Descontrair condicional na agregação do MongoDb?

var pipeline = [];
pipeline.push(
    {   // SELECT
    $project : { "sex" : 1,
             "salesIndex":1
            }
    },
    {   // WHERE
        $match: {"salesIndex": {$gte: index}}
    }
);
if (filteringByDepartment) {
    pipeline.push(
        { $unwind: '$departments' },
        { $match: { departments: departmentId }}
    );
}    
pipeline.push(
    {   // GROUP BY y agregadores
        $group: {
            _id      : "$sex",
            sexCount : { $sum: 1 }
        }
    },
    { $sort: { sexCount: -1 } }
);

models.Users.aggregate(pipeline, function(err, dbres) {
    //...
});