# dokumentasi Django 2.2.9.dev20191202104323

# Kode sumber untuk django.db.models.aggregates

    """
    Classes to represent the definitions of aggregate functions.
    """
    from django.core.exceptions import FieldError
    from django.db.models.expressions import Case, Func, Star, When
    from django.db.models.fields import IntegerField
    from django.db.models.functions.mixins import (
        FixDurationInputMixin, NumericOutputFieldMixin,
    )

    __all__ = [
        'Aggregate', 'Avg', 'Count', 'Max', 'Min', 'StdDev', 'Sum', 'Variance',
    ]

    class Aggregate(Func):
        template = '%(function)s(%(distinct)s%(expressions)s)'
        contains_aggregate = True
        name = None
        filter_template = '%s FILTER (WHERE %%(filter)s)'
        window_compatible = True
        allow_distinct = False

        def __init__(self, *expressions, distinct=False, filter=None, **extra):
            if distinct and not self.allow_distinct:
                raise TypeError("%s does not allow distinct." % self.__class__.__name__)
            self.distinct = distinct
            self.filter = filter
            super().__init__(*expressions, **extra)

        def get_source_fields(self):
            # Don't return the filter expression since it's not a source field.
            return [e._output_field_or_none for e in super().get_source_expressions()]

        def get_source_expressions(self):
            source_expressions = super().get_source_expressions()
            if self.filter:
                return source_expressions + [self.filter]
            return source_expressions

        def set_source_expressions(self, exprs):
            self.filter = self.filter and exprs.pop()
            return super().set_source_expressions(exprs)

        def resolve_expression(self, query=None, allow_joins=True, reuse=None, summarize=False, for_save=False):
            # Aggregates are not allowed in UPDATE queries, so ignore for_save
            c = super().resolve_expression(query, allow_joins, reuse, summarize)
            c.filter = c.filter and c.filter.resolve_expression(query, allow_joins, reuse, summarize)
            if not summarize:
                # Call Aggregate.get_source_expressions() to avoid
                # returning self.filter and including that in this loop.
                expressions = super(Aggregate, c).get_source_expressions()
                for index, expr in enumerate(expressions):
                    if expr.contains_aggregate:
                        before_resolved = self.get_source_expressions()[index]
                        name = before_resolved.name if hasattr(before_resolved, 'name') else repr(before_resolved)
                        raise FieldError("Cannot compute %s('%s'): '%s' is an aggregate" % (c.name, name, name))
            return c

        @property
        def default_alias(self):
            expressions = self.get_source_expressions()
            if len(expressions) == 1 and hasattr(expressions[0], 'name'):
                return '%s__%s' % (expressions[0].name, self.name.lower())
            raise TypeError("Complex expressions require an alias")

        def get_group_by_cols(self):
            return []

        def as_sql(self, compiler, connection, **extra_context):
            extra_context['distinct'] = 'DISTINCT ' if self.distinct else ''
            if self.filter:
                if connection.features.supports_aggregate_filter_clause:
                    filter_sql, filter_params = self.filter.as_sql(compiler, connection)
                    template = self.filter_template % extra_context.get('template', self.template)
                    sql, params = super().as_sql(
                        compiler, connection, template=template, filter=filter_sql,
                        **extra_context
                    )
                    return sql, params + filter_params
                else:
                    copy = self.copy()
                    copy.filter = None
                    source_expressions = copy.get_source_expressions()
                    condition = When(self.filter, then=source_expressions[0])
                    copy.set_source_expressions([Case(condition)] + source_expressions[1:])
                    return super(Aggregate, copy).as_sql(compiler, connection, **extra_context)
            return super().as_sql(compiler, connection, **extra_context)

        def _get_repr_options(self):
            options = super()._get_repr_options()
            if self.distinct:
                options['distinct'] = self.distinct
            if self.filter:
                options['filter'] = self.filter
            return options

    class Avg(FixDurationInputMixin, NumericOutputFieldMixin, Aggregate):
        function = 'AVG'
        name = 'Avg'

    class Count(Aggregate):
        function = 'COUNT'
        name = 'Count'
        output_field = IntegerField()
        allow_distinct = True

        def __init__(self, expression, filter=None, **extra):
            if expression == '*':
                expression = Star()
            if isinstance(expression, Star) and filter is not None:
                raise ValueError('Star cannot be used with filter. Please specify a field.')
            super().__init__(expression, filter=filter, **extra)

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

    class Max(Aggregate):
        function = 'MAX'
        name = 'Max'

    class Min(Aggregate):
        function = 'MIN'
        name = 'Min'

    class StdDev(NumericOutputFieldMixin, Aggregate):
        name = 'StdDev'

        def __init__(self, expression, sample=False, **extra):
            self.function = 'STDDEV_SAMP' if sample else 'STDDEV_POP'
            super().__init__(expression, **extra)

        def _get_repr_options(self):
            return {**super()._get_repr_options(), 'sample': self.function == 'STDDEV_SAMP'}

    class Sum(FixDurationInputMixin, Aggregate):
        function = 'SUM'
        name = 'Sum'

    class Variance(NumericOutputFieldMixin, Aggregate):
        name = 'Variance'

        def __init__(self, expression, sample=False, **extra):
            self.function = 'VAR_SAMP' if sample else 'VAR_POP'
            super().__init__(expression, **extra)

        def _get_repr_options(self):
            return {**super()._get_repr_options(), 'sample': self.function == 'VAR_SAMP'}