GeoDjango distance filter with distance value stored within model - query

Question

I have an Order model, that has an origin PointField and a range IntegerField. Furthermore, there is an UserProfile model, which has a geo_location PointField. Now, I have an User instance, user. I want to select all Orders, whose distance between Order.origin and user.userprofile.geo_location is less then the value (meters) in the Order.range model field.

So again, simplified models:

class Order(models.Model):
    origin = models.PointField()
    range = models.IntegerField(blank=True, default=10000)

class UserProfile(models.Model):
    geo_location = models.PointField()

I've got this working (passing the distance statically):

>>> Order.objects.filter(origin__distance_lte=(user.profile.geo_location, D(m=3000)))

My next (unsuccessful) try was to use an F() expression to use the value from the Order.range field:

>>> Order.objects.filter(origin__distance_lte=(user.profile.geo_location, D(m=F('range'))))
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/Users/danger/devel/.virtualenvs/proj/lib/python2.7/site-packages/django/contrib/gis/measure.py", line 165, in __init__
self.m, self._default_unit = self.default_units(kwargs)
  File "/Users/danger/devel/.virtualenvs/proj/lib/python2.7/site-packages/django/contrib/gis/measure.py", line 49, in default_units
if not isinstance(value, float): value = float(value)
 TypeError: float() argument must be a string or a number

I think the problem is that the D() isn't ran lazily - I can understand that. So I tried to just take the raw value from the range field (integer) which I supposed to work, but:

>>> Order.objects.filter(origin__distance_lte=(user.profile.geo_location, F('range')))
Traceback (most recent call last):
File "<console>", line 1, in <module>
File "/Users/danger/devel/.virtualenvs/proj/lib/python2.7/site-packages/django/db/models/query.py", line 69, in __repr__
data = list(self[:REPR_OUTPUT_SIZE + 1])
File "/Users/danger/devel/.virtualenvs/proj/lib/python2.7/site-packages/django/db/models/query.py", line 84, in __len__
self._result_cache.extend(self._iter)
File "/Users/danger/devel/.virtualenvs/proj/lib/python2.7/site-packages/django/db/models/query.py", line 273, in iterator
for row in compiler.results_iter():
File "/Users/danger/devel/.virtualenvs/proj/lib/python2.7/site-packages/django/db/models/sql/compiler.py", line 680, in results_iter
for rows in self.execute_sql(MULTI):
File "/Users/danger/devel/.virtualenvs/proj/lib/python2.7/site-packages/django/db/models/sql/compiler.py", line 725, in execute_sql
sql, params = self.as_sql()
File "/Users/danger/devel/.virtualenvs/proj/lib/python2.7/site-packages/django/db/models/sql/compiler.py", line 68, in as_sql
where, w_params = self.query.where.as_sql(qn=qn, connection=self.connection)
File "/Users/danger/devel/.virtualenvs/proj/lib/python2.7/site-packages/django/db/models/sql/where.py", line 92, in as_sql
sql, params = child.as_sql(qn=qn, connection=connection)
File "/Users/danger/devel/.virtualenvs/proj/lib/python2.7/site-packages/django/db/models/sql/where.py", line 95, in as_sql
sql, params = self.make_atom(child, qn, connection)
File "/Users/danger/devel/.virtualenvs/proj/lib/python2.7/site-packages/django/contrib/gis/db/models/sql/where.py", line 47, in make_atom
spatial_sql = connection.ops.spatial_lookup_sql(data, lookup_type, params_or_value, lvalue.field, qn)
File "/Users/danger/devel/.virtualenvs/proj/lib/python2.7/site-packages/django/contrib/gis/db/backends/postgis/operations.py", line 531, in spatial_lookup_sql
raise ValueError('Argument type should be %s, got %s instead.' % (arg_type, type(value[1])))
ValueError: Argument type should be (<class 'decimal.Decimal'>, <class 'django.contrib.gis.measure.Distance'>, <type 'float'>, <type 'int'>, <type 'long'>), got <class 'django.db.models.expressions.F'> instead.

So how can I accomplish what I am trying to? Any help is appreciated!

Answer 1

I think you're going to have to drop some SQL in to do what you want, as the GeoDjango helpers don't have a way of you making a suitable Distance object that is simultaneously a Django F object (ie field lookup). You don't say which database you're using, but here's how you do it with PostGIS:

Order.objects.all().extra(
    where=['ST_Distance(origin, ST_PointFromText(%s, 4326)) <= CAST(range AS double precision) / 1000'],
    params=[user.profile.geo_location.wkt]
)

Let's explain what's happening here, because it's pretty thorny. Going from the left:

• .extra() allows you to add extra fields and restrictions into a query; here we're adding a restriction
• ST_Distance() is the PostGIS function that the GeoDjango distance_lte operator converts into (from the Django documentation)
• ST_PointFromText() converts from WKT syntax into a PostGIS Geometry object
• 4326 is the default SRID for Django, but not for PostGIS so we have to specify it
• we have to CAST your field to double precision because you're using an integer
• then we have to divide by 1000 to convert from metres into kilometres, which I believe is what we need
• GeoDjango Point fields have an accessor .wkt which gives their WKT representation, which we needed earlier in our ST_PointFromText() call

Note that according to the PostGIS documentation, ST_Distance() doesn't use indexes, so you may want to investigate using ST_DWithin() instead (it's documented right after ST_Distance()).