# documentação Django 2.0.8

# Código fonte para django.contrib.postgres.aggregates.statistics

    from django.db.models import FloatField, IntegerField
    from django.db.models.aggregates import Aggregate

    __all__ = [
        'CovarPop', 'Corr', 'RegrAvgX', 'RegrAvgY', 'RegrCount', 'RegrIntercept',
        'RegrR2', 'RegrSlope', 'RegrSXX', 'RegrSXY', 'RegrSYY', 'StatAggregate',
    ]

    class StatAggregate(Aggregate):
        output_field = FloatField()

        def __init__(self, y, x, output_field=None, filter=None):
            if not x or not y:
                raise ValueError('Both y and x must be provided.')
            super().__init__(y, x, output_field=output_field, filter=filter)

        def resolve_expression(self, query=None, allow_joins=True, reuse=None, summarize=False, for_save=False):
            return super().resolve_expression(query, allow_joins, reuse, summarize)

    class Corr(StatAggregate):
        function = 'CORR'

    class CovarPop(StatAggregate):
        def __init__(self, y, x, sample=False, filter=None):
            self.function = 'COVAR_SAMP' if sample else 'COVAR_POP'
            super().__init__(y, x, filter=filter)

    class RegrAvgX(StatAggregate):
        function = 'REGR_AVGX'

    class RegrAvgY(StatAggregate):
        function = 'REGR_AVGY'

    class RegrCount(StatAggregate):
        function = 'REGR_COUNT'
        output_field = IntegerField()

        def convert_value(self, value, expression, connection):
            return 0 if value is None else value

    class RegrIntercept(StatAggregate):
        function = 'REGR_INTERCEPT'

    class RegrR2(StatAggregate):
        function = 'REGR_R2'

    class RegrSlope(StatAggregate):
        function = 'REGR_SLOPE'

    class RegrSXX(StatAggregate):
        function = 'REGR_SXX'

    class RegrSXY(StatAggregate):
        function = 'REGR_SXY'

    class RegrSYY(StatAggregate):
        function = 'REGR_SYY'