Question

I have a model with PointField for location coordinates. I have a MySQL function that calculates the distance between two points called dist. I use extra() “select” to calculate distance for each returned object in the queryset. I also use extra() “where” to filter those objects that are within a specific range. Like this

query = queryset.extra(
    select={
        "distance":"dist(geomfromtext('%s'),geomfromtext('%s'))"%(loc1, loc2)
    },
    where=["1 having `distance` <= %s"%(km)]
) #simplified example

This works fine for getting and reading the results, except when I try counting the resultset I get the error that ‘distance’ is not a field. After exploring a bit further, it seems that count ignores the “select” from extra and just uses “where”. While the full SQL query looks like this:

SELECT (dist(geomfromtext('POINT (-4.6858300000000003 36.5154300000000021)'),geomfromtext('POINT (-4.8858300000000003 36.5154300000000021)'))) AS `distance`, `testmodel`.`id`, `testmodel`.`name`, `testmodel`.`email`, (...) FROM `testmodel` WHERE 1 having `distance` <= 50.0

The count query is much shorter and doesn’t have the dist selection part:

SELECT COUNT( `testmodel`.`id`) FROM `testmodel` WHERE 1 having `distance` <= 50.0

Logically, MySQL gives an error because “distance” is undefined. Is there a way to tell Django it has to include the extra select for the count?

Thanks for any ideas!

Answer 1

You could use a raw query if you are not plannig to use any other database system.

    params = {'point1':wktpoint1, 'point2':wktpoint2}
    query = """
        SELECT
            dist(%(point1)s, %(point2)s)
        FROM
            testmodel
    ;"""
    query_set = self.raw(query, params)

Also, if you need more GIS support, you should evaluate PostgreSQL+PostGIS (If you don’t like to reinvent the wheel, you should not make your own dist function)

Django offers GIS support through GeoDjango. There you got functions like distance. You should check support here

In order to use GeoDjango you need to add a field on yout model, to tell them to use the GeoManager, Then you can start doing geoqueries, and you should have no problems with count.

with mysql you cando something like this using geodjango

### models.py
from django.contrib.gis.db import models
class YourModel(models.Model):
    your_geo_field=models.PolygonField()
    #your_geo_field=models.PointField()
    #your_geo_field=models.GeometryField()
    objects = models.GeoManager()

### your code
from django.contrib.gis.geos import *
from django.contrib.gis.measure import D
a_geom=fromstr('POINT(-96.876369 29.905320)', srid=4326)
distance=5
YoourModel.objects.filter(your_geo_field__distance_lt=(a_geom, D(m=distance))).count()

you can see better examples here and the reference here