django1/django/contrib/gis/db/models/query.py

import warnings

from django.contrib.gis.db.models.fields import (
    GeometryField, LineStringField, PointField, get_srid_info,
)
from django.contrib.gis.db.models.lookups import GISLookup
from django.contrib.gis.db.models.sql import (
    AreaField, DistanceField, GeomField, GMLField,
)
from django.contrib.gis.geometry.backend import Geometry
from django.contrib.gis.measure import Area, Distance
from django.db import connections
from django.db.models.expressions import RawSQL
from django.db.models.fields import Field
from django.db.models.query import QuerySet
from django.utils import six
from django.utils.deprecation import RemovedInDjango20Warning


class GeoQuerySet(QuerySet):
    "The Geographic QuerySet."

    # ### GeoQuerySet Methods ###
    def area(self, tolerance=0.05, **kwargs):
        """
        Returns the area of the geographic field in an `area` attribute on
        each element of this GeoQuerySet.
        """
        # Performing setup here rather than in `_spatial_attribute` so that
        # we can get the units for `AreaField`.
        procedure_args, geo_field = self._spatial_setup(
            'area', field_name=kwargs.get('field_name'))
        s = {'procedure_args': procedure_args,
             'geo_field': geo_field,
             'setup': False,
             }
        connection = connections[self.db]
        backend = connection.ops
        if backend.oracle:
            s['procedure_fmt'] = '%(geo_col)s,%(tolerance)s'
            s['procedure_args']['tolerance'] = tolerance
            s['select_field'] = AreaField('sq_m')  # Oracle returns area in units of meters.
        elif backend.postgis or backend.spatialite:
            if backend.geography:
                # Geography fields support area calculation, returns square meters.
                s['select_field'] = AreaField('sq_m')
            elif not geo_field.geodetic(connection):
                # Getting the area units of the geographic field.
                s['select_field'] = AreaField(Area.unit_attname(geo_field.units_name(connection)))
            else:
                # TODO: Do we want to support raw number areas for geodetic fields?
                raise Exception('Area on geodetic coordinate systems not supported.')
        return self._spatial_attribute('area', s, **kwargs)

    def centroid(self, **kwargs):
        """
        Returns the centroid of the geographic field in a `centroid`
        attribute on each element of this GeoQuerySet.
        """
        return self._geom_attribute('centroid', **kwargs)

    def difference(self, geom, **kwargs):
        """
        Returns the spatial difference of the geographic field in a `difference`
        attribute on each element of this GeoQuerySet.
        """
        return self._geomset_attribute('difference', geom, **kwargs)

    def distance(self, geom, **kwargs):
        """
        Returns the distance from the given geographic field name to the
        given geometry in a `distance` attribute on each element of the
        GeoQuerySet.

        Keyword Arguments:
         `spheroid`  => If the geometry field is geodetic and PostGIS is
                        the spatial database, then the more accurate
                        spheroid calculation will be used instead of the
                        quicker sphere calculation.

         `tolerance` => Used only for Oracle. The tolerance is
                        in meters -- a default of 5 centimeters (0.05)
                        is used.
        """
        return self._distance_attribute('distance', geom, **kwargs)

    def envelope(self, **kwargs):
        """
        Returns a Geometry representing the bounding box of the
        Geometry field in an `envelope` attribute on each element of
        the GeoQuerySet.
        """
        return self._geom_attribute('envelope', **kwargs)

    def force_rhr(self, **kwargs):
        """
        Returns a modified version of the Polygon/MultiPolygon in which
        all of the vertices follow the Right-Hand-Rule.  By default,
        this is attached as the `force_rhr` attribute on each element
        of the GeoQuerySet.
        """
        return self._geom_attribute('force_rhr', **kwargs)

    def geojson(self, precision=8, crs=False, bbox=False, **kwargs):
        """
        Returns a GeoJSON representation of the geometry field in a `geojson`
        attribute on each element of the GeoQuerySet.

        The `crs` and `bbox` keywords may be set to True if the user wants
        the coordinate reference system and the bounding box to be included
        in the GeoJSON representation of the geometry.
        """
        backend = connections[self.db].ops
        if not backend.geojson:
            raise NotImplementedError('Only PostGIS 1.3.4+ and SpatiaLite 3.0+ '
                                      'support GeoJSON serialization.')

        if not isinstance(precision, six.integer_types):
            raise TypeError('Precision keyword must be set with an integer.')

        options = 0
        if crs and bbox:
            options = 3
        elif bbox:
            options = 1
        elif crs:
            options = 2
        s = {'desc': 'GeoJSON',
             'procedure_args': {'precision': precision, 'options': options},
             'procedure_fmt': '%(geo_col)s,%(precision)s,%(options)s',
             }
        return self._spatial_attribute('geojson', s, **kwargs)

    def geohash(self, precision=20, **kwargs):
        """
        Returns a GeoHash representation of the given field in a `geohash`
        attribute on each element of the GeoQuerySet.

        The `precision` keyword may be used to custom the number of
        _characters_ used in the output GeoHash, the default is 20.
        """
        s = {'desc': 'GeoHash',
             'procedure_args': {'precision': precision},
             'procedure_fmt': '%(geo_col)s,%(precision)s',
             }
        return self._spatial_attribute('geohash', s, **kwargs)

    def gml(self, precision=8, version=2, **kwargs):
        """
        Returns GML representation of the given field in a `gml` attribute
        on each element of the GeoQuerySet.
        """
        backend = connections[self.db].ops
        s = {'desc': 'GML', 'procedure_args': {'precision': precision}}
        if backend.postgis:
            s['procedure_fmt'] = '%(version)s,%(geo_col)s,%(precision)s'
            s['procedure_args'] = {'precision': precision, 'version': version}
        if backend.oracle:
            s['select_field'] = GMLField()

        return self._spatial_attribute('gml', s, **kwargs)

    def intersection(self, geom, **kwargs):
        """
        Returns the spatial intersection of the Geometry field in
        an `intersection` attribute on each element of this
        GeoQuerySet.
        """
        return self._geomset_attribute('intersection', geom, **kwargs)

    def kml(self, **kwargs):
        """
        Returns KML representation of the geometry field in a `kml`
        attribute on each element of this GeoQuerySet.
        """
        s = {'desc': 'KML',
             'procedure_fmt': '%(geo_col)s,%(precision)s',
             'procedure_args': {'precision': kwargs.pop('precision', 8)},
             }
        return self._spatial_attribute('kml', s, **kwargs)

    def length(self, **kwargs):
        """
        Returns the length of the geometry field as a `Distance` object
        stored in a `length` attribute on each element of this GeoQuerySet.
        """
        return self._distance_attribute('length', None, **kwargs)

    def mem_size(self, **kwargs):
        """
        Returns the memory size (number of bytes) that the geometry field takes
        in a `mem_size` attribute  on each element of this GeoQuerySet.
        """
        return self._spatial_attribute('mem_size', {}, **kwargs)

    def num_geom(self, **kwargs):
        """
        Returns the number of geometries if the field is a
        GeometryCollection or Multi* Field in a `num_geom`
        attribute on each element of this GeoQuerySet; otherwise
        the sets with None.
        """
        return self._spatial_attribute('num_geom', {}, **kwargs)

    def num_points(self, **kwargs):
        """
        Returns the number of points in the first linestring in the
        Geometry field in a `num_points` attribute on each element of
        this GeoQuerySet; otherwise sets with None.
        """
        return self._spatial_attribute('num_points', {}, **kwargs)

    def perimeter(self, **kwargs):
        """
        Returns the perimeter of the geometry field as a `Distance` object
        stored in a `perimeter` attribute on each element of this GeoQuerySet.
        """
        return self._distance_attribute('perimeter', None, **kwargs)

    def point_on_surface(self, **kwargs):
        """
        Returns a Point geometry guaranteed to lie on the surface of the
        Geometry field in a `point_on_surface` attribute on each element
        of this GeoQuerySet; otherwise sets with None.
        """
        return self._geom_attribute('point_on_surface', **kwargs)

    def reverse_geom(self, **kwargs):
        """
        Reverses the coordinate order of the geometry, and attaches as a
        `reverse` attribute on each element of this GeoQuerySet.
        """
        s = {'select_field': GeomField()}
        kwargs.setdefault('model_att', 'reverse_geom')
        if connections[self.db].ops.oracle:
            s['geo_field_type'] = LineStringField
        return self._spatial_attribute('reverse', s, **kwargs)

    def scale(self, x, y, z=0.0, **kwargs):
        """
        Scales the geometry to a new size by multiplying the ordinates
        with the given x,y,z scale factors.
        """
        if connections[self.db].ops.spatialite:
            if z != 0.0:
                raise NotImplementedError('SpatiaLite does not support 3D scaling.')
            s = {'procedure_fmt': '%(geo_col)s,%(x)s,%(y)s',
                 'procedure_args': {'x': x, 'y': y},
                 'select_field': GeomField(),
                 }
        else:
            s = {'procedure_fmt': '%(geo_col)s,%(x)s,%(y)s,%(z)s',
                 'procedure_args': {'x': x, 'y': y, 'z': z},
                 'select_field': GeomField(),
                 }
        return self._spatial_attribute('scale', s, **kwargs)

    def snap_to_grid(self, *args, **kwargs):
        """
        Snap all points of the input geometry to the grid.  How the
        geometry is snapped to the grid depends on how many arguments
        were given:
          - 1 argument : A single size to snap both the X and Y grids to.
          - 2 arguments: X and Y sizes to snap the grid to.
          - 4 arguments: X, Y sizes and the X, Y origins.
        """
        if False in [isinstance(arg, (float,) + six.integer_types) for arg in args]:
            raise TypeError('Size argument(s) for the grid must be a float or integer values.')

        nargs = len(args)
        if nargs == 1:
            size = args[0]
            procedure_fmt = '%(geo_col)s,%(size)s'
            procedure_args = {'size': size}
        elif nargs == 2:
            xsize, ysize = args
            procedure_fmt = '%(geo_col)s,%(xsize)s,%(ysize)s'
            procedure_args = {'xsize': xsize, 'ysize': ysize}
        elif nargs == 4:
            xsize, ysize, xorigin, yorigin = args
            procedure_fmt = '%(geo_col)s,%(xorigin)s,%(yorigin)s,%(xsize)s,%(ysize)s'
            procedure_args = {'xsize': xsize, 'ysize': ysize,
                              'xorigin': xorigin, 'yorigin': yorigin}
        else:
            raise ValueError('Must provide 1, 2, or 4 arguments to `snap_to_grid`.')

        s = {'procedure_fmt': procedure_fmt,
             'procedure_args': procedure_args,
             'select_field': GeomField(),
             }

        return self._spatial_attribute('snap_to_grid', s, **kwargs)

    def svg(self, relative=False, precision=8, **kwargs):
        """
        Returns SVG representation of the geographic field in a `svg`
        attribute on each element of this GeoQuerySet.

        Keyword Arguments:
         `relative`  => If set to True, this will evaluate the path in
                        terms of relative moves (rather than absolute).

         `precision` => May be used to set the maximum number of decimal
                        digits used in output (defaults to 8).
        """
        relative = int(bool(relative))
        if not isinstance(precision, six.integer_types):
            raise TypeError('SVG precision keyword argument must be an integer.')
        s = {
            'desc': 'SVG',
            'procedure_fmt': '%(geo_col)s,%(rel)s,%(precision)s',
            'procedure_args': {
                'rel': relative,
                'precision': precision,
            }
        }
        return self._spatial_attribute('svg', s, **kwargs)

    def sym_difference(self, geom, **kwargs):
        """
        Returns the symmetric difference of the geographic field in a
        `sym_difference` attribute on each element of this GeoQuerySet.
        """
        return self._geomset_attribute('sym_difference', geom, **kwargs)

    def translate(self, x, y, z=0.0, **kwargs):
        """
        Translates the geometry to a new location using the given numeric
        parameters as offsets.
        """
        if connections[self.db].ops.spatialite:
            if z != 0.0:
                raise NotImplementedError('SpatiaLite does not support 3D translation.')
            s = {'procedure_fmt': '%(geo_col)s,%(x)s,%(y)s',
                 'procedure_args': {'x': x, 'y': y},
                 'select_field': GeomField(),
                 }
        else:
            s = {'procedure_fmt': '%(geo_col)s,%(x)s,%(y)s,%(z)s',
                 'procedure_args': {'x': x, 'y': y, 'z': z},
                 'select_field': GeomField(),
                 }
        return self._spatial_attribute('translate', s, **kwargs)

    def transform(self, srid=4326, **kwargs):
        """
        Transforms the given geometry field to the given SRID.  If no SRID is
        provided, the transformation will default to using 4326 (WGS84).
        """
        if not isinstance(srid, six.integer_types):
            raise TypeError('An integer SRID must be provided.')
        field_name = kwargs.get('field_name')
        self._spatial_setup('transform', field_name=field_name)
        self.query.add_context('transformed_srid', srid)
        return self._clone()

    def union(self, geom, **kwargs):
        """
        Returns the union of the geographic field with the given
        Geometry in a `union` attribute on each element of this GeoQuerySet.
        """
        return self._geomset_attribute('union', geom, **kwargs)

    # ### Private API -- Abstracted DRY routines. ###
    def _spatial_setup(self, att, desc=None, field_name=None, geo_field_type=None):
        """
        Performs set up for executing the spatial function.
        """
        # Does the spatial backend support this?
        connection = connections[self.db]
        func = getattr(connection.ops, att, False)
        if desc is None:
            desc = att
        if not func:
            raise NotImplementedError('%s stored procedure not available on '
                                      'the %s backend.' %
                                      (desc, connection.ops.name))

        # Initializing the procedure arguments.
        procedure_args = {'function': func}

        # Is there a geographic field in the model to perform this
        # operation on?
        geo_field = self._geo_field(field_name)
        if not geo_field:
            raise TypeError('%s output only available on GeometryFields.' % func)

        # If the `geo_field_type` keyword was used, then enforce that
        # type limitation.
        if geo_field_type is not None and not isinstance(geo_field, geo_field_type):
            raise TypeError('"%s" stored procedures may only be called on %ss.' % (func, geo_field_type.__name__))

        # Setting the procedure args.
        procedure_args['geo_col'] = self._geocol_select(geo_field, field_name)

        return procedure_args, geo_field

    def _spatial_attribute(self, att, settings, field_name=None, model_att=None):
        """
        DRY routine for calling a spatial stored procedure on a geometry column
        and attaching its output as an attribute of the model.

        Arguments:
         att:
          The name of the spatial attribute that holds the spatial
          SQL function to call.

         settings:
          Dictonary of internal settings to customize for the spatial procedure.

        Public Keyword Arguments:

         field_name:
          The name of the geographic field to call the spatial
          function on.  May also be a lookup to a geometry field
          as part of a foreign key relation.

         model_att:
          The name of the model attribute to attach the output of
          the spatial function to.
        """
        warnings.warn(
            "The %s GeoQuerySet method is deprecated. See GeoDjango Functions "
            "documentation to find the expression-based replacement." % att,
            RemovedInDjango20Warning, stacklevel=2
        )
        # Default settings.
        settings.setdefault('desc', None)
        settings.setdefault('geom_args', ())
        settings.setdefault('geom_field', None)
        settings.setdefault('procedure_args', {})
        settings.setdefault('procedure_fmt', '%(geo_col)s')
        settings.setdefault('select_params', [])

        connection = connections[self.db]

        # Performing setup for the spatial column, unless told not to.
        if settings.get('setup', True):
            default_args, geo_field = self._spatial_setup(
                att, desc=settings['desc'], field_name=field_name,
                geo_field_type=settings.get('geo_field_type'))
            for k, v in six.iteritems(default_args):
                settings['procedure_args'].setdefault(k, v)
        else:
            geo_field = settings['geo_field']

        # The attribute to attach to the model.
        if not isinstance(model_att, six.string_types):
            model_att = att

        # Special handling for any argument that is a geometry.
        for name in settings['geom_args']:
            # Using the field's get_placeholder() routine to get any needed
            # transformation SQL.
            geom = geo_field.get_prep_value(settings['procedure_args'][name])
            params = geo_field.get_db_prep_lookup('contains', geom, connection=connection)
            geom_placeholder = geo_field.get_placeholder(geom, None, connection)

            # Replacing the procedure format with that of any needed
            # transformation SQL.
            old_fmt = '%%(%s)s' % name
            new_fmt = geom_placeholder % '%%s'
            settings['procedure_fmt'] = settings['procedure_fmt'].replace(old_fmt, new_fmt)
            settings['select_params'].extend(params)

        # Getting the format for the stored procedure.
        fmt = '%%(function)s(%s)' % settings['procedure_fmt']

        # If the result of this function needs to be converted.
        if settings.get('select_field'):
            select_field = settings['select_field']
            if connection.ops.oracle:
                select_field.empty_strings_allowed = False
        else:
            select_field = Field()

        # Finally, setting the extra selection attribute with
        # the format string expanded with the stored procedure
        # arguments.
        self.query.add_annotation(
            RawSQL(fmt % settings['procedure_args'], settings['select_params'], select_field),
            model_att)
        return self

    def _distance_attribute(self, func, geom=None, tolerance=0.05, spheroid=False, **kwargs):
        """
        DRY routine for GeoQuerySet distance attribute routines.
        """
        # Setting up the distance procedure arguments.
        procedure_args, geo_field = self._spatial_setup(func, field_name=kwargs.get('field_name'))

        # If geodetic defaulting distance attribute to meters (Oracle and
        # PostGIS spherical distances return meters).  Otherwise, use the
        # units of the geometry field.
        connection = connections[self.db]
        geodetic = geo_field.geodetic(connection)
        geography = geo_field.geography

        if geodetic:
            dist_att = 'm'
        else:
            dist_att = Distance.unit_attname(geo_field.units_name(connection))

        # Shortcut booleans for what distance function we're using and
        # whether the geometry field is 3D.
        distance = func == 'distance'
        length = func == 'length'
        perimeter = func == 'perimeter'
        if not (distance or length or perimeter):
            raise ValueError('Unknown distance function: %s' % func)
        geom_3d = geo_field.dim == 3

        # The field's get_db_prep_lookup() is used to get any
        # extra distance parameters.  Here we set up the
        # parameters that will be passed in to field's function.
        lookup_params = [geom or 'POINT (0 0)', 0]

        # Getting the spatial backend operations.
        backend = connection.ops

        # If the spheroid calculation is desired, either by the `spheroid`
        # keyword or when calculating the length of geodetic field, make
        # sure the 'spheroid' distance setting string is passed in so we
        # get the correct spatial stored procedure.
        if spheroid or (backend.postgis and geodetic and
                        (not geography) and length):
            lookup_params.append('spheroid')
        lookup_params = geo_field.get_prep_value(lookup_params)
        params = geo_field.get_db_prep_lookup('distance_lte', lookup_params, connection=connection)

        # The `geom_args` flag is set to true if a geometry parameter was
        # passed in.
        geom_args = bool(geom)

        if backend.oracle:
            if distance:
                procedure_fmt = '%(geo_col)s,%(geom)s,%(tolerance)s'
            elif length or perimeter:
                procedure_fmt = '%(geo_col)s,%(tolerance)s'
            procedure_args['tolerance'] = tolerance
        else:
            # Getting whether this field is in units of degrees since the field may have
            # been transformed via the `transform` GeoQuerySet method.
            srid = self.query.get_context('transformed_srid')
            if srid:
                u, unit_name, s = get_srid_info(srid, connection)
                geodetic = unit_name.lower() in geo_field.geodetic_units

            if geodetic and not connection.features.supports_distance_geodetic:
                raise ValueError(
                    'This database does not support linear distance '
                    'calculations on geodetic coordinate systems.'
                )

            if distance:
                if srid:
                    # Setting the `geom_args` flag to false because we want to handle
                    # transformation SQL here, rather than the way done by default
                    # (which will transform to the original SRID of the field rather
                    #  than to what was transformed to).
                    geom_args = False
                    procedure_fmt = '%s(%%(geo_col)s, %s)' % (backend.transform, srid)
                    if geom.srid is None or geom.srid == srid:
                        # If the geom parameter srid is None, it is assumed the coordinates
                        # are in the transformed units.  A placeholder is used for the
                        # geometry parameter.  `GeomFromText` constructor is also needed
                        # to wrap geom placeholder for SpatiaLite.
                        if backend.spatialite:
                            procedure_fmt += ', %s(%%%%s, %s)' % (backend.from_text, srid)
                        else:
                            procedure_fmt += ', %%s'
                    else:
                        # We need to transform the geom to the srid specified in `transform()`,
                        # so wrapping the geometry placeholder in transformation SQL.
                        # SpatiaLite also needs geometry placeholder wrapped in `GeomFromText`
                        # constructor.
                        if backend.spatialite:
                            procedure_fmt += (', %s(%s(%%%%s, %s), %s)' % (
                                backend.transform, backend.from_text,
                                geom.srid, srid))
                        else:
                            procedure_fmt += ', %s(%%%%s, %s)' % (backend.transform, srid)
                else:
                    # `transform()` was not used on this GeoQuerySet.
                    procedure_fmt = '%(geo_col)s,%(geom)s'

                if not geography and geodetic:
                    # Spherical distance calculation is needed (because the geographic
                    # field is geodetic). However, the PostGIS ST_distance_sphere/spheroid()
                    # procedures may only do queries from point columns to point geometries
                    # some error checking is required.
                    if not backend.geography:
                        if not isinstance(geo_field, PointField):
                            raise ValueError('Spherical distance calculation only supported on PointFields.')
                        if not str(Geometry(six.memoryview(params[0].ewkb)).geom_type) == 'Point':
                            raise ValueError(
                                'Spherical distance calculation only supported with '
                                'Point Geometry parameters'
                            )
                    # The `function` procedure argument needs to be set differently for
                    # geodetic distance calculations.
                    if spheroid:
                        # Call to distance_spheroid() requires spheroid param as well.
                        procedure_fmt += ",'%(spheroid)s'"
                        procedure_args.update({'function': backend.distance_spheroid, 'spheroid': params[1]})
                    else:
                        procedure_args.update({'function': backend.distance_sphere})
            elif length or perimeter:
                procedure_fmt = '%(geo_col)s'
                if not geography and geodetic and length:
                    # There's no `length_sphere`, and `length_spheroid` also
                    # works on 3D geometries.
                    procedure_fmt += ",'%(spheroid)s'"
                    procedure_args.update({'function': backend.length_spheroid, 'spheroid': params[1]})
                elif geom_3d and connection.features.supports_3d_functions:
                    # Use 3D variants of perimeter and length routines on supported backends.
                    if perimeter:
                        procedure_args.update({'function': backend.perimeter3d})
                    elif length:
                        procedure_args.update({'function': backend.length3d})

        # Setting up the settings for `_spatial_attribute`.
        s = {'select_field': DistanceField(dist_att),
             'setup': False,
             'geo_field': geo_field,
             'procedure_args': procedure_args,
             'procedure_fmt': procedure_fmt,
             }
        if geom_args:
            s['geom_args'] = ('geom',)
            s['procedure_args']['geom'] = geom
        elif geom:
            # The geometry is passed in as a parameter because we handled
            # transformation conditions in this routine.
            s['select_params'] = [backend.Adapter(geom)]
        return self._spatial_attribute(func, s, **kwargs)

    def _geom_attribute(self, func, tolerance=0.05, **kwargs):
        """
        DRY routine for setting up a GeoQuerySet method that attaches a
        Geometry attribute (e.g., `centroid`, `point_on_surface`).
        """
        s = {'select_field': GeomField()}
        if connections[self.db].ops.oracle:
            s['procedure_fmt'] = '%(geo_col)s,%(tolerance)s'
            s['procedure_args'] = {'tolerance': tolerance}
        return self._spatial_attribute(func, s, **kwargs)

    def _geomset_attribute(self, func, geom, tolerance=0.05, **kwargs):
        """
        DRY routine for setting up a GeoQuerySet method that attaches a
        Geometry attribute and takes a Geoemtry parameter.  This is used
        for geometry set-like operations (e.g., intersection, difference,
        union, sym_difference).
        """
        s = {
            'geom_args': ('geom',),
            'select_field': GeomField(),
            'procedure_fmt': '%(geo_col)s,%(geom)s',
            'procedure_args': {'geom': geom},
        }
        if connections[self.db].ops.oracle:
            s['procedure_fmt'] += ',%(tolerance)s'
            s['procedure_args']['tolerance'] = tolerance
        return self._spatial_attribute(func, s, **kwargs)

    def _geocol_select(self, geo_field, field_name):
        """
        Helper routine for constructing the SQL to select the geographic
        column.  Takes into account if the geographic field is in a
        ForeignKey relation to the current model.
        """
        compiler = self.query.get_compiler(self.db)
        opts = self.model._meta
        if geo_field not in opts.fields:
            # Is this operation going to be on a related geographic field?
            # If so, it'll have to be added to the select related information
            # (e.g., if 'location__point' was given as the field name).
            # Note: the operation really is defined as "must add select related!"
            self.query.add_select_related([field_name])
            # Call pre_sql_setup() so that compiler.select gets populated.
            compiler.pre_sql_setup()
            for col, _, _ in compiler.select:
                if col.output_field == geo_field:
                    return col.as_sql(compiler, compiler.connection)[0]
            raise ValueError("%r not in compiler's related_select_cols" % geo_field)
        elif geo_field not in opts.local_fields:
            # This geographic field is inherited from another model, so we have to
            # use the db table for the _parent_ model instead.
            parent_model = geo_field.model._meta.concrete_model
            return self._field_column(compiler, geo_field, parent_model._meta.db_table)
        else:
            return self._field_column(compiler, geo_field)

    # Private API utilities, subject to change.
    def _geo_field(self, field_name=None):
        """
        Returns the first Geometry field encountered or the one specified via
        the `field_name` keyword. The `field_name` may be a string specifying
        the geometry field on this GeoQuerySet's model, or a lookup string
        to a geometry field via a ForeignKey relation.
        """
        if field_name is None:
            # Incrementing until the first geographic field is found.
            for field in self.model._meta.fields:
                if isinstance(field, GeometryField):
                    return field
            return False
        else:
            # Otherwise, check by the given field name -- which may be
            # a lookup to a _related_ geographic field.
            return GISLookup._check_geo_field(self.model._meta, field_name)

    def _field_column(self, compiler, field, table_alias=None, column=None):
        """
        Helper function that returns the database column for the given field.
        The table and column are returned (quoted) in the proper format, e.g.,
        `"geoapp_city"."point"`.  If `table_alias` is not specified, the
        database table associated with the model of this `GeoQuerySet` will be
        used.  If `column` is specified, it will be used instead of the value
        in `field.column`.
        """
        if table_alias is None:
            table_alias = compiler.query.get_meta().db_table
        return "%s.%s" % (compiler.quote_name_unless_alias(table_alias),
                          compiler.connection.ops.quote_name(column or field.column))