Django inheritance - accessing child class fields

Question

I have the following Django models:

class Image(TimeStamp):
     hash = models.CharField(max_length=33,
                            unique=True,
                            editable=False)

     filesize = models.PositiveIntegerField(blank=True,
                                           null=True,
                                           editable=False)

class Image1(Image):
    image = models.ImageField(upload_to='image1/')

class Image2(Image):
    image = models.ImageField(upload_to='image2/')

I want to be able to automatically compute filesize and hash upon image creation and the most reasonable place seems to me in a super class. However, I need to be able to access child class image field from the super class in order to compute hash and filesize. Is there a way to achieve this?

I added this save method to the superclass, but of course it doesn't know about image:

def save(self, *args, **kwargs):
    super(Image, self).save(*args, **kwargs)

    self.hash = hashlib.md5(self.image.read()).hexdigest()
    self.filesize = self.image.size

Did you try making an image field in the parent? Then you're simply overriding the upload_to in each child. — Rob L
– Rob L, Commented Oct 26, 2017 at 18:00
Is the only difference between those models the upload_to location? If so you should just use a single model with a callable for that attribute. — Daniel Roseman
– Daniel Roseman, Commented Oct 26, 2017 at 18:18
Yes, upload_to is the only thing in common for now, but later I might add different height, width and max_file_size arguments to different child classes. — foobar
– foobar, Commented Oct 27, 2017 at 8:30
Is it worth introducing new DB fields for those so that they can be overridden? (I meant max_length, height_field and width_field of the models.ImageField in the previous comment) — foobar
– foobar, Commented Oct 27, 2017 at 8:37

foobar · Accepted Answer · 2017-10-27 11:07:15Z

Here's what I ended up doing to solve this. Thank everyone for the feedback and please let me know if this code can be improved further:

def get_image_directory(instance, filename):
    return os.path.join(instance.upload_to, filename)


class Image(TimeStamp):
    hash = models.CharField(max_length=33,
                            unique=True,
                            editable=False)

    file_size = models.PositiveIntegerField(blank=True,
                                           null=True,
                                           editable=False)

    image = models.ImageField(upload_to=get_image_directory)

    def __str__(self):
        return self.image.url.split('?')[0]

    def save(self, *args, **kwargs):
        self.hash = hashlib.md5(self.image.read()).hexdigest()
        self.file_size = self.image.size
        super(Image, self).save(*args, **kwargs)

    class Meta:
        abstract = True


class Image1(Image):
    upload_to = 'image1/'

class Image2(Image):
    upload_to = 'image2/'

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