How to trigger download with Rails send_data from AJAX post - ruby-on-rails

Question

I'm trying to use send_data to return a PNG image as the response for a ajax post request. How do I get the browser to trigger a download on the success callback?
Details
I'm generating a large base64 image using canvas.toDataURL(), and then posting it to Rails (v3.2.6). Rails decodes it to a binary PNG, and sends the image back to the client.
I've also tried send_file but it has the same issue.
Other options
Download image client side: We can't do this because (1) Safari crashes on large base64 URLs, and (2) Safari does not yet support the download attribute on anchor tags which I would need to specify the downloaded image filename.
Use a $.get instead of $.post: We can't do this because we need to send our canvas.toDataURL() with the request to the server. GET requests URIs have size limitations.

Answer 1

create a function in controller
def ajax_download
send_file "path_to_file/" + params[:file]
end
and then in controller action
respond_to do |format|
#java_url = "/home/ajax_download?file=#{file_name}"
format.js {render :partial => "downloadFile"}
end
and make a partial in view folder name with _downloadFile.js.erb and write this line
window.location.href = "<%=#java_url %>"

Answer 2

You can't download a file to disk from JS. It's a security concern. See the blog post below for a good workaround.
http://johnculviner.com/post/2012/03/22/Ajax-like-feature-rich-file-downloads-with-jQuery-File-Download.aspx

Answer 3

Do not just copy and paste the accepted answer. It is a massive security risk that cannot be understated. Although the technique is clever, delivering a file based on a parameter anybody can enter allows access to any file anybody can imagine is lying around.
Here's an example of a more secure way to use the same technique. It assumes there is a user logged in who has an API token, but you should be able to adapt it to your own scenario.
In the action:
current_user.pending_download = file_name
current_user.save!
respond_to do |format|
#java_url = "/ajax_download?token=#{current_user.api_token}"
format.js {render :partial => "downloadFile"}
end
Create a function in the controller
def ajax_download
if params[:token] == current_user.api_token
send_file "path_to_file/" + current_user.pending_download
current_user.pending_download = ''
current_user.save!
else
redirect_to root_path, notice: "Unauthorized"
end
end
Make a partial in view folder name with _downloadFile.js.erb
window.location.href = "<%=#java_url %>"
And of course you will need a route that points to /ajax_download in routes.rb
get 'ajax_download', to: 'controller#ajax_download'