Geographic database functions with two parameters should accept expressions in one or the other arguments

[Antoine Auberger]

Context

With the release of Django 1.9 database functions were introduced to simplify measurements, operations, lookups, etc. within the queryset.

Some of the functions introduced take two arguments : Difference, Distance, Intersection, SymDifference and Union.
The doc (1) states these functions "Accepts two geographic fields or expressions and returns the distance between them".

Problem

As they all inherit from GeoFuncWithGeoParam (2), they actually expect expr1 to be an expression (column or value), and expr2 to be a GEOSGeometry object, despite stated otherwise.

If an expression is passed as expr2, this error is raised :

In [0]: MyClass.objects.annotate(distance=Distance('field1', 'field2'))

[...]

ValueError: Please provide a geometry attribute with a defined SRID.

Solutions

1. Edit the doc to mention these limitations
2. (much better) Fix the code to accept expressions in expr2 as documented (and GEOSGeometry objects as expr1). Most of the backends support geographic measurements and operations between two fields, so there is not much to change.

Happy to work on a patch if the bug is validated.

(1) ​https://docs.djangoproject.com/en/dev/ref/contrib/gis/functions/#distance
(2) ​https://github.com/django/django/blob/1.9/django/contrib/gis/db/models/functions.py#L91

[Simon Charette]

I'm not that familar with the GIS stuff but I could see queries using expression such as Distance('field1', 'field2') being useful and I can't see why we'd like to prevent that.

[Sergey Fedoseev]

Duplicate of #25605.