Paperclip (3.0) and Rails (3.2): f.file_field loses file path on 'new' action after a validation error

Question

I set up a story model with an image attachment handled by Paperclip, that looks like:

class Story < ActiveRecord::Base  
  has_attached_file :image # [...]
  attr_accessible :user_id, :title, :image, :image_file_name
  belongs_to: user

  validates_presence_of :user_id
  validates :title,     :length => { :maximum => 50 }
  validates_attachment_size :image, :less_than => 2.megabytes, :unless => Proc.new { |story| story[:image].nil? }
  # [...] 
end

When I fill my story form, that looks like:

<%= form_for @story, html: { multipart: true } do |f| %>
<% if @story.errors.any? %>
<div id="error-explanation">
    <ul>
        <% @story.errors.full_messages.each do |msg| %>
        <li class="error-mess">Error: <%= msg.downcase %></li>
        <% end %>
    </ul>
</div>
<% end %>

<%= f.text_field :title %></td>
<%= f.file_field :image %>
<%= f.submit t('.send') %>

<% end %>

If validation fails for a story.title too long the form is redisplayed correctly along with the proper error message and the invalid title already filled in, but the file_field is now blank and I have to click again on it in order to re-select the file I want to upload.

And here is how my stories_controller.rb looks like:

def create
  @story = @current_user.stories.new(params[:story]) 
  if @story.save                                                                                                                   
    redirect_to thanks_path    
  else
    # [email protected] so I render action 'new' again just to
    # bang my head against this 'anomaly' 
    render action: "new"
  end              
end

How can I avoid users having to re-select the file to upload after a validation error?

Answer 1

The way HTTP file uploads work in browsers, the file has been uploaded to your app already on first submit – so you should store it somewhere, so that you still have access to it later on a second form submit. (At least in PHP an uploaded file get’s deleted after the script has run, if it’s not explicitly been moved somewhere else – I don’t know if that applies for RoR as well.)

You can’t pre-fill an input type=file field in HTML – for security reasons. And even if the user picks the file again, they would have to send it a second time – waste of user’s and your bandwidth.

So either store it somewhere on first submit, or try to do your validations on the client-side too with JavaScript before allowing submit (as far as possible), so that you minimize the form submits that actually fail validation on the server side.

Answer 2

CBroe is right, the best solution is to store the file temporarily. What I would do to do this is: - Move the file to a temp directory, and name it with the id of the user that was trying to upload it. - When the form is posted and there is no file uploaded, try to use the temp file for that user (if it exists). - If the Story is succesfully saved, delete any temp files for that user.

I think that should do the trick.