Clarify docs on manually setting and saving a FileField.

[Gordon Wrigley]

The documentation talks about how to set a filefield on a new object and how to set a filefield using a form but I couldn't find anything that talked about how to directly set a filefield on an existing object.
​https://docs.djangoproject.com/en/3.1/topics/http/file-uploads/#handling-uploaded-files-with-a-model

Maybe those docs could use some extra detail on how you can set an existing file field by either assigning a ContentFile (with a name) to the field and saving the instance, or by calling save on the field and passing a ContentFile and a name. And particularly how the name is required in both cases and will be passed through upload_to.

Further figuring this out for myself was substantially complicated by the following behaviour:

In [24]: e=MyModel.objects.first()

In [25]: e.my_file
Out[25]: <FieldFile: None>

In [26]: e.my_file=ContentFile(content=b"fred")

In [27]: e.save()

In [28]: e=MyModel.objects.first()

In [29]: e.my_file
Out[29]: <FieldFile: None>

In [30]: e.my_file=ContentFile(content=b"bob", name="bob.txt")

In [31]: e.save()

In [32]: e=MyModel.objects.first()

In [33]: e.my_file
Out[33]: <FieldFile: files/5bc2fe4c-4262-4134-9397-c740de5a7edf/bob.txt>

In [34]: e.my_file.open().read()
Out[34]: b'bob'

Particularly 26-29 where setting the filefield to a ContentFile with no name and then saving is effectively just ignored with no error.
Is this expected behaviour?

[Carlton Gibson]

Hi Gordon. Thanks for the report.

I agree this is an area that folks find confusing. Happy to review suggested changes.

I think the right place for a clarification would be in the ​Using files in models section of the Files topic documentation.

Perhaps a review of the tests in ​tests/files to ensure we've covered all the relevant cases would also be in order.

[Hasanul Islam]

Hi Carlton,
As the file is not being saved without the name attribute of the file, shouldn't we generate the name property randomly if not provided by the user?

If yes, I will update the relevant codes implementing random name generation, otherwise, I will update the documentation mentioning name is required. However, I think we should not enforce the user to provide the name attribute. I would prefer to update the code.

[Jesse]

Replying to Hasanul Islam:

Hi Carlton,
As the file is not being saved without the name attribute of the file, shouldn't we generate the name property randomly if not provided by the user?

If yes, I will update the relevant codes implementing random name generation, otherwise, I will update the documentation mentioning name is required. However, I think we should not enforce the user to provide the name attribute. I would prefer to update the code.

TL;DR: Allow Files (not just ContentFiles) with a blank/None name. In FileField.save(), add a check after the generate_filename() call to make sure the file actually has a name once we're ready to save it to disk.

When saving the model it seems to still use the upload_to argument. If you pass something for upload_to it will overwrite whatever was passed for name.

Here's an example of my use case. A user can upload a picture through the admin panel. Whenever a new picture is uploaded I create a thumbnail for it.

def photo_upload_path(instance, original_filename):
    filename = hashlib.sha256(str(time.time()).encode("utf-8")).hexdigest()
    filename = filename[:16]
    return f"photos/{filename}.jpg"


def thumbnail_upload_path(instance, original_filename):
    filename = os.path.basename(instance.photo.name)
    return f"photos/thumbnails/{filename}"


class MyModel(models.Model):
    photo = models.ImageField(upload_to=photo_upload_path)

    # Hidden in Django admin. Managed by us.
    photo_thumbnail = models.ImageField(upload_to=thumbnail_upload_path)

    def save(self):
        img = Image.open(self.photo.path)
        img.thumbnail(settings.THUMBNAIL_SIZE)

        contents = io.BytesIO()
        img.save(contents, "JPEG")

        self.photo_thumbnail = ContentFile(contents.getvalue(), name="foo")

        super.save()

In this case, the photo gets renamed to photos/<hash>.jpg, and I want thumbnails to go to photos/thumbnails/<hash>.jpg.

I resize the image, save its contents to a BytesIO object, and create the ContentFile object. However I have to pass in a bogus name for the ContentFile even though it will use the upload_to to overwrite it. If I pass an empty name Django seems to throw the file away and it doesn't get saved.

[Joshua Massover]

#33388 was closed as a duplicate to this one. I included an example but not in the ​Using files in models as Carlton suggested, I put it in ​Handling uploaded files Glad to move it where ever if this does not suffice.

​https://github.com/django/django/pull/15245

[Mariusz Felisiak]

See ​comment.

[Joshua Massover]

Replying to Mariusz Felisiak:

See ​comment.

Updated the PR

[Joshua Massover]

PR updated from more feedback

[Joshua Massover]

Updated again from more feedback!

[Mariusz Felisiak <felisiak.mariusz@…>]

In c9d6e35:

Fixed #32243 -- Added docs examples for manually saving Files.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 76c80d9:

[4.0.x] Fixed #32243 -- Added docs examples for manually saving Files.

Backport of c9d6e3595cfd0aa58cde1656bd735ecfcd7a872b from main

[GitHub <noreply@…>]

In 25514b60:

Refs #32243 -- Fixed typo in docs/topics/files.txt.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 3714b44:

[4.0.x] Refs #32243 -- Fixed typo in docs/topics/files.txt.

Backport of 25514b604a64686ba603bf10a8a63390dc38b79d from main