How can I read just one field in MongoDB C#?-CodePudding

Im trying to read just one field from the MongoDB with a given ID, but its giving me all the object... How can I do it ? What should I do? I tried with this repository but its returning me all the object

Here is my class:

public class Hall
{
  [BsonId]
  [BsonRepresentation(BsonType.ObjectId)]
  public string Id { get; set; }
  public string Name { get; set; }
  public List<string> Surveys { get; set; }
}

And here is my repository with the connection to Mongo:

public Task<List<Hall>> ReadSurveysHall(string hallId)
{
  var filter = Builders<Hall>.Filter.(x => x.Id, hallId);
  return _mongoManager.Find(filter, MongoConstants.HallCollection);
}

I just want it to give me just the list of Survey's string. Thank you in advance.

CodePudding user response：

In order to return only the list of surveys, you can change your method like this:

public async Task<List<string>> ReadSurveysHall(string hallId)
{
  var filter = Builders<Hall>.Filter.(x => x.Id, hallId);
  var halls = await _mongoManager.Find(filter, MongoConstants.HallCollection);
  var hall = halls.FirstOrDefault();
  if (hall == null)
    return new List<string>();
  return hall.Surveys;
}

You can further optimize this by using a projection so that MongoDB only delivers the data that you are interested in, e.g.:

var projection = Builders<Hall>.Projection.Exclude(x => x.Id).Include(x => x.Surveys);
var options = new FindOptions<Hall, Hall>() 
{
  Projection = projection,
};

How you use that projection depends on the capabilities of _mongoManager. Take a look at the Find methods and check whether any of these accepts a projection or some kind of FindOptions as a parameter.