What is the best way to count all distinct values of ArrayField using django with djongo mapper

Working with Data Python Frameworks
1

What is the best way to count all distinct values in mongodb ArrayField?
Here is my model:

class tweet(models.Model):              
   topic = models.JSONField()

3 sample documents:

1. ["investment", "economy"]
2. ["investment"]
3. ["economy", "politics"]

Desire result:

[{"investment":2},{"economy":2},{"politics":1}]

Maybe something like this:

tweet.objects.annotate(t=Func(F('topic'), function='$unwind')).values_list('t').annotate(c=Count('_id'))

any help would be appreciated

I have also posted on stackoverflow: