Django object manager with Haversine distance annotation method (aka filter by distance with mysql)

models.py

from django.db import models
from with_distance_manager import WithDistanceManager

class Foo(models.Model):
    longitude = models.DecimalField(max_digits=19, decimal_places=10, null=True)
    latitude = models.DecimalField(max_digits=19, decimal_places=10, null=True)

    objects = WithDistanceManager()

with_distance_manager.py

class WithDistanceManager(models.Manager):
    def with_distance(self, latitude, longitude):
        """
        Returns a QuerySet of locations annotated with their distance from the
        given point. This can then be filtered.
        Usage:
            Foo.objects.within(lat, lon).filter(distance__lt=10).count()
        @see http://stackoverflow.com/a/31715920/1373318
        """
        class Sin(Func):
            function = 'SIN'
        class Cos(Func):
            function = 'COS'
        class Acos(Func):
            function = 'ACOS'
        class Radians(Func):
            function = 'RADIANS'

        radlat = Radians(latitude) # given latitude
        radlong = Radians(longitude) # given longitude
        radflat = Radians(F('latitude'))
        radflong = Radians(F('longitude'))

        # Note 3959.0 is for miles. Use 6371 for kilometers
        Expression = 3959.0 * Acos(Cos(radlat) * Cos(radflat) *
                                   Cos(radflong - radlong) +
                                   Sin(radlat) * Sin(radflat))

        return self.get_queryset()\
            .exclude(latitude=None)\
            .exclude(longitude=None)\
            .annotate(distance=Expression)

Note this is a packaging of http://stackoverflow.com/a/31715920/1373318