Rails nested association and restarted numbering

Question

If I have a nested resource like so:

resources :users
  resources :posts
end

and a user has_many posts

Answer 1

It can be quite a bit of work, but basically you can do it with these steps:

1. Create a migration to add a new attribute to store the specific user-post count. (I used user_post_id)
2. Override Post's to_param method to use the new value you just created. (It has to be a string.)
   • to_param is the method that the url and path helpers use.
3. Create a before_save filter that will actually increment the user_post_id value for each new post.

4. Change all your controller methods to find on user_post_id

   @user = User.find(params[:user_id])
   @post = @user.posts.where(:user_post_id => (params[:id])).first
   

5. Change all your Views that might not work now

You can see the source here: Custom Nested Resource URL example

Code

migration:

class AddUserPostIdToPosts < ActiveRecord::Migration
  def change
    add_column :posts, :user_post_id, :integer
  end
end

post.rb:

class Post < ActiveRecord::Base
  before_save :set_next_user_post_id
  belongs_to :user

  validates :user_post_id, :uniqueness => {:scope => :user_id}

  def to_param
    self.user_post_id.to_s
  end

private
  def set_next_user_post_id
    self.user_post_id ||= get_new_user_post_id
  end

  def get_new_user_post_id
    user = self.user
    max = user.posts.maximum('user_post_id') || 0
    max + 1
  end
end

A couple controller methods posts_controller.rb:

class PostsController < ApplicationController
  respond_to :html, :xml

  before_filter :find_user

  def index
    @posts = @user.posts.all
    respond_with @posts
  end

  def show
    @post = @user.posts.where(:user_post_id => (params[:id])).first
    respond_with [@user, @post]
  end
  ...
end