Rails 4: How to upload files with AJAX - ruby-on-rails

Question

I want to upload files with AJAX. In the past I accomplished this by using the magical jQuery form plugin and it worked great. Currently I'm building a Rails app and trying to do things "The Rails Way" so I'm using the Form Helper and the paperclip gem to add file attachments.
The rails docs warn that the Form Helper does not work for AJAX file uploads:
Unlike other forms making an asynchronous file upload form is not as
simple as providing form_for with remote: true. With an Ajax form the
serialization is done by JavaScript running inside the browser and
since JavaScript cannot read files from your hard drive the file
cannot be uploaded. The most common workaround is to use an invisible
iframe that serves as the target for the form submission.
It seems clear there's no off-the-shelf solution. So I'm wondering what's the smartest thing to do. Seems like I have several options:
Use the form helper and the iframe trick.
Use the form helper + load jQuery form plugin to submit the file (not sure if this will play nice with Rails's authenticity token, etc)
Use the form helper + paperclip + [some other gem] to extend it's functionality to allow AJAX form submission.
All three seem possible. I know the least about #3, specifically the [some other gem] part. I found two similar questions (this and this) which mention a branch of Pic-Upload called Uploadify but those are both 2 years old and deal with Rails 2 and 3 (and Uploadify hasn't been updated in years). So given how much has changed, I think this is really a whole new question:
What's the best way to upload files with AJAX in Rails 4?

Answer 1

Have a look into the remotipart gem: https://github.com/JangoSteve/remotipart -- may get you all of the way there with very little work!

Answer 2

Using #rails/ujs.
view (.html.erb):
<%= file_field_tag :file, { id: "ajax_file_upload"} %>
controller(_controller.rb):
def update
#record = YourModel.find(params[:id])
respond_to do |format|
if #record.update_attributes(params[:your_model])
format.json { render json: { success: true } }
else
error_messages = #record.errors.messages.values.flatten
format.json { render json: { success: false, errors: error_messages } }
end
end
end
javascript(.js)
const uploadFile = element => {
const formData = new FormData();
formData.append("your_model[attribute_name]", element.target.files[0]);
Rails.ajax({
url: "your_model/:id",
type: "PUT",
beforeSend(xhr, options) {
options.data = formData;
return true;
},
success: response => {
if (response.success) {
alert("File uploaded successfully");
}
else {
alert(response.errors.join("<br>"));
}
},
error: () => {
alert("ajax send error");
}
});
};
const documentOnReady = () => {
const fileField = document.getElementById("ajax_file_upload");
if (fileField) {
fileField.addEventListener("change", uploadFile);
}
}
document.addEventListener("turbolinks:load", documentOnReady);
Note: No need to setRequestHeader in ajax while using FormData.
FormData uses the same format a form would use if the encoding type were set to "multipart/form-data"

Answer 3

IMHO Rails is not perfect when dealing with upload files using AJAX, especially if you want a progress bar. My suggestion is to use Javascript for the form submission over an AJAX request like you suggested in (2). If you are comfortable with Javascript you will not have many problems.
I recently used the same approach by using this very simple JS library https://github.com/hayageek/jquery-upload-file and I wrote more details here http://www.alfredo.motta.name/upload-video-files-with-rails-paperclip-and-jquery-upload-file/
For an application with a form to upload a movie with title and description the JS code looks like follow:
$(document).ready(function() {
var uploadObj = $("#movie_video").uploadFile({
url: "/movies",
multiple: false,
fileName: "movie[video]",
autoSubmit: false,
formData: {
"movie[title]": $('#movie_title').text(),
"movie[description]": $('#movie_description').text()
},
onSuccess:function(files,data,xhr)
{
window.location.href = data.to;
}
});
$("#fileUpload").click(function(e) {
e.preventDefault();
$.rails.disableFormElements($($.rails.formSubmitSelector));
uploadObj.startUpload();
});
});
Far from perfect, but gives you flexibility on your frontend.