Rails 4: chaining associated objects in Views - ruby-on-rails-4

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I'm building this app along a little outdated training video on udemy.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

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 2

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 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.