#29955: Support dwithin lookup on F expressions for distance stored in model
-------------------------------------+-------------------------------------
Reporter: Peter Bex | Owner: nobody
Type: New | Status: new
feature |
Component: GIS | Version: 2.1
Severity: Normal | Keywords: distance, query
Triage Stage: | builder, F expressions
Unreviewed | Has patch: 0
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
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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')))
}}}
--
Ticket URL: <https://code.djangoproject.com/ticket/29955>
Django <https://code.djangoproject.com/>
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