How to use custom managers in chain queries?

I made a custom manager that has to randomize my query:

class RandomManager(models.Manager):

    def randomize(self):        
        count = self.aggregate(count=Count('id'))['count']
        random_index = random.randint(0, count - 1)
        return self.all()[random_index]

When I use the method defined in my manager in the first place, it's works ok:

>>> PostPages.random_objects.randomize()
>>> <PostPages: post 3>

I need to randomize the already filtered query. When I tried to use the manager and the method in chain I got an error:

PostPages.random_objects.filter(image_gallary__isnull=False).randomize()
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
/home/i159/workspace/shivaroot/shivablog/<ipython-input-9-98f654c77896> in <module>()
----> 1 PostPages.random_objects.filter(image_gallary__isnull=False).randomize()

AttributeError: 'QuerySet' object has no attribute 'randomize'

Result of filtering is not an instance of model class, but it's django.db.models.query.QuerySet, so that it does not have my manager and method, respectively. Is there a way to use custom manager in chain query?

Looks like this snippet provides a solution to your situation: Custom managers with chainable filters.