如何在 C# 的 MongoDB 驅動程序中獲取數組中一個字段的值

Question

我有課程：

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; }
}

我試圖通過投影來獲得它：

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();

而且我每次都會收到DetailsNeededFields作為空列表。 我想要像標准聚合一樣的字符串列表：

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

Answer 1

可以使用AsQueryable接口輕松實現，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();