Why do I get an "undefined method for `has_attached_file` when installing PaperClip?

I just installed the plugin for Paperclip and I am getting the following error message but I am not sure why:

NoMethodError (undefined method `has_attached_file' for #<Class:0x10338acd0>):
  /Users/bgadoci/.gem/ruby/1.8/gems/will_paginate-2.3.12/lib/will_paginate/finder.rb:170:in `method_missing'
  app/models/post.rb:2
  app/controllers/posts_controller.rb:50:in `show'

It is referencing the will_paginate gem. From what I can find, it seems that either there is something wrong with my PostsController#index or perhaps a previously attempt at installing the gem instead of the plugin, in which case I have read I should be able to remedy through the /config/environments.rb file somehow.

I didn't think that previous gem installation would matter as I did it in an old version of the site that I trashed before installing the plugin. In the current version of the site I show that the table has been updated with the Paperclip columns after migration. Here is my code:

PostsConroller#show:

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

    respond_to do |format|
      format.html # show.html.erb
      format.xml  { render :xml => @post }
    end
  end

Post model:

class Post < ActiveRecord::Base
  
  has_attached_file :photo
  validates_presence_of :body, :title
  has_many :comments, :dependent => :destroy
  has_many :tags, :dependent => :destroy
  has_many :votes, :dependent => :destroy
  belongs_to :user
  after_create :self_vote
      def self_vote
       # I am assuming you have a user_id field in `posts` and `votes` table.
       self.votes.create(:user => self.user)
      end
       
  cattr_reader :per_page 
    @@per_page = 10
 
end

/views/posts/new.html.erb:

<h1>New post</h1>
<%= link_to 'Back', posts_path %>
<% form_for(@post, :html => { :multipart => true}) do |f| %>
  <%= f.error_messages %>

  <p>
    <%= f.label :title %><br />
    <%= f.text_field :title %>
  </p>
  <p>
    <%= f.label :body %><br />
    <%= f.text_area :body %>
  </p>
  <p>
	<%= f.file_field :photo %>
  </p>

  <p>
    <%= f.submit 'Create' %>
  </p>
<% end %>

It is very important that you restart your server after installing new gems/plugins. This should solve your problem

I'd suggest installing paperclip gem. Then you'd just need to add config.gem 'paperclip' to your environment.rb and run sudo rake gems:install.

create the file paperclip.rb inside the config/initializers/paperclip.rb

Add the below lines and restart the server

require "paperclip/railtie"

Paperclip::Railtie.insert

I got this error spontaneously on 2 different dev machines after Paperclip was working fine for weeks.

spring stop

then restarted my rails console was needed

I guess this should have been obvious, but I'm using mongo/mongoid as my data layer and needed to install mongoid paperclip for it to work.