Class Count

django/db/models/aggregates.py:295–325

class Count(Aggregate):
    function = "COUNT"
    name = "Count"
    output_field = IntegerField()
    allow_distinct = True
    empty_result_set_value = 0
    arity = 1
    allows_composite_expressions = 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 resolve_expression(self, *args, **kwargs):
        result = super().resolve_expression(*args, **kwargs)
        source_expressions = result.get_source_expressions()

        # In case of composite primary keys, count the first column.
        if isinstance(expr := source_expressions[0], ColPairs):
            if self.distinct:
                raise ValueError(
                    "COUNT(DISTINCT) doesn't support composite primary keys"
                )

            source_expressions[0] = expr.get_cols()[0]
            result.set_source_expressions(source_expressions)

        return result


class Max(Aggregate):