我知道这个问题还有其他版本，但我很难让这个问题符合要求。我有类似的文档：

[
{_id: 1, date: "2022-04-08T23:30:12.000Z", books: [{author: "Johnson", title: "First Title"}, {author: "Smith", title: "Second Title}]},
{_id: 2, date: "2022-04-22T23:30:12.000Z", books: [{author: "Johnson", title: "Some Other Title"}]},
{_id: 3, date: "2022-05-05T23:30:12.000Z", books: [{author: "Smith", title: "Title Round 2"}]},
{_id: 4, date: "2022-05-15T23:30:12.000Z", books: [{author: "Johnson", title: "Found a Title", {author: "Smith", title: "Wrote again"}, {author: "Brooks", title: "New Title"}]}
]

我尝试按月-年对文档进行分组，然后计算不同值在author字段中显示的次数。到目前为止，我有一个管道，看起来像：

{
          "$unwind": "$books"
        },
        {
          $project: {
            _id: 1,
            books: 1,
            month: {
              "$month": "$date"
            },
            year: {
              "$year": "$date"
            }
          }
        },
        {
          $project: {
            _id: 1,
            books: 1,
            date: {
              $concat: [
                {
                  $substr: [
                    "$year",
                    0,
                    4
                  ]
                },
                "-",
                {
                  $substr: [
                    "$month",
                    0,
                    2
                  ]
                },
                
              ]
            }
          }
        },
        {
          $group: {
            _id: {
              date: "$date",
              books: {
                freq: {
                  $sum: 1
                }
              }
            }
          }
        },
        {
          $project: {
            "_id": 1,
            "date": 1,
            "books": 1
          }
        },
        
      ]
    }

我的目标是最终的输出，看起来像：

[
{date: "2022-04", authors: { "Johnson": 2, "Smith": 1}},
{date: "2022-05", authors: {"Johnson": 1, "Smith": 2, "Brooks": 1}}
]

我已经看到了计算子文档的方法，但是在尝试实现时，我丢失了我的组的日期或只是得到了错误。我已经看够了，知道它的可行性，只是失去了试图让它恰到好处。任何帮助都很感激。