Question

what I’m trying to do is have a upload button to upload files to our storage system. I’m using Google App Engine with Python. Also HTML and Javascript for views.

For that, we have a HTML, and a.js that asks the user if he’s sure that he wants to overwrite a file. For that overwriting question, I need to ask the Database to know if it exists,and so if the question should be asked or not…

The thing is I don’t know even where to start. I have this confirm() text shown to the user, and a GQL database, but I don’t know how to make a question. For example, I upload via a URL, but then I don’t have a response for that, and also I don’t want to pass a question (name of the file,…) to the database via URL…

Do you have any idea of what path should I follow? Am I trying something impossible or without any sense?

Thanks a lot!

I add some code:

this is the HTML form where we ask the user to upload a file:

<form id="up_file" enctype="multipart/form-data" method="post">
                <input type="hidden" name="user_id" value="{{ current_user.id }}"/>
                <input type="hidden" name="group_id" value="{{ group.id }}"/>
                <p>File: <input type="file" name="filename" id="file_name"/></p>
                <p><input type="button" value="Upload" onClick="seguro_sobreescribir(filename,{{ current_user.id }},{{ group.id }})"/></p>
            </form> 

and this is the javascript that currently tries to send information to our application in google engine when someone clicks the upload button:

function Request(function_name, opt_argv) {
  if (!opt_argv)
    opt_argv = new Array();

  // Find if the last arg is a callback function; save it
  var callback = null;
  var len = opt_argv.length;
  if (len > 0 && typeof opt_argv[len-1] == 'function') {
    callback = opt_argv[len-1];
    opt_argv.length--;
  }
  var async = (callback != null);

  // Build an Array of parameters, w/ function_name being the first parameter
  var params = new Array(function_name);
  for (var i = 0; i < opt_argv.length; i++) {
    params.push(opt_argv[i]);
  }
  var body = JSON.stringify(params);

  // Create an XMLHttpRequest 'POST' request w/ an optional callback handler
  var req = new XMLHttpRequest();

  req.open('POST', 'https://safeshareapp.appspot.com/upload', async);

  req.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
  req.setRequestHeader("Content-length", body.length);
  req.setRequestHeader("Connection", "close");

  if (async) {
    req.onreadystatechange = function() {
      if(req.readyState == 4 && req.status == 200) {
        var response = null;
        try {
         response = JSON.parse(req.responseText);
        } catch (e) {
         response = req.responseText;
        }
        callback(response);
      }
    }
  }

  // Make the actual request
  req.send(body);
}


// Comprobar si existe, y si existe, preguntar si se quiere sobreescribir
function seguro_sobreescribir(filename,user_id,group_id)
{   

        var resp=confirm("Seguro que quiere sobreescribir el archivo "+filename.value+" del usuario "+user_id+" del grupo "+group_id+"?");      

        if(resp)
        {           
            var result = Request('Upload',[filename,user_id,group_id]);
            alert("Hemos hecho request "+ result);
        }       

}

And this is the RequestHandler that should handle our request:

class RPCHandler(webapp.RequestHandler):
    """ Allows the functions defined in the RPCMethods class to be RPCed."""
    def __init__(self):
        webapp.RequestHandler.__init__(self)
        self.methods = RPCMethods()

    def post(self):

        args = simplejson.loads(self.request.body)
        func, args = args[0], args[1:]

        if func[0] == '_':
            self.error(403) # access denied
            return

        func = getattr(self.methods, func, None)
        if not func:
            self.error(404) # file not found
            return

        result = func(*args)
        self.response.out.write(simplejson.dumps(result))

class RPCMethods:

    def Upload(self, *args):

        status = -1
        fileitem = args[0]        
        userid = args[1]
        groupid=args[2]    

        return status

def main():
    app = webapp.WSGIApplication([('/upload', RPCHandler)], debug=True)
    util.run_wsgi_app(app)

if __name__ == '__main__':
    main()

The fact is that the return status gives us a undefined back at the javascript.
We don’t know if we are not uploading the file and if so, how to do it. That’s because we have 2 things that we don’t know how to put together:

• The normal “input type=file, method=post, and submit input type=submit button of a HTML form

• Our connection via RequestHandler to the google app engine etc.

Do you have any ideas?

Answer 1

Here’s a possibility:

Implement a servlet that can answer the question given a filename; it could return a ‘0’ or ‘1’ (or whatever you choose) as its HTTP response depending on whether the file exists. Then make an XmlHttpRequest POST to that servlet from your javascript with the filename as a POST parameter. Depending on the return of the XmlHttpRequest, show UI to the user asking to confirm.