################## model ###############

class Term < ActiveRecord::Base
 belongs_to :post
 belongs_to :category
end

class Post < ActiveRecord::Base
 belongs_to :user
 has_many :terms
 
 has_many :categories, :through => :terms
 
 has_many :comments

 def to_s
  title
 end
end

class Category < ActiveRecord::Base

 has_many :posts
 has_many :posts, :through => :terms

 def to_s
  title
 end

end

################## views ###############

########## categories/show ############# 

<h1>Category <em><%= @category %></em></h1>

<section class="listing">
 <%= render partial: 'posts/post', collection: @category.posts.order('created_at desc') %>
</section>