1 from django.db.models import Aggregate

     2 from django.contrib.gis.db.backend import SpatialBackend

     3 from django.contrib.gis.db.models.sql import GeomField

     4 

     5 class GeoAggregate(Aggregate):

     6 

     7     def add_to_query(self, query, alias, col, source, is_summary):

     8         if hasattr(source, 'geom_type'):

     9             # Doing additional setup on the Query object for spatial aggregates.

    10             aggregate = getattr(query.aggregates_module, self.name)

    11             

    12             # Adding a conversion class instance and any selection wrapping

    13             # SQL (e.g., needed by Oracle).

    14             if aggregate.conversion_class is GeomField:

    15                 query.extra_select_fields[alias] = GeomField()

    16                 if SpatialBackend.select:

    17                     query.custom_select[alias] = SpatialBackend.select

    18      

    19         super(GeoAggregate, self).add_to_query(query, alias, col, source, is_summary)

    20 

    21 class Collect(GeoAggregate):

    22     name = 'Collect'

    23 

    24 class Extent(GeoAggregate):

    25     name = 'Extent'

    26 

    27 class MakeLine(GeoAggregate):

    28     name = 'MakeLine'

    29 

    30 class Union(GeoAggregate):

    31     name = 'Union'