// Controller

  def create
    @doc = Doc.new(doc_params)

    upload = params[:file]

    @doc.path = "public/files/#{upload.original_filename}"
    File.open(@doc.path, "wb") { |f| f.write(upload['datafile'].read)}

    respond_to do |format|
      if @doc.save
        format.html { redirect_to @doc, notice: 'Doc was successfully created.' }
        format.json { render :show, status: :created, location: @doc }
      else
        format.html { render :new }
        format.json { render json: @doc.errors, status: :unprocessable_entity }
      end
    end
  end


// View

<%= form_for(@doc) do |f| %>
  <% if @doc.errors.any? %>
    <div id="error_explanation">
      <h2><%= pluralize(@doc.errors.count, "error") %> prohibited this doc from being saved:</h2>

      <ul>
      <% @doc.errors.full_messages.each do |message| %>
        <li><%= message %></li>
      <% end %>
      </ul>
    </div>
  <% end %>

  <div class="field">
    <%= f.label :title %><br>
    <%= f.text_field :title %>
  </div>
  <div class="field">
      <%= file_field_tag :file %>
  </div>
  <div class="actions">
    <%= f.submit %>
  </div>
<% end %>