MongoDB聚合查询返回未定义

Question

我正在尝试通过匹配书集中的书ID字段来从评论集中获取评论。这是我的汇总查询，

static async getBookByID(id) {

return await books.aggregate([
        {
          $lookup:
          {
            from: "Comments",
            let: { 'book_id': id },
            pipeline: [
              {
                $match:
                  { $expr: { $eq: ['$book_id', '$$book_id'] } }
              },
               { $project: { 'title': 1, 'comments': 1 } },
              {
                $sort: {
                  'date': -1
                }
              }
            ],
            as: 'comments'
          }
        }
      ]);
}

我的测试失败

 Comparing two different types of values. Expected string but received undefined.

编辑

评论集合

{"_id":"5a9427658b0beebeb696b14d",
"name":"John Bishop",
"email":"john@fakegmail.com",
"book_id":"573a13a6f29313caabd17bd5",
"text":"Sed odit incidunt totam dolorem nam doloribus. Dignissimos dolore accusamus vel. Cupiditate deserunt esse quisquam quisquam cum.",
"date":"1976-03-16T19:54:28.000Z"}

图书收藏

{"_id":"573a13a6f29313caabd17bd5","title":"Once Upon a Time in Mexico"}