nil is not a symbol when uploading file using Carrierwave - ruby-on-rails-3

Question

i have searched a lot.
I'm facing this error
Create method in Controller is
def create
#category = Category.new(params[:category])
respond_to do |format|
if #category.save
format.html { redirect_to #category, notice: 'Category was successfully created.' }
format.json { render json: #category, status: :created, location: #category }
else
format.html { render action: "new" }
format.json { render json: #category.errors, status: :unprocessable_entity }
end
end
end
My Form is
<%= form_for #category, :html => { :multipart => true } do |f| %>
<% if #category.errors.any? %>
<div id="error_explanation">
<h2><%= pluralize(#category.errors.count, "error") %> prohibited this category from being saved:</h2>
<ul>
<% #category.errors.full_messages.each do |msg| %>
<li><%= msg %></li>
<% end %>
</ul>
</div>
<% end %>
<div class="field">
<%= f.label :name %><br />
<%= f.text_field :name %>
</div>
<div class="field">
<%= f.label :description %><br />
<%= f.text_area :description%>
</div>
<div class="field">
<%= f.file_field :image %>
</div>
<div class="actions">
<%= f.submit %>
</div>
<% end %>
When i select the file to upload and click submit button not proceed
My Uploader is
# encoding: utf-8
require 'carrierwave/processing/rmagick'
class ImageUploader < CarrierWave::Uploader::Base
#Include RMagick or MiniMagick support
include CarrierWave::RMagick
# Choose what kind of storage to use for this uploader:
storage :file
# This is a sensible default for uploaders that are meant to be mounted:
def store_dir
"uploads/#{model.class.to_s.underscore}/#{mounted_as}/#{model.id}"
end
def scale(width, height)
# do something
end
# Create different versions of your uploaded files:
version :thumb do
process :resize_to_fill => [80, 80]
end
end

Answer 1

i have Create the new project, apply everything in the same way and its working, still don't know why this error occurred but now i have no problem.