Custom imagekit cache backend

gistfile1.py

import os

from django.db.models.signals import pre_save

from celery.execute import send_task


class CustomCacheStateBackend(object):

    def has_changed(self, file):
        if hasattr(file.instance, "_image_changed"):
            return file.instance._image_changed
        return True

    def is_invalid(self, file):
        if not getattr(file, '_file', None):
            # No file on object. Have to check storage.
            return not file.storage.exists(file.name)
        return False

    def validate(self, file):
        """
        Generates a new image by running the processors on the source file.

        """
        if self.is_invalid(file):
            content = file.generate()
            if content:
                file.storage.save(file.name, content)

    def invalidate(self, file):
        from myapp.models import SomeModel
        if self.has_changed(file):
            file.delete(save=False)

            if isinstance(file.instance, SomeModel):
                send_task("myapp.models.regenerate_spec_images", [file.instance.pk, ])

    def clear(self, file):
        file.delete(save=False)

    def register_field(self, model_class, field, attname):
        pre_save.connect(_pre_save_handler, sender=model_class)


def _pre_save_handler(sender, instance, **kwargs):
    from myapp.models import SomeModel
    if isinstance(instance, SomeModel):
        obj = SomeModel.objects.get(pk=instance.pk)
        if os.path.dirname(instance.image.path) != os.path.dirname(obj.image.path):
            instance._image_changed = True
        else:
            instance._image_changed = False