Can't use PostGIS function ST_OrderingEquals with GeoDjango

[Evan Heidtmann]

Unless I'm missing something, Django 1.8 provides no facility in the ORM to use the ST_OrderingEquals() function to compare two geometries. I would like to use this function in a query.

Am I correct? If so, I may attempt a patch or at least a workaround.

Thanks!

Ref: ​http://postgis.net/docs/manual-1.3/ch06.html#ST_OrderingEquals

P.S. It's another question entirely whether the 'exact' lookup should use this function. For comparison, two LineString objects with opposite vertex ordering are not considered equal, so why should they be considered equal at the DB level?

[Evan Heidtmann]

For other users who may find this, here's my workaround that allows me to find the geometry that matches an input geometry, both in vertex values and in vertex ordering. A Django wizard may be able to improve this and I would welcome any criticism.

class SegmentModel(models.Model):
    ...

    @classmethod
    def get_or_create_for_line(cls, line):
        from django.contrib.gis.db.backends.postgis.adapter import PostGISAdapter

        # We have to run a query outside of Manager.raw() because Manager.raw()
        # always supplies a params argument to cursor.execute(), and sometimes
        # the binary form of the line contains a percent sign.
        from django.db import connection
        with connection.cursor() as cursor:
            sql = 'SELECT id FROM ride_segment WHERE line ~= {line_serialized} AND ST_OrderingEquals(line, {line_serialized})'.format(
                line_serialized=str(PostGISAdapter(line)))
            cursor.execute(sql)
            ids = cursor.fetchmany()

        if len(ids) > 1:
            raise cls.MultipleObjectsReturned('Multiple segments for line {}'.format(line.ewkt))
        elif len(ids) == 0:
            return (cls.objects.create(line=line), True)
        else:
            return (cls.objects.get(id=ids[0][0]), False)