Manually populate an ImageField - ajax

Question

I have a models.ImageField which I sometimes populate with the corresponding forms.ImageField. Sometimes, instead of using a form, I want to update the image field with an ajax POST. I am passing both the image filename, and the image content (base64 encoded), so that in my api view I have everything I need. But I do not really know how to do this manually, since I have always relied in form processing, which automatically populates the models.ImageField.
How can I manually populate the models.ImageField having the filename and the file contents?
EDIT
I have reached the following status:
instance.image.save(file_name, File(StringIO(data)))
instance.save()
And this is updating the file reference, using the right value configured in upload_to in the ImageField.
But it is not saving the image. I would have imagined that the first .save call would:
Generate a file name in the configured storage
Save the file contents to the selected file, including handling of any kind of storage configured for this ImageField (local FS, Amazon S3, or whatever)
Update the reference to the file in the ImageField
And the second .save would actually save the updated instance to the database.
What am I doing wrong? How can I make sure that the new image content is actually written to disk, in the automatically generated file name?
EDIT2
I have a very unsatisfactory workaround, which is working but is very limited. This illustrates the problems that using the ImageField directly would solve:
# TODO: workaround because I do not yet know how to correctly populate the ImageField
# This is very limited because:
# - only uses local filesystem (no AWS S3, ...)
# - does not provide the advance splitting provided by upload_to
local_file = os.path.join(settings.MEDIA_ROOT, file_name)
with open(local_file, 'wb') as f:
f.write(data)
instance.image = file_name
instance.save()
EDIT3
So, after some more playing around I have discovered that my first implementation is doing the right thing, but silently failing if the passed data has the wrong format (I was mistakingly passing the base64 instead of the decoded data). I'll post this as a solution

Answer 1

Just save the file and the instance:
instance.image.save(file_name, File(StringIO(data)))
instance.save()
No idea where the docs for this usecase are.

Answer 2

You can use InMemoryUploadedFile directly to save data:
file = cStringIO.StringIO(base64.b64decode(request.POST['file']))
image = InMemoryUploadedFile(file,
field_name='file',
name=request.POST['name'],
content_type="image/jpeg",
size=sys.getsizeof(file),
charset=None)
instance.image = image
instance.save()