Opened 7 years ago

Last modified 3 weeks ago

#28696 assigned New feature

Add the ability to natively filter geometries by geom_type

Reported by: Geoffrey Fairchild Owned by: Ahmed Ibrahim
Component: GIS Version: dev
Severity: Normal Keywords:
Cc: Triage Stage: Accepted
Has patch: yes Needs documentation: no
Needs tests: no Patch needs improvement: yes
Easy pickings: no UI/UX: no

Description

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.

Change History (11)

comment:1 by Simon Charette, 7 years ago

Triage Stage: UnreviewedAccepted
Version: 1.11master

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'},
)

in reply to:  1 comment:2 by Geoffrey Fairchild, 7 years ago

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

Last edited 7 years ago by Geoffrey Fairchild (previous) (diff)

comment:3 by Simon Charette, 7 years ago

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

comment:4 by Sergey Fedoseev, 7 years ago

Owner: changed from nobody to Sergey Fedoseev
Status: newassigned

comment:5 by Sergey Fedoseev, 7 years ago

Owner: Sergey Fedoseev removed
Status: assignednew

comment:6 by Sergey Fedoseev, 7 years ago

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

in reply to:  6 comment:7 by Ahmed Ibrahim, 2 months ago

Replying to Sergey Fedoseev:

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

I'll handle this ticket

Do we have an interface to handle returning different field types per database? or do I need to override as_sql ?

comment:8 by Ahmed Ibrahim, 2 months ago

Owner: set to Ahmed Ibrahim
Status: newassigned

comment:9 by Simon Charette, 2 months ago

Ahmed, you'll need to define a specialized transform for this to work similar to the Func discussed in comment:1. To deal with Oracle you'll have to define an as_oracle method that uses a CASE/WHEN to map back the integer return values to the proper string types.

in reply to:  9 comment:10 by Ahmed Ibrahim, 2 months ago

Replying to Simon Charette:

Ahmed, you'll need to define a specialized transform for this to work similar to the Func discussed in comment:1. To deal with Oracle you'll have to define an as_oracle method that uses a CASE/WHEN to map back the integer return values to the proper string types.

Thank you Simon for your valuable information, this helped me, I will be doing the PR soon

comment:11 by Mariusz Felisiak, 3 weeks ago

Has patch: set
Patch needs improvement: set
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