Upload Image without any gemfile Rails4 - ruby-on-rails-4

Question

I am new in Ruby on Rails.
I trying to upload images without any gemfile. I am following http://guides.rubyonrails.org/form_helpers
Here is my books_controller.rb file:
class BooksController < ApplicationController
before_action :set_book, only: [:show, :edit, :update, :destroy]
def index
#books = Book.all
end
def show
end
def new
#book = Book.new
end
def edit
end
def create
#book = Book.new(book_params)
respond_to do |format|
if #book.save
format.html { redirect_to #book, notice: 'Book was successfully created.' }
format.json { render action: 'show', status: :created, location: #book }
else
format.html { render action: 'new' }
format.json { render json: #book.errors, status: :unprocessable_entity }
end
end
end
def update
respond_to do |format|
if #book.update(book_params)
format.html { redirect_to #book, notice: 'Book was successfully updated.' }
format.json { head :no_content }
else
format.html { render action: 'edit' }
format.json { render json: #book.errors, status: :unprocessable_entity }
end
end
end
def destroy
#book.destroy
respond_to do |format|
format.html { redirect_to books_url }
format.json { head :no_content }
end
end
def upload
uploaded_io = params[:book][:image_url]
File.open(Rails.root.join('public', 'uploads', uploaded_io.original_filename), 'wb') do |file|
file.write(uploaded_io.read)
end
end
private
def set_book
#book = Book.find(params[:id])
end
def book_params
params.require(:book).permit(:title, :description, :image_url)
end
end
and here is my form.html.erb file:
<%= form_for #book do |f| %>
<% if #book.errors.any? %>
<div id="error_explanation">
<h2><%= pluralize(#book.errors.count, "error") %> prohibited this book from being saved:</h2>
<ul>
<% #book.errors.full_messages.each do |msg| %>
<li><%= msg %></li>
<% end %>
</ul>
</div>
<% end %>
<div class="field">
<%= f.label :title %><br>
<%= f.text_field :title %>
</div>
<div class="field">
<%= f.label :description %><br>
<%= f.text_area :description %>
</div>
<div class="field">
<%= f.label :image_url %><br>
<%= f.file_field :image_url %>
</div>
<div class="actions">
<%= f.submit %>
</div>
<% end %>
Here is my schema.rb file:
ActiveRecord::Schema.define(version: 20140724173638) do
create_table "books", force: true do |t|
t.string "title"
t.text "description"
t.string "image_url"
t.datetime "created_at"
t.datetime "updated_at"
end
end
i also create a dir(public/uploads).
But its not working.when i submit "create button" showing something like this:
TypeError: can't cast ActionDispatch::Http::UploadedFile to string: INSERT INTO "books" ("created_at", "description", "image_url", "title", "updated_at") VALUES (?, ?, ?, ?, ?, ?, ?)
What's the wrong??
Thanks,
Mezbah