If you want to add tags, categories, filters, and so on, to your Rails 5 models, the acts-as-taggable-on gem is the way to go. Using it with Rails 5 is fairly straightforward.

1. Add it to your Gemfile:

gem 'acts-as-taggable-on', '~> 6.0'

2. Run bundle install.

3. Install its migrations:

rake acts_as_taggable_on_engine:install:migrations

4. Run rake db:migrate.

5. Add acts_as_taggable_on to your model. You don’t need to create a migration for your tags – acts-as-taggable-on handles that for you. You can change :tags to anything you’d like – :categories, :codes, etc.

# post.rb
class Post < ApplicationRecord
  acts_as_taggable_on :tags
end

6. Then, inside your controller, you’ll need to accept :tag_list (or :category_list, or :code_list, etc.) as a strong param. It’s the singular version of the word you used in your model, appended with _list.

# posts_controller.rb
class PostsController < ApplicationController
  # ...

  def post_params
    params.require(:post).permit(:title, :body, :tag_list)
  end
end

7. Now you can add the tags to a form. Sometimes, you may be able to get away with <%= f.input :tag_list %>, but I’ve had to use a slightly more complicated version in the past. Users can add multiple tags by separating them with commas:

<!-- views/posts/_form.html.erb -->
<%= form.label :tag_list, "Tags" %>
<%= 
  form.text_field :tag_list, 
  value: @post.send(:tag_list).to_s, 
  placeholder: 'rails, development'
%>

8. You can search by tagged filters, as well:

# routes.rb
get '/tagged', to: "posts#tagged", as: :tagged

# posts_controller.rb
def tagged
  if params[:tag].present?
    @posts = Post.tagged_with(params[:tag])
  else
    @posts = Post.all
  end
end

9. And then list each post, plus the option to sort by tags in your view:

<!-- views/posts/index.html.erb -->
<% @posts.each do |p| %>
<h2><%= p.title %></h2>
<div class="tags">
  <% p.tag_list.each do |t| %>
    <%= link_to t, tagged_path(tag: t) %>
  <% end %>
</div>
<% end %>

Once you understand how it works, acts-as-taggable-on is a pretty easy gem to use. It has a lot of other features, as well, that you can learn more about on its GitHub page. Have fun tagging!