How to get few specific fields for all documents in a collection With MongoDb .net Driver

Question

I am working with "MongoDB.Driver" Version="2.9.3". I am trying to request only for specified fields. I need to perform fallowing request(works by the pypline ):

db.Segments.find({ "geometry" : { "$nearSphere" : { "$geometry" : { "type" : "Point", "coordinates" : [6.7889999999999997, 64.412599999999998] }, "$minDistance" : 0, "$maxDistance" : 10000 } } }, {_id : 0, 'properties.be_id' : 1,'properties.rel_ltt' : 1 ,'properties.rp_co' : 1, 'properties.wenum' : 1} ).pretty();

I need to create Bson document which I can use to find specified data with restricted fields. So far I tried to parse from Json to Bson

var doc = BsonDocument.Parse("{ 'geometry' : { '$nearSphere' : { '$geometry' : { 'type' : 'Point', 'coordinates' :" +
                    " [" + geolocation.Longitude.ToString(culture) + "," + geolocation.Latitude.ToString(culture) + "] }, '$minDistance' : 0, '$maxDistance' : 10 } }}, " +
                    " {'properties.be_id' : 1,'properties.rel_lttr' : 1 ,'properties.rp_co' : 1, 'properties.wenum' : 1}}}");

And then use find method :

SegmentResult.AddRange(RScollection.Find<Segment>(doc).ToList());

This approach doesn't seems to work. If anyone can help me to fix this problem I would be extremely grateful.

Answer 1

This is a simple console application.

using MongoDB.Bson;
using MongoDB.Bson.Serialization;
using MongoDB.Driver;
using System.Collections.Generic;
using System.Linq;

public class Program
{           
    public static void Main()
    {
            var tempPipeline = new List<BsonDocument>();

            List<string> selectFields = new List<string>();
            selectFields.Add("<your requested field 1>");
            selectFields.Add("<your requested field 2>");

            var projection = new Dictionary<string, dynamic>();

            //To include fields: Specify the field name and set to 0 in the project document.
            //Ex:- Exclue _id field
            if (!selectFields.Contains("_id"))
            {
                projection.Add("_id", 0);
            }
            //Only the fields specified in the project document are returned. The _id field is returned unless it is set to 0 in the Project document.

            //To include fields: Specify the field name and set to 1 in the project document.

            foreach (var field in selectFields)
            {
                projection.Add(field, 1);       
                //or else
                //projection.Add(field, $"${field}");
            }


            var projectStage = new BsonDocument("$project", projection.ToBsonDocument());
            tempPipeline.Add(projectStage.ToBsonDocument());

            PipelineDefinition<BsonDocument, BsonDocument> aggregatonPipeline = tempPipeline;

            var cursor =  GetDatabase().GetCollection<BsonDocument>("<your collection>").Aggregate(aggregatonPipeline);

           IList<dynamic> results = new List<dynamic>();

            while (cursor.MoveNext())
            {
                foreach (var document in cursor.Current)
                {
                    results.Add(BsonSerializer.Deserialize<dynamic>(document));
                }
            }
            cursor.Dispose();

            results.ToList();
    }

    public static IMongoDatabase GetDatabase()
        {
            var settings = new MongoClientSettings
            {
                // setup your db settings
            };

            var client = new MongoClient(settings);
            return client.GetDatabase("<your database>");
    }
}

Read more to set which fields are returned: https://docs.mongodb.com/compass/master/query/project/

And more :

https://docs.mongodb.com/manual/crud/

https://docs.mongodb.com/manual/tutorial/query-documents/