Rails 4 + carrierwave/paperclip + nested model upload multiple images solution

To do just follow these Steps.

Step 1) rails new multiple_image_upload_carrierwave

Step 2) In gem file

             gem 'carrierwave'

Step 3) bundle install

Step 4) rails generate uploader Avatar

Step 5) Create post scaffold

Step 6) rails g scaffold title:string

Step 7) Create post_attachment scaffold

Step 8) rails g scaffold post_attachment post_id:integer avatar:string

Step 9) rake db:migrate

Step 10) In post.rb

class Post < ActiveRecord::Base
 has_many :post_attachments accepts_nested_attributes_for :post_attachments
end

Step 11) In post_attachment.rb

class PostAttachment < ActiveRecord::Base
 mount_uploader :avatar, AvatarUploader belongs_to :post
end

Step 12) In post_controller.rb

def show
  @post_attachments = @post.post_attachments.all
end

def new
 @post = Post.new
 @post_attachment = @post.post_attachments.build
end

def create
  @post = Post.new(post_params)
     respond_to do |format|
       if @post.save
         params[:post_attachments]['avatar'].each do |a|
          @post_attachment = @post.post_attachments.create!(:avatar => a, :post_id => @post.id)
       end
         format.html { redirect_to @post, notice: 'Post was successfully created.' }
      else
         format.html { render action: 'new' }
     end
   end
 end

private
 def post_params
    params.require(:post).permit(:title, post_attachments_attributes: [:id, :post_id, :avatar])
 end

Step 13) In views/posts/_form.html.erb

<%= form_for(@post, :html => { :multipart => true }) do |f| %>

   <%= f.label :title %>
   <%= f.text_field :title %>

   <%= f.fields_for :post_attachments do |p| %>

     <%= p.label :avatar %>
     <%= p.file_field :avatar, :multiple => true, name: "post_attachments[avatar][]" %>

   <% end %>

  <%= f.submit %>

<% end %>

Step 14 )   In views/posts/show.html.erb

<%= notice %>

Title: <%= @post.title %>

Step 15)

<% @post_attachments.each do |p| %>
   <%= image_tag p.avatar_url %>
<% end %>
 <%= link_to 'Edit', edit_post_path(@post) %> | <%= link_to 'Back', posts_path %>

In rails 3 no need to define strong parameters and as you can define attribute_accessible in both the model and accept_nested_attribute to post model because attribute accessible is deprecated in rails 4.