Question

Hi Stackoverflow people,

I am confused with m2m queries in Django. I have a model RadioStations which lists radio stations around a continent (simply name and the available country) and has the following declaration:

class Station(models.Model):
    name = models.CharField(_('Station Name'), max_length=255
    reference = models.URLField(_('Link'), blank=True, verify_exists=True)  
    country = models.ManyToManyField(WorldBorder)

The class WorldBorder follows the GeoDjango example here.

Now I would like to search for all stations in the US.
If I use:

s = Station.objects.filter(country__name__contains = "United States")

I get all stations in the US. However, if I now search with a user location, e.g.

pnt = fromstr('POINT(-96.876369 29.905320)', srid=4326)
s = Station.objects.filter(country__mpoly__contains = pnt)

the result of the query is empty (even so the point is located in the U.S.
Is that related to the way of doing a m2m query? Why would the results of the query being empty? Is there a different way of addressing the m2m relationship?

Thank you for your suggestions!

Answer 1

The solution to the question is the follows:

Instead of going from Stations to WorldBorder, I ended up going the other way.
Django allows the reversed look up through the attribute_set.all() method.

The solution is to look up which country contains the Point with

country = WorldBorder.objects.get(mpoly__contains = ref_point)

and then look up all Stations which contain the country with

station_list = country.stations_set.all()

Note that the set.all() requires a get query, and not a filter query.
More background on the set.all() method can be found here.