MongoDB Driver for C# 如何获取数组中一个字段的值

Question

我有 类:

public class Whole
{
  public ObjectId Id { get; set; }
  public string NeededField { get; set; }
  public List<Detail> Details { get; set; }
  public string SomeUnnecesaryField { get; set; } 
}

public class Detail
{
  public string NeededField { get; set; }
  public string NotNeededField { get; set; }
}

[BsonNoId]
[BsonIgnoreExtraElements]
public class MyNeededInformations
{
  public string NeededField { get; set; }
  [BsonElement("Details.NeededField")]
  public List<string> DetailsNeededFields { get; set; }
}

我试图通过投影获得它：

var filter = someFilter;
var projection = Builders<Whole>.Projection
  .Include(w => w.NeededField)
  .Include(w => w.Details)
  .Exclude("_id");
return Collection
  .Find(filter)
  .Project(projection)
  .As<MyNeededInformations>()
  .ToList();

而且我每次都收到 DetailsNeededFields 作为空列表。我想要像标准聚合一样的字符串列表：

db.collection.aggregate([
  {$match: someFilter},
  {$project: {"_id": 0, "NeededField": 1, "DetailsNeededFields": "$Details.NeededField"}}
])

Answer 1

可以通过 AsQueryable 界面轻松实现，如下所示：

var results = await collection
    .AsQueryable()
    .Where(_ => true)
    .Select(w => new MyNeededInformations
    {
        NeededField = w.NeededField,
        DetailsNeededFields = (List<string>)w.Details.Select(d => d.NeededField)
    })
    .ToListAsync();