Django注释无法访问模型的floatfield

Question

I am trying annotate a queryset with the object's distance from a user's provided location. Here is what I have so far:

lat1 = request.POST['lat']
lon1 = request.POST['lon']

locations = Location.objects.annotate(distance_away = posDifference('lat', 'lon', lat1, lon1)).order_by('distance_away')[0]

Here is the posDifference function, it returns the distance between the two points in miles.

def posDifference(lat1, lon1, lat2, lon2):

    EARTH_RADIUS_IN_MILES = 3958.761

    lat1 = float(lat1)
    lon1 = float(lon1)
    lat2 = float(lat2)
    lon2 = float(lon2)

    lat1 = radians(lat1)
    lat2 = radians(lat2)

    delta_lon = radians(lon1 - lon2)
    cos_x = (sin(lat1) * sin(lat2) + cos(lat1) * cos(lat2) * cos(delta_lon)) # Distance formula
    distance = acos(cos_x) * EARTH_RADIUS_IN_MILES # Conversion to miles

    return distance

The error I am getting is ValueError: could not convert string to float: lat at line lat1 = float(lat1) in posDifference . I believe the strings 'lat' and 'lon' are being passed to the function instead of the Location object's fields lat and lon .

Answer 1

Of course they are just passed into function. You're simply calling posDifference with that parameters. I'm not sure why are you expecting other result...

You should build your function in different way, using F for referring to correct db column and Func for referring to some SQL functions ( sin , cos , acos ). It can look like this:

from django.db.models import Func, F

# It's better to build some library of used database functions first:
class Radians(Func):
    function = 'RADIANS'

class Sin(Func):
    function = 'SIN'

class Cos(Func):
    function = 'COS'

class Acos(Func):
    function = 'ACOS'

def posDifference(lat1, lon1, lat2, lon2):

    EARTH_RADIUS_IN_MILES = 3958.761

    lat1 = F(lat1)
    lon1 = F(lon1)
    lat2 = float(lat2)
    lon2 = float(lon2)

    lat1 = Radians(lat1)
    lat2 = radians(lat2)

    delta_lon = Radians(lon1) - radians(lon2)
    cos_x = (Sin(lat1) * sin(lat2) + Cos(lat1) * cos(lat2) * Cos(delta_lon)) # Distance formula
    distance = Acos(cos_x) * EARTH_RADIUS_IN_MILES # Conversion to miles

    return distance

That should work fine. More on that you can learn from django documentation

Answer 2

You sholud better use GeoDjango for that measurements. There are ready for use functions for calculating distances, comparing distances, checking if point on map is inside some range or rectangle etc. All is done on database level and requires only some database extension.