example of a many to many relationship in rails

post.rb

class Post < ActiveRecord::Base
  has_many :post_tags
  has_many :tags, through: :post_tags
end

post_tag.rb

class PostTag < ActiveRecord::Base
  belongs_to :post
  belongs_to :tag
end

posts-_form.html.erb

<!-- app/views/posts/_form.html.erb -->
<%= form_for @post do |f| %>
  <div class='row'>
    <div class='col-md-6'>
      <div class='form-group'>
        <%= f.label :title, class: 'form-label' %>
        <%= f.text_field :title, class: 'form-control' %>
      </div>
    </div>
    <div class='col-md-6'>
      <div class='form-group'>
        <%= f.label :tags, class: 'form-label' %>
        <%= f.select :tag_ids, options_from_collection_for_select(Tag.all, 'id', 'name'), {}, { multiple: true, class: 'form-control' } %>
      </div>
    </div>
    <div class='col-md-12'>
      <div class='form-group'>
        <%= f.label :content, class: 'form-label' %>
        <%= f.text_area :content, class: 'form-control', rows: 20 %>
      </div>
    </div>
    <div class='col-md-12'>
      <%= f.submit 'Submit', class: 'btn btn-primary' %>
    </div>
  </div>
<% end %>

posts-index.html.erb

<!-- app/views/posts/index.html.erb -->
<h1>Posts</h1>
<div class='row'>
  <div class='col-md-7'>
    <% @posts.each do |post| %>
      <div class='post'>
        <h2><%= post.title %></h2>
        <hr>
        <div>
          <%= truncate(post.content, length: 200) %>
        </div>
        <% if post.tags.any? %>
          <div class='tags'>
            <strong>Tags:</strong>
            <% post.tags.each do |tag| %>
              <%= link_to tag do %>
                <span class='label label-default'><%= tag.name %></span>
              <% end %>
            <% end %>
          </div>
        <% end %>
      </div>
    <% end %>
  </div>
  <div class='col-md-5'>
    <%= link_to 'New Post', new_post_path, class: 'btn btn-warning pull-right' %>
  </div>
</div>

posts_controller.rb

class PostsController < ApplicationController

  respond_to :html

  def index
    @posts = Post.all
  end

  def new
    @post = Post.new
  end

  def create
    @post = Post.create(safe_params)
    respond_with @post
  end


  private

  def safe_params
    safe_attributes = [
      :title,
      :content,
      tag_ids: [],
    ]
    params.require(:post).permit(*safe_attributes)
  end

end

shell.sh

bin/rails g model Post title content:text
bin/rails g model Tag name
bin/rails g model PostTag post:belongs_to tag:belongs_to
bin/rake db:migrate

show.html.erb

<!-- app/views/tags/show.html.erb -->
<h1><%= "#{@tag.name.pluralize} Posts" %></h1>
<ul>
  <% @tag.posts.each do |post| %>
    <li><%= link_to post.title, post %></li>
  <% end %>
</ul>

tag.rb

class Tag < ActiveRecord::Base
  has_many :post_tags
  has_many :posts, through: :post_tags
end

tags-_form.html.erb

<!-- app/views/tags/_form.html.erb -->
<%= form_for @tag do |f| %>
  <div class='form-group'>
    <div class='col-md-8'>
      <%= f.label :name, class: 'form-label' %>
      <%= f.text_field :name, class: 'form-control' %>
    </div>
    <div class='col-md-4'>
      <%= f.submit 'Submit', class: 'btn btn-primary input-without-label' %>
    </div>
  </div>
<% end %>

tags-index.html.erb

<!-- app/views/tags/index.html.erb -->
<div class='row'>
  <div class='col-md-6'>
    <h1>Tags</h1>
    <ul>
      <% @tags.each do |tag| %>
        <li><%= tag.name %></li>
      <% end %>
    </ul>
  </div>
  <div class='col-md-6'>
    <h1>New Tag</h1>
    <%= render 'form' %>
  </div>
</div>

tags_controller.rb

class TagsController < ApplicationController

  respond_to :html

  def index
    @tags = Tag.all
    @tag = Tag.new
  end

  def show
    @tag = Tag.find(params[:id])
  end

  def create
    @tag = Tag.create(safe_params)
    respond_with @tag, location: tags_url, action: :index
  end


  private

  def safe_params
    params.require(:tag).permit(:name)
  end

end