# Τεκμηρίωση του Django - 1.10.7

# Πηγαίος κώδικας για το django.contrib.gis.db.models.aggregates

    from django.contrib.gis.db.models.fields import ExtentField
    from django.db.models.aggregates import Aggregate

    __all__ = ['Collect', 'Extent', 'Extent3D', 'MakeLine', 'Union']

    class GeoAggregate(Aggregate):
        function = None
        is_extent = False

        def as_sql(self, compiler, connection):
            # this will be called again in parent, but it's needed now - before
            # we get the spatial_aggregate_name
            connection.ops.check_expression_support(self)
            self.function = connection.ops.spatial_aggregate_name(self.name)
            return super(GeoAggregate, self).as_sql(compiler, connection)

        def as_oracle(self, compiler, connection):
            if not hasattr(self, 'tolerance'):
                self.tolerance = 0.05
            self.extra['tolerance'] = self.tolerance
            if not self.is_extent:
                self.template = '%(function)s(SDOAGGRTYPE(%(expressions)s,%(tolerance)s))'
            return self.as_sql(compiler, connection)

        def resolve_expression(self, query=None, allow_joins=True, reuse=None, summarize=False, for_save=False):
            c = super(GeoAggregate, self).resolve_expression(query, allow_joins, reuse, summarize, for_save)
            for expr in c.get_source_expressions():
                if not hasattr(expr.field, 'geom_type'):
                    raise ValueError('Geospatial aggregates only allowed on geometry fields.')
            return c

        def convert_value(self, value, expression, connection, context):
            return connection.ops.convert_geom(value, self.output_field)

    class Collect(GeoAggregate):
        name = 'Collect'

    class Extent(GeoAggregate):
        name = 'Extent'
        is_extent = '2D'

        def __init__(self, expression, **extra):
            super(Extent, self).__init__(expression, output_field=ExtentField(), **extra)

        def convert_value(self, value, expression, connection, context):
            return connection.ops.convert_extent(value, context.get('transformed_srid'))

    class Extent3D(GeoAggregate):
        name = 'Extent3D'
        is_extent = '3D'

        def __init__(self, expression, **extra):
            super(Extent3D, self).__init__(expression, output_field=ExtentField(), **extra)

        def convert_value(self, value, expression, connection, context):
            return connection.ops.convert_extent3d(value, context.get('transformed_srid'))

    class MakeLine(GeoAggregate):
        name = 'MakeLine'

    class Union(GeoAggregate):
        name = 'Union'