Add the ability to natively filter geometries by geom_type

[Geoffrey Fairchild]

First, version information just in case it matters:

• Python 3.6.3
• PostgreSQL 9.6.5
• PostGIS 2.4.0
• Django 1.11.6
• psycopg2 2.7.3.1

I have a some models that essentially look like this:

class Location(models.Model):
    name = models.CharField(max_length=255)
    # other fields

class LocationBorder(BoundaryBase):
    geometry = models.GeometryField()
    location = models.ForeignKey(Location, on_delete=models.CASCADE)

Locations can therefore have multiple borders, and locations can be of different types (e.g., polygon, multipolygon, point).

What I want to do is pull all location borders that are either polygons or multipolygons because I need to run an analysis on just those types of locations. I know all my borders have geom_type attributes:

> for location_border in LocationBorder.objects.all():
>    print(location_border.geometry.geom_type)
Polygon
MultiPolygon
Polygon
Point
...

So I thought that I could filter on that attribute, but it turns out I can't:

> for location_border in LocationBorder.objects.filter(Q(geometry__geom_type='Polygon') | Q(geometry__geom_type='MultiPolygon'))
>    print(location_border.geometry.geom_type)
...
django.core.exceptions.FieldError: Unsupported lookup 'geom_type' for GeometryField or join on the field not permitted.

I did some searching, and I came across ​this Stack Overflow thread, which essentially says that it's not possible to do this except by using extra(). That Stack Overflow thread was originally updated 2014, so I was shocked to see that this still isn't possible. Why is it that I can't filter on the geom_type attribute?

I should mention that using extra() does indeed work for me:

> for location_border in LocationBorder.objects.extra(where=["GeometryType(geometry)='POLYGON' OR GeometryType(geometry)='MULTIPOLYGON'"]):
>    print(location_border.geometry.geom_type)
Polygon
MultiPolygon
Polygon
Polygon
...

While this works, it's verbose and not very Pythonic/Djangonic.

[Simon Charette]

The reason why you can't filter against GeometryType is simply that this lookup have not been aded yet.

In the mean time you could use the expression API to achieve the same thing without relying on extra().

from django.db.models import CharField, Func

class GeometryType(Func):
    function = 'GeometryType'
    output_field = CharField()

LocationBorder.objects.annotate(
    geom_type=GeometryType('geometry'),
).filter(
    geom_type__in={'POLYGON', 'MULTIPOLYGON'},
)

[Geoffrey Fairchild]

Nice! This is cleaner than what I'm currently doing.

[Simon Charette]

It looks like this could be added on all supported backends by using GeometryType on PostGIS, Spatialite, and MySQLGIS and SDO_GTYPE ​on OracleGIS.

[Sergey Fedoseev]

GeometryType works differently on PostGIS and SpatiaLite for 3d geometries, SDO_GTYPE returns integer, but not string.