mongoose aggregate group cond 报错 The $cond accumulator is a unary operator

[一只小迷糊虫]

{
   $group: {
        _id: {
              uid: '$uid',
              role: '$role'
        },
        nickname: { $cond: [{ $eq: ["$role", 2] }, '$nickname', '测试'] },
   }
}

报错

  The $cond accumulator is a unary operator

不知道哪里出了错误，请大佬帮忙看一下，多谢

[一只小迷糊虫]

已解决，在$project中使用

{
     $project: {
              _id: 0,
              uid: '$uid',
              role: '$role',
              nickname: { $cond: [{ $eq: ["$role", 2] }, '$nickname', '测试'] },
     }
}