Rails 4: chaining associated objects in Views

Question

Ok, I searched all over the web and found no answer. I am looking for a way to display a name of a 'category' in the show view of a post (I have to mention I'm rookie in Rails).

I have....

a model "Post"

class Post < ActiveRecord::Base
    has_many :categories
    belongs_to :user
end

a model "Category"

class Category < ActiveRecord::Base
    belongs_to :post
end

and the "show" view of the "post" has a line like this

<%= @post.category.name %>

The error message as screen shot: NoMethodError in Posts#Show - undefined method `category' for #

The "show" action in "Posts" controller:

def show
    @post = Post.find(params[:id])
end

I'm building this app along a little outdated training video on udemy. In this video there's in the category model a line with "attr_accessible"

class Category < ActiveRecord::Base

    attr_accessible :name           <---------------------- this
    has_many :posts
end

...but this does no longer exists since Rails 4.0. Is there another way to retrieve the category name of the post?

Thank you in advance :-)

Answer 1

The method category not exists because the Post model has many "categories" not one "category". The Post should have the method "categories". Then if you want to show a first "category" of post in the view:

<%= @post.categories.first.name %>

If you want to show all "categories" of post, then you iterate the collection:

<% @post.categories.each do |category| %>
<%= category.name %>
<% end %>

Answer 2

I got the answer. I found out, that every way, to get the data out of the table categories in the view for products won't work. I than thougt, showing the category in the category show view is simple working. With this idea in mind I took the same code, this one:

app/views/categories/show.html.erb

<p>
  <strong>Name:</strong>
  <%= @category.category_name %>          <--------this line
</p>

...from the category show view and put it into the post show view. I than got again an error but different:

--> NoMethodError in Posts#show

Ok, this says the instance variable "@category" isn't available for the post show view. To change that was easy. I copied the object from the categories controller's show action into the posts controller's the show action. Like this:

class PostsController < ApllicationController
 .
 .
 .
  def show
    @post = Post.find(params[:id])
    @category = Category.find(params[:id])
  end

And: it works!!!! But now, is there something wrong to do it like this?

Cheers.

Answer 3

I tried again and found the real error in my code. the foreign key was not set by Rails. I had to do manually. The table which has a "belongs_to" (in my case the posts table) needs a foreign key added (like category_id).

First creating the migration file:

rails g migration add_foreign_key_to_posts_table

Second adding the migration code to the migration file:

class AddForeignKeyToPostsTable < ActiveRecord::Migration
  def change
    add_foreign_key :posts, :categories
  end
end

Third raking the db migration with:

rake db:migrate

Fourth adding the foreign key for other resources by following steps one to three. Now everything works fine.