hashable.py

class Hashable(models.Model):

    hash_method_prefix = 'compute_hash_'
    default_hash_method = 'compute_hash_default'
    hashable_fields = []

    class Meta:
        abstract = True    

    def compute_hash_default(self, field_name, hash_function):
        return hash_function(getattr(self, field_name), self)

    def save(self, *args, **kwargs):

        for hash_infos in self.hashable_fields:

            field_name, hash_field_name = hash_infos[:2]
            hash_function = hash_infos[2] if len(hash_infos) == 3 else None

            hash_method = getattr(self, '%s%s' % (self.hash_method_prefix, field_name),
                                  getattr(self, self.default_hash_method))
            setattr(self, hash_field_name, hash_method(field_name, hash_function))

        return super().save(*args, **kwargs)


# instance not needed by default but we could imagine some use case
def md5_hash_file(file_field, instance, default=settings.EMPTY_FILE_HASH):
    try:
        md5 = hashlib.md5()
        for chunk in file_field.chunks():
            md5.update(chunk)
        return md5.hexdigest()
    except:
        return default


class MyModel(Hashable):
    img = models.ImageField(...)
    hash = models.CharField(...)

    hashable_fields = [
        ('img', 'hash', md5_hash_file)
    ]


class OtherModel(Hashable):
    file = models.FileField(...)
    hash_file = models.CharField(...)

    file2 = models.FileField(...)
    hash_file2 = models.CharFiel(...)

    hashable_fields = [
        ('file', 'hash_file', md5_hash_file),
        ('file2', 'hash_file2')
    ]

    def compute_hash_file2(self, file_name, hash_function):
        # here some complicated code
        return some_hash