Rails 4 many to many association not working

Question

Ruby on rails newbie here. Trying to create a starter blog app and having trouble with many to many association between my models.

I have 2 models - Post, Category that have a many to many association between each other.

My problem: When I create a new post, the Post gets saved but the post-category association does not get saved in the categories_posts table.

My code is as below.

I appreciate your inputs on this.

post.rb

class Post < ActiveRecord::Base
  validates_presence_of :title, :body, :publish_date
  belongs_to :user
  has_and_belongs_to_many :categories
end

category.rb

class Category < ActiveRecord::Base
  validates_presence_of :name
  has_and_belongs_to_many :posts
end

categories_posts.rb

class CategoriesPosts < ActiveRecord::Base
end

Migrations - create_posts.rb

class CreatePosts < ActiveRecord::Migration
  def change
    create_table :posts do |t|
     t.string :title
     t.text :body
     t.date :publish_date
     t.integer :user_id

     t.timestamps
    end
  end
end 

Migrations - create_categories.rb

class CreateCategories < ActiveRecord::Migration
  def change
    create_table :categories do |t|
      t.string :name
      t.timestamps
    end 
  end
end

Migrations - create_categories_posts.rb

class CreateCategoriesPosts < ActiveRecord::Migration
  def change
    create_table :categories_posts do |t|
      t.integer :category_id
      t.integer :post_id
      t.timestamps
    end
  end
end

Post Controller - create and new methods

#GET /posts/new
def new
  @post = Post.new
end

def create
  @post = Post.new(post_params)

  #User id is not a form field and hence is not assigned in the view. It is assigned when control is transferred back here after Save is pressed
  @post.user_id = current_user.id

  respond_to do |format|
    if @post.save
      format.html { redirect_to @post, notice: 'Post was successfully created.' }
      format.json { render action: 'show', status: :created, location: @post }
    else
      format.html { render action: 'new' }
      format.json { render json: @post.errors, status: :unprocessable_entity }
    end
  end
end

Post View(for creating a new Post):

<%= simple_form_for @post, :html => { :class => 'form-horizontal' } do |f| %>
  <%= f.input :title %>
  <%= f.input :body %>
  <%= f.input :publish_date %>
  <%= f.association :categories, :as => :check_boxes %>
  <div class="form-actions">
    <%= f.button :submit, :class => 'btn-primary' %>
    <%= link_to t('.cancel', :default => t("helpers.links.cancel")),
            posts_path, :class => 'btn' %>
  </div>
<% end %>

Thanks, Mike

Answer 1

When using the has_and_belongs_to_many association you need a unique index on your join table. Your migration should look like this:

class CreateCategoriesPosts < ActiveRecord::Migration
  def change
    create_table :categories_posts do |t|
      t.integer :category_id
      t.integer :post_id
      t.timestamps
    end
    add_index :categories_posts, [:category_id, :post_id]
  end
end

You can also get rid of the CategoriesPost model, that is only needed if you wanted to implement a :has_many, :through association. That should answer your question.

---

And just to be thorough, if you wanted to use a :has_many, :through association with a CategoriesPost model you can implement that like so:

class Post < ActiveRecord::Base
  has_many :categoriesposts
  has_many :categories, :through => :categoriesposts
end
class Category < ActiveRecord::Base
  has_many :categoriesposts
  has_many :posts, :through => :categoriesposts
end
class CategoriesPost < ActiveRecord::Base
  belongs_to :post
  belongs_to :category
end

Implementing this method allows you to add more attributes to your categoriespost model if you wanted.