AVG Decimal simplejson (Decimal is not JSON serializable)

If you use AVG aggregation function on Integer field in MySQL,

hotels = Hotel.objects.filter(city = "Paris").extra(
    select={
        'avg_rate': 'SELECT AVG(hotel_review.rate) FROM hotel_review WHERE hotel_review.is_active = 1 AND hotel_review.hotel_id = hotel_hotel.id',
    },
)

you will have in a result { “avg_rate” : Decimal(’2.6667′)} and an “Exception Value: Decimal(’2.6667′) is not JSON serializable”.
In this case you can do this:

from django.utils import simplejson
from decimal import Decimal

class MyJSONEncoder(simplejson.JSONEncoder):
    """JSON encoder which understands decimals."""

    def default(self, obj):
        '''Convert object to JSON encodable type.'''
        if isinstance(obj, Decimal):
            return "%d" % obj
        return simplejson.JSONEncoder.default(self, obj)

hotels_json = simplejson.dumps(list(hotels.values()), cls = MyJSONEncoder)

You can read about extra search fields and JSON serialization in our article