More Blobstore upload woes with standard Django

Question

I'm implementing an image upload feature for my Django app (plain Django 1.4 , NOT the non-rel version) running on Google App Engine. The uploaded image is wrapped in a Django model which allows the user to add attributes like a caption and search tags.

The upload is performed by creating a Blobstore upload url through the function call blobstore.create_upload_url(url). The function argument is the url to which the Bobstore redirects when the upload is complete. I want this to be the url of the default Django form handler that performs the save/update of the model that wraps the image so I don't have to duplicate default Django behaviour for form validation, error reporting and database update.

I tried supplying reverse('admin:module_images_add') to create_upload_url() but this doesn't work as it throws an [Errno 30] Read-only file system exception. I presume this originates from the default Django form handler again trying to upload the file the standard Django way but then hits the brick wall of Google App Engine not allowing access to the file system.

At the moment, the only way I can see to get this working without duplicating code is by strictly separating processes: one for defining an image model instance and the second for uploading the actual image. Not very intuitive.

See also this question and answer which I posted earlier.

Any suggestions on how to get this working using one form and reusing Django default form handlers?

EDIT:

I've been reading up on decorators (I'm relatively new to Python) and from what I read, decorators appear to able to modify the behaviour of existing Python code. Would it be possible to change the runtime behaviour of the existing form handler to solve the above using a decorator? I obviously have to (1) develop the decorator and (2) attach it to the default handler. I'm not sure if (2) is possible as it has to be done runtime. I cannot patch the Django code running on GAE...

Answer 1

Well, I finally managed to get this working. Here's what I did in case anyone runs into this as well:

(1) I removed the ImageFile attribute from my model. It ended up causing Django to try and do a file upload from the file system which is not allowed in GAE.

(2) I added a Blobstore key to my model which is basically the key to the GAE BlobStore blob and is required to be able to serve the image at a later stage. On a side note: this attribute has limited length using the GAE SDK but is considerably longer in GAE production. I ended up defining a TextField for it.

(3) Use storage.py with Daniel Roseman's adaption from this question and add the BlobstoreFileUploadHandler to the file handlers in your SETTINGS.PY. It will ensure that the Blobstore key is there in the request for you to save with your model.

(4) I created a custom admin form which contains an ImageField named "image". This is required as it allows you to pick a file. The ImageField is actually "virtual" as its only purpose on the form is to allow me to pick a file for uploading. This is crucial as per (1).

(5) I overwrote render_change_form() method of my ModelAdmin class which will prepare a Blobstore upload url. The upload url has two versions: one for adding new images and one saving changes to existing. Upload urls are passed to the template via the context object.

(6) I modified the change_form.html to include the Blobstore upload url from (5) as the form's action.

(7) I overwrote the save_model() method of my ModelAdmin:

def save_model(self, request, obj, form, change):
    if request.FILES.has_key("blobkey"):
        blob_key = request.FILES["blobkey"].blobstore_info._BlobInfo__key
        obj.blobstore_key = blob_key
    super(PhotoFeatureAdmin, self).save_model(request, obj, form, change)

This allows me to retrieve the blob key as set by the upload handler and set it as a property of my model.

For deletion of image models, I added a special function which is triggered by the delete signal of the model. This will keep the Blobstore in sync with the image models in the app.

That's it. The above allows to upload images to the blob store of GAE where each blob is neatly wrapped in a Django model object which admin users can maintain. The good thing is that there's no need to duplicate standard Django behaviour and the model object of the image can easily be extended with attributes in the future.

Final word: in my opinion the support for blobs in plain Django on GAE is currently very poor considering the above. It should be much easier to achieve this, without having to rely on Django non-rel code and a rather long list of modifications; alternatively Google should state something about this in their developer documents. Unless I missed something, this is undocumented territory.