Processing file uploads before object is saved

问题:

I've got a model like this:

class Talk(BaseModel):
  title        = models.CharField(max_length=200)
  mp3          = models.FileField(upload_to = u'talks/', max_length=200)
  seconds      = models.IntegerField(blank = True, null = True)

I want to validate before saving that the uploaded file is an MP3, like this:

def is_mp3(path_to_file):
  from mutagen.mp3 import MP3
  audio = MP3(path_to_file)
  return not audio.info.sketchy

Once I'm sure I've got an MP3, I want to save the length of the talk in the seconds attribute, like this:

audio = MP3(path_to_file)
self.seconds = audio.info.length

The problem is, before saving, the uploaded file doesn't have a path (see this ticket, closed as wontfix), so I can't process the MP3.

I'd like to raise a nice validation error so that ModelForms can display a helpful error ("You idiot, you didn't upload an MP3" or something).

Any idea how I can go about accessing the file before it's saved?

^{p.s. If anyone knows a better way of validating files are MP3s I'm all ears - I also want to be able to mess around with ID3 data (set the artist, album, title and probably album art, so I need it to be processable by mutagen).}

回答1:

You can access the file data in request.FILES while in your view.

I think that best way is to bind uploaded files to a form, override the forms clean method, get the UploadedFile object from cleaned_data, validate it anyway you like, then override the save method and populate your models instance with information about the file and then save it.

回答2:

a cleaner way to get the file before be saved is like this:

from django.core.exceptions import ValidationError

#this go in your class Model
def clean(self):
    try:
        f = self.mp3.file #the file in Memory
    except ValueError:
        raise ValidationError("A File is needed")
    f.__class__ #this prints <class 'django.core.files.uploadedfile.InMemoryUploadedFile'>
    processfile(f)

and if we need a path, ther answer is in this other question

回答3:

You could follow the technique used by ImageField where it validates the file header and then seeks back to the start of the file.

class ImageField(FileField):
    # ...    
    def to_python(self, data):
        f = super(ImageField, self).to_python(data)
        # ...
        # We need to get a file object for Pillow. We might have a path or we might
        # have to read the data into memory.
        if hasattr(data, 'temporary_file_path'):
            file = data.temporary_file_path()
        else:
            if hasattr(data, 'read'):
                file = BytesIO(data.read())
            else:
                file = BytesIO(data['content'])

        try:
            # ...
        except Exception:
            # Pillow doesn't recognize it as an image.
            six.reraise(ValidationError, ValidationError(
                self.error_messages['invalid_image'],
                code='invalid_image',
            ), sys.exc_info()[2])
        if hasattr(f, 'seek') and callable(f.seek):
            f.seek(0)
        return f