Question

I have a categories table and a posts table. A post belongs to a category. I want to output a list of all categories with the latest post in that category.

I feel kind of silly, but I’ve been working on this for a few hours now. I’ve tried join and include when querying the categories, but I had problems limiting to only the latest post of each category.

I then tried to create my own hash or array, but I just kept running in to problems. So before I waste anymore time, I thought that the following would be the next cleanest way I can imagine it to work.

I would really appreciate any help I can get on how to achieve this.

The following is my code (stripped to the bare minimum).

db/schema.rb

ActiveRecord::Schema.define(:version => yyyymmddhhmmss) do

  create_table "categories", :force => true do |t|
    t.string   "name"
    t.datetime "created_at",    :null => false
    t.datetime "updated_at",    :null => false
  end

  create_table "posts", :force => true do |t|
    t.string   "title"
    t.integer  "category_id"
    t.datetime "created_at",   :null => false
    t.datetime "updated_at",   :null => false
  end

end

app/models/category.rb

class Category < ActiveRecord::Base
  ...
  has_many :posts
  ...
end

app/models/post.rb

class Post < ActiveRecord::Base
  ...
  belongs_to :category
  ...
end

app/controller/categories_controller.rb

class CategoriesController < ApplicationController
  ...
  def index
    @categories = Category.all
    # The following loop is what my question is about
    @categories.each do |c|
      latest_post = Post.where(:category_id => c.id).order('published_at DESC').first
      # "Inject" post.id and post.title in to the current @categories hash
    end
  end
  ...
end

app/views/categories/index.html.erb

<% @categories.each do |c| %>
  ...
  <h4><a href="<%= category_path(c) %>"><%= c.name %></a></h4>
  # The following line is how I envision the output to work
  <p><a href="<%= post_path(c.post_id) %>"><%= c.post_title %></a></p>
  ...
<% end %>

raise @categories.to_yaml before

---
- !ruby/object:Category
  attributes:
    id: 1
    name: General
    created_at: 2013-01-10 22:08:57.291758000 Z
    updated_at: 2013-01-10 22:09:02.414022000 Z
...

The following is hypothetical.
raise @categories.to_yaml after

---
- !ruby/object:Category
  attributes:
    id: 1
    name: General
    created_at: 2013-01-10 22:08:57.291758000 Z
    updated_at: 2013-01-10 22:09:02.414022000 Z
    post_id: 80
    post_title: Lorem Ipsum
...

Answer 1

Create a one to one association first :

class Category
  has_one :latest_post, :order => "created_at DESC", class_name => "Post"
end

and then eager load:

@categories = Category.includes(:latest_post).all

And… Voilà!