book superhero, Batman first appeared in Detective Comics #27 (May 1939), and since then has appeared primarily in publications by DC Comics. Originally referred to as "The Bat-Man" and still referred to at times as "The Batman", he is additionally known as "The Caped Crusader", "The Dark Knight", and the "World's Greatest Detective," among other titles. (from Wikipedia)

superman = Article.create! name: "Superman", content: <<-ARTICLE
Superman is a fictional comic book superhero appearing in publications by DC Comics, widely considered to be an American cultural icon. Created by American writer Jerry Siegel and Canadian-born American artist Joe Shuster in 1932 while both were living in Cleveland, Ohio, and sold to Detective Comics, Inc. (later DC Comics) in 1938, the character first appeared in Action Comics #1 (June 1938) and subsequently appeared in various radio serials, television programs, films, newspaper strips, and video games. (from Wikipedia)

krypton = Article.create! name: "Krypton", content: <<-ARTICLE
Krypton is a fictional planet in the DC Comics universe, and the native world of the super-heroes Superman and, in some tellings, Supergirl and Krypto the Superdog. Krypton has been portrayed consistently as having been destroyed just after Superman's flight from the planet, with exact details of its destruction varying by time period, writers and franchise. Kryptonians were the dominant people of Krypton. (from Wikipedia)

lex_luthor = Article.create! name: "Lex Luthor", content: <<-ARTICLE
Lex Luthor is a fictional character, a supervillain who appears in comic books published by DC Comics. He is the archenemy of Superman, and is also a major adversary of Batman and other superheroes in the DC Universe. Created by Jerry Siegel and Joe Shuster, he first appeared in Action Comics #23 (April 1940). Luthor is described as "a power-mad, evil scientist" of high intelligence and incredible technological prowess. (from Wikipedia)

robin = Article.create! name: "Robin", content: <<-ARTICLE
Robin is the name of several fictional characters appearing in comic books published by DC Comics, originally created by Bob Kane, Bill Finger and Jerry Robinson, as a junior counterpart to DC Comics superhero Batman. The team of Batman and Robin is commonly referred to as the Dynamic Duo or the Caped Crusaders. (from Wikipedia)

Create the tag model with name attribute.

rails g model tag name

Create the tagging model.

rails g model tagging tag:belongs_to article:belongs_to

Add indexes to the generated migrations.

class CreateTaggings < ActiveRecord::Migration[5.0]
  def change
    create_table :taggings do |t|
      t.belongs_to :tag, foreign_key: true
      t.belongs_to :article, foreign_key: true

    add_index :taggings, :tag_id
    add_index :taggings, :article_id
rails db:migrate

You will get the error:

StandardError: An error has occurred, this and all later migrations canceled:

Index name 'index_taggings_on_tag_id' on table 'taggings' already exists


add_index :taggings, :tag_id
add_index :taggings, :article_id

Migrate the database.

rails db:migrate

Setup the associations in the models. In tag model:

class Tag < ApplicationRecord
  has_many :taggings
  has_many :articles, through: :taggings

In article model:

class Article < ApplicationRecord
  has_many :taggings
  has_many :tags, through: :taggings

  def self.tagged_with(name)
    Tag.find_by!(name: name).articles

  def self.tag_counts
    Tag.select('tags.*, count(taggings.tag_id) as count').joins(:taggings).group('taggings.tag_id')

  def tag_list
    tags.map(&:name).join(', ')

  def tag_list=(names)
    self.tags = names.split(',').map do |n|
      Tag.where(name: n.strip).first_or_create!

Create the articles controller with index, new, show and edit actions.

rails g controller articles index new show edit

The articles controller is straightforward.

class ArticlesController < ApplicationController
  def index
    @articles = if params[:tag]

  def new
    @article = Article.new

  def show
    @article = Article.find(params[:id])

  def create
    @article = Article.new(article_params)
    if @article.save
      redirect_to @article, notice: 'Created article.'
      render :new

  def edit
    @article = Article.find(params[:id])

  def update
    @article = Article.find(params[:id])
    if @article.update_attributes(article_params)
      redirect_to @article, notice: 'Updated article.'
      render :edit


  def article_params
    params.require(:article).permit(:name, :published_on, :content)

In app/views/articles/index.html.erb, add the code to display the tags for an article:

<%= raw article.tags.map(&:name).map { |t| link_to t, tag_path(t) }.join(', ') %>

It will look like this:


<div id="tag_cloud">
  <% tag_cloud Article.tag_counts, %w[s m l] do |tag, css_class| %>
    <%= link_to tag.name, tag_path(tag.name), class: css_class %>
  <% end %>

<div id="articles">
  <% @articles.each do |article| %>
    <h2><%= link_to article.name, article %></h2>
    <%= simple_format article.content %>
      Tags: <%= raw article.tags.map(&:name).map { |t| link_to t, tag_path(t) }.join(', ') %>
    <p><%= link_to "Edit Article", edit_article_path(article) %></p>
  <% end %>

<p><%= link_to "New Article", new_article_path %></p>

Add css for tag cloud to articles.scss:

#tag_cloud {
  width: 400px;
  line-height: 1.6em;
  .s { font-size: 0.8em; }
  .m { font-size: 1.2em; }
  .l { font-size: 1.8em; }

For layout.scss, refer the git repository for the source. Here is the implementation for the tag_cloud.

module ApplicationHelper
  def tag_cloud(tags, classes)
    max = tags.sort_by(&:count).last
    tags.each do |tag|
      index = tag.count.to_f / max.count * (classes.size - 1)
      yield(tag, classes[index.round])

Populate sample data using seeds.rb:

rails db:seed

Define the resource in routes.rb.

Rails.application.routes.draw do
  resources :articles

  root 'articles#index'

Start the rails server.

rails s

Go to localhost:3000. Edit an article and add some tags. It will fail with error:

Unpermitted parameter: tag_list

Add tag_list to the article_params method:

def article_params
  params.require(:article).permit(:name, :published_on, :content, :tag_list)

Now the tag will get saved. If you go the articles index page, you will get the error:

Showing /Users/bparanj/projects/tug/app/views/articles/index.html.erb where line #5 raised:

undefined method `tag_path' for #<#<Class:0x007ff0a30e0e20>:0x007ff09d7d4a70>
Did you mean?  tag_option


get 'tags/:tag', to: 'articles#index', as: :tag

to routes.rb. Now you will be able to see the tags for all the articles and the tag cloud.

Performance Tip by Oren Dobzinski: There's no need to sort the array only to pick the largest element. In application helper for the tag_cloud method:

max = tags.max_by(&:count)

The form partial needs to have a text field to accept tags that are separated by commas.

<%= form_for @article do |f| %>
  <% if @article.errors.any? %>
    <div id="error_explanation">
      <h2><%= pluralize(@article.errors.count, "error") %> prohibited this article from being saved:</h2>
      <% @article.errors.full_messages.each do |msg| %>
        <li><%= msg %></li>
      <% end %>
  <% end %>

  <div class="field">
    <%= f.label :name %><br />
    <%= f.text_field :name %>
  <div class="field">
    <%= f.label :tag_list, "Tags (separated by commas)" %><br />
    <%= f.text_field :tag_list %>
  <div class="field">
    <%= f.label :content %><br />
    <%= f.text_area :content %>
  <div class="actions">
    <%= f.submit %>
<% end %>

You can download the source code for this article from tug.

Bug Fix

If the tags have a dot within the word, for instance Rails 5.1, this solution will break with the error message:

Error: ActionController::UnknownFormat (ActionController::UnknownFormat)

Because Rails will treat whatever it finds after the dot, in this case 1 as an extension. To fix it, change the routes as follows:

get 'tags/:tag', to: 'articles#index', as: :tag, :constraints  => { :tag => /[^\/]+/ }

The constraint tells Rails not to get confused if it sees a dot and then a number. So, it will not be treated as an extension for a format. The entire query parameter will now consist of the string 'Rails 5.1' instead of getting chopped off like this: 'Rails 5'.


In this article, you learned how to implement tagging functionality for just one model in a Rails 5 app.

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