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

Increment/Decrement Field value

Incrementing or decrementing field values can be done by using the F() function.
UPDATE FIELD = FIELD + 1 WHERE… ?

from django.db.models import F

# views.py ...
offer = Offer.objects.get(...)
# SQL: UPDATE field_to_increment = field_to_increment + 1 ...
offer.field_to_increment = F('field_to_increment') + 1
offer.save()

Accessing Request Object In Form

Something that I use quite regularly is accessing the request object in a form.

It is actually pretty simple, in your form you add a custom __init__ function that sets the request as a parameter:

class PasswordRecoverForm(PasswordResetForm):
    def __init__(self, *args, **kwargs):
    self.request = kwargs.pop('request', None)
    super(PasswordRecoverForm, self).__init__(*args, **kwargs)

def clean_email(self):
    """Do something with the request"""
    print self.request

Then in your view code you can pass the request as a parameter.

form = PasswordRecoverForm(request=request)

Change Django Auth Group Ordering

Following on from my last post on changing the behaviour of the Django contrib auth models. Another thing you might like to do is to change the default ordering:

from django.contrib.auth.models import User
User._meta.ordering = ['-email']

or

from django.contrib.auth.models import Group
Group._meta.ordering = ['-id']

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