Please read the following bits of documentation:
https://docs.djangoproject.com/en/dev/topics/db/sql/#performing-raw-sql-queries
https://docs.djangoproject.com/en/dev/ref/models/querysets/#django.db.models.query.QuerySet.extra
Cheers,
AT
On Tue, Jul 24, 2012 at 9:03 PM, jondbaker <jonathandavidba...@gmail.com>wrote:
> I've implemented the spherical law of cosines to aid in proximity-based
> searching. Everything works correctly, but I'm a bit stumped as to how I
> can return the calculated distance for a record given that it's not a field
> in the model. In the Model Manager below, row[1] represents the dynamically
> calculated distance, but I need to find a way to "attach" each calculation
> to it's corresponding record so that I can print the distance in a template.
>
> class LocationManager(models.Manager):
> '''
> '''
> def nearby_locations(self, latitude, longitude, radius):
> '''
> '''
> cursor = connection.cursor()
> if settings.DATABASES['default']['ENGINE'] ==
> 'django.db.backends.sqlite3':
> # sqlite doesn't natively support math functions, so add them
> connection.connection.create_function('acos', 1, math.acos)
> connection.connection.create_function('cos', 1, math.cos)
> connection.connection.create_function('radians', 1,
> math.radians)
> connection.connection.create_function('sin', 1, math.sin)
>
> sql = """SELECT id, (3959 * acos(cos(radians(%f)) *
> cos(radians(latitude)) * cos(radians(longitude) -
> radians(%f)) +
> sin(radians(%f)) * sin(radians(latitude))))
> AS distance FROM locations_location
> GROUP BY id HAVING distance < %d
> ORDER BY distance ASC""" % (latitude, longitude, latitude,
> int(radius))
> cursor.execute(sql)
> data = [(row[0], row[1]) for row in cursor.fetchall()]
> ids = [i[0] for i in data]
> return self.filter(id__in=ids)
>
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