id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
29955	Support dwithin lookup on F expressions for distance stored in model	Peter Bex	Simon Charette	"Today I had to find all points of interest which occur within distance of a trajectory LineString, where each point of interest object stores how far away it should be from the trajectory in order to be included (we use it for determining which fuel stations are allowed for refueling).

I wanted to write the ""obvious"" thing like this:

{{{
import django.db.models import Model, TextField, F
from django.contrib.gis.db.models import PointField
from django.contrib.gis.geos import LineString

class PointOfInterest(Model):
    name = TextField()
    location = PointField(geography=True)

PointOfInterest.objects.filter(location__dwithin=(LineString(....), F('allowed_distance')))
}}}

Unfortunately this gives an error. The only thing that's allowed as the second object in the dwithin lookup is a `Distance()` object.

I tried to get around this but couldn't figure out exactly how the internals are supposed to work. With my limited understanding of the Django query internals, I came up with this custom hack (which you'll probably find godawful):

{{{
# Hack because __dwithin does not accept fields, only fixed Distance()
# values.
@BaseSpatialField.register_lookup
class DWithinFieldLookup(DistanceLookupBase):
	lookup_name = 'dwithin_field'
	sql_template = '%(func)s(%(lhs)s, %(rhs)s)'

	def get_rhs_op(self, connection, rhs):
		return connection.ops.gis_operators['dwithin']

	# Taken from django.db.models.lookups (super-superclass)
	def get_db_prep_lookup(self, value, connection):
		if isinstance(value, GEOSGeometry):
			return super().get_db_prep_lookup(value, connection)
		else:
			return ('%s', [value])

	# Taken from django.db.models.lookups (super-superclass)
	def process_rhs(self, compiler, connection):
		value1 = self.rhs
		value2 = self.rhs_params[0]
		if self.bilateral_transforms:
			if self.rhs_is_direct_value():
				# Do not call get_db_prep_lookup here as the value will be
				# transformed before being used for lookup
				value1 = Value(value1, output_field=self.lhs.output_field)
			value1 = self.apply_bilateral_transforms(value1)
			value1 = value1.resolve_expression(compiler.query)

		if self.bilateral_transforms:
			if self.rhs_is_direct_value():
				# Do not call get_db_prep_lookup here as the value will be
				# transformed before being used for lookup
				value2 = Value(value2, output_field=self.lhs.output_field)
			value2 = self.apply_bilateral_transforms(value2)
			value2 = value2.resolve_expression(compiler.query)

		if hasattr(value1, 'as_sql'):
			sql1, params1 = compiler.compile(value1)
			sql1 = '(' + sql1 + ')'
		else:
			sql1, params1 = self.get_db_prep_lookup(value1, connection)

		if hasattr(value2, 'resolve_expression'):
			value2 = value2.resolve_expression(query=compiler.query, allow_joins=True, reuse=None, summarize=False, for_save=False)
			sql2, params2 = compiler.compile(value2)
		elif hasattr(value2, 'as_sql'):
			sql2, params2 = compiler.compile(value2)
			sql2 = '(' + sql2 + ')'
		else:
			sql2, params2 = self.get_db_prep_lookup(value2, connection)

		return sql1 + ',' + sql2, params1 + params2
}}}

This allows me to express exactly as I originally wanted to:

{{{
PointOfInterest.objects.filter(location__dwithin_field=(LineString(....), F('allowed_distance')))
}}}"	New feature	closed	GIS	2.1	Normal	fixed	distance, query builder, F expressions	Sergey Fedoseev	Accepted	1	0	0	0	0	0