Direct to S3 upload using aws-sdk gem and jQuery-File-Upload on heroku

I’m trying to achieve direct to Amazon S3 upload in Rails using jQuery-File-Upload and the aws-sdk gem, and following heroku’s direct to S3 instructions. This is the upload form produced in the html:

<form id="pic-upload"
class="directUpload"
data-form-data="{
"key":"uploads/59c99e44-6bf2-4937-9680-02c839244b33/${filename}",
"success_action_status":"201",
"acl":"public-read",
"policy":"eyJle...In1dfQ==",
"x-amz-credential":"AKIAJCOB5HQVW5IUPYGQ/20160101/us-east-1/s3/aws4_request",
"x-amz-algorithm":"AWS4-HMAC-SHA256",
"x-amz-date":"20160101T010335Z",
"x-amz-signature":"0f32ae...238e"}"
data-url="https://websmash.s3.amazonaws.com"
data-host="websmash.s3.amazonaws.com"
enctype="multipart/form-data"
action="/users/bazley/update_pictures"
accept-charset="UTF-8"
method="post">

``

This is the corresponding jQuery:

$(function() {
  $('.directUpload').find("input:file").each(function(i, elem) {
    var fileInput    = $(elem);
    var form         = $(fileInput.parents('form:first'));
    var submitButton = form.find('input[type="submit"]');
    var progressBar  = $("<div class='bar'></div>");
    var barContainer = $("<div class='progress'></div>").append(progressBar);
    fileInput.after(barContainer);
    fileInput.fileupload({
      fileInput:       fileInput,
      url:             form.data('url'),
      type:            'POST',
      autoUpload:       true,
      formData:         form.data('form-data'),
      paramName:        'file', // S3 does not like nested name fields i.e. name="user[avatar_url]"
      dataType:         'XML',  // S3 returns XML if success_action_status is set to 201
      replaceFileInput: false,
      progressall: function (e, data) {
        var progress = parseInt(data.loaded / data.total * 100, 10);
        progressBar.css('width', progress + '%')
      },
      start: function (e) {
        submitButton.prop('disabled', true);
        progressBar.
          css('background', 'green').
          css('display', 'block').
          css('width', '0%').
          text("Loading...");
      },
      done: function(e, data) {
        submitButton.prop('disabled', false);
        progressBar.text("Uploading done");
        // extract key and generate URL from response
        var key   = $(data.jqXHR.responseXML).find("Key").text();
        var url   = '//' + form.data('host') + '/' + key;
        // create hidden field
        var input = $("<input />", { type:'hidden', name: fileInput.attr('name'), value: url })
        form.append(input);
      },
      fail: function(e, data) {
        submitButton.prop('disabled', false);
        progressBar.
          css("background", "red").
          text("Failed");
      }
    });
  });
});

``

Trying to upload a file produces these logs:

Started POST "/users/bazley/update_pictures" for ::1 at 2016-01-01 21:26:59 +0000 Processing by CharactersController#update_pictures as HTML
Parameters: {
    "utf8"=>"✓",
    "authenticity_token"=>"rvhu...fhdg==",
    "standardpicture"=>{
        "picture"=>#<ActionDispatch::Http::UploadedFile:0x0000010b32f530
            @tempfile=#<Tempfile:/var/folders/19/_vdcl1r913g6fzvk1l56x4km0000gn/T/RackMultipart20160101-49946-7t94p.jpg>,
            @original_filename="europe.jpg",
            @content_type="image/jpeg",
            @headers="Content-Disposition: form-data; name=\"standardpicture[picture]\"; filename=\"europe.jpg\"\r\nContent-Type: image/jpeg\r\n">
    },
    "commit"=>"Upload pictures",
    "callsign"=>"bazley"
}

``

The form submits successfully, but it isn’t working because Rails doesn’t save the correct location (“picture”, a string) on S3; instead it thinks the location is

"picture"=>#<ActionDispatch::Http::UploadedFile:0x0000010b32f530

``

You can see this in the submitted parameters. It should be something like:

"picture"=>"//websmash.s3.amazonaws.com/uploads/220f5378-1e0f-4823-9527-3d1170089a49/europe.jpg"}, "commit"=>"Upload pictures"}

``

What I don’t understand is why it’s getting the parameters wrong when all the correct information seems to be present in the form. It clearly says

data-url="https://websmash.s3.amazonaws.com"

``

in the form, and the jQuery includes

url:  form.data('url'),

``

so what’s going wrong?

For completeness: in the controller:

before_action :set_s3_direct_post
.
.
def set_s3_direct_post
  @s3_direct_post = S3_BUCKET.presigned_post(key: "uploads/#{SecureRandom.uuid}/${filename}", success_action_status: '201', acl: 'public-read')
end

``

The form:

<%= form_for :standardpicture, url: update_pictures_user_path,
             html: {  id: "pic-upload", class: "directUpload",
                      data: { 'form-data' => (@s3_direct_post.fields),
                              'url' => @s3_direct_post.url,
                              'host' => URI.parse(@s3_direct_post.url).host }
                   } do |f| %>
  <div class="field">
    <%= f.label :picture %>
    <%= f.file_field :picture, accept: 'image/jpeg,image/gif,image/png' %>
  </div>
  <%= f.submit "Upload pictures", class: "btn btn-primary" %>
<% end %>

``

aws.rb initializer:

Aws.config.update({
  region: 'us-east-1',
  credentials: Aws::Credentials.new(ENV['AWS_ACCESS_KEY_ID'], ENV['AWS_SECRET_ACCESS_KEY']),
})
S3_BUCKET = Aws::S3::Resource.new.bucket(ENV['S3_BUCKET'])

``