How to get values of one field in array in MongoDB Driver for C#

Question

I have the classes:

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; }
}

And I trying to obtain it with projection:

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();

And I receive DetailsNeededFields as empty list every time. I want to have list of strings like with standard aggeregate:

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

Shouldn't the element that you're trying to call "Details.NeededField" not simply be "Details" and be a List<Detail> rather than a List<string>? — ProgrammingLlama
– ProgrammingLlama, Commented Mar 29, 2021 at 6:31
@Llama It is small reproduceable example, in real project Details have many fields, where I need only one of them — Leszek Mazur
– Leszek Mazur, Commented Mar 29, 2021 at 6:44
But you're asking for the equivalent of an console aggregate command that I'm pretty sure is invalid. You should at least make that part of your question valid. — ProgrammingLlama
– ProgrammingLlama, Commented Mar 29, 2021 at 6:45
@Llama Projection stage in example is working now - thank you — Leszek Mazur
– Leszek Mazur, Commented Mar 29, 2021 at 7:04

Dĵ ΝιΓΞΗΛψΚ · Accepted Answer · 2021-03-29 09:05:00Z

1

can be achieved easily with the AsQueryable interface like so:

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

answered Mar 29, 2021 at 9:05

Dĵ ΝιΓΞΗΛψΚ

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3 Comments

Leszek Mazur Over a year ago

Will the (List<string>)w.Details.Select(d => d.NeededField) be translated to project only one field, or all details will be downloaded? Is it better than w.Details.Select(d => d.NeededField).ToList()?

Dĵ ΝιΓΞΗΛψΚ Over a year ago

@LeszekMazur yes only the needed field values will be downloaded. and you can't call ToList() as it's not translatable by the driver.

Chris Dueck Over a year ago

Is there a way to do this with an IAggregateFluent<T> from an aggregation? AsQueryable() can't be used in this case.

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