yii::$app->mongodb->GetCollection('sessions')->Aggregate([
    [
        '$lookup' => [
            'from' => 'users',
            'localField' => '_userId',
            'foreignField' => '_id',
            'let' => ['sRoleId' => '$_roleId'],
            'pipeline' => [
                [
                    '$match' => [
                        'enable' => true,
                    ],
                ],
                [
                    '$lookup' => [
                        'from' => 'roles',
                        'pipeline' => [
                            [
                                '$match' => [
                                    //'$expr' => ['$_id' => '$$sRoleId'],          //Error
                                    //'$expr' => ['x' => 'y'],                    //Always is true and return all docs
                                    '$expr' => ['$eq' => ['$_id','$$sRoleId']],  //OK
                                ],
                            ],
                        ],
                        'as' => 'role',
                    ],
                ],
            ],
            'as' => 'user',
        ],
    ],
]);