c# - MongoDB $first 不支持的错误 C# 驱动程序

Question

我在 C# 驱动程序中有一个聚合查询，如下所示:

var result = await _records.Aggregate()
                .Match(record => record.Statuses.Any(status => status.Status == currentStatus))
                .Unwind(record => record.Statuses)
                .Sort(Builders<BsonDocument>.Sort.Descending("statuses.date"))
                .Group(doc => doc["_id"], group => new { Id = group.Key, CurrentStatus = group.First()["statuses"] })
                .Match(arg => arg.CurrentStatus["status"] == BsonValue.Create(currentStatus.ToString()))
                .ToListAsync();

与以下 shell 语法匹配:

db.records.aggregate(
    [
        {
            $match : { "statuses.status" : "Finished" }
        },
        {
            $unwind: "$statuses"
        },
        {
            $sort: { "statuses.date":-1 }
        },
        {
            $group: {
                _id: "$_id",
                current_status: { $first: "$statuses" }
            }
        },
        {
            $match : { "current_status.status" : "Finished" }
        }
    ])

我认为 C# 版本由于 CurrentStatus = group.First()["statuses"] 而无法正常工作，异常(exception)情况是:

System.NotSupportedException: get_Item of type MongoDB.Bson.BsonValue is an unsupported method in a $project or $group pipeline operator.

我对 First() 约定的方法是基于此:

MongoDB C# Driver First Expression Reference

对如何以司机喜欢的方式翻译有什么想法吗？

Answer 1

在玩了一个类似的例子之后，看起来你可以通过首先在你的组上调用 Select 来获取 statuses 字段然后调用First() 关于那个:

var result = await _records.Aggregate()
  .Match(record => record.Statuses.Any(status => status.Status == currentStatus))
  .Unwind(record => record.Statuses)
  .Sort(Builders<BsonDocument>.Sort.Descending("statuses.date"))
  .Group(doc => doc["_id"], group => new 
  { 
      Id = group.Key, 
      CurrentStatus = group.Select(x => x["statuses"]).First() 
  })
  .Match(arg => arg.CurrentStatus["status"] == BsonValue.Create(currentStatus.ToString()))
  .ToListAsync();