How to update documents using another field's value using MongoDB and C#?

Question

I'm trying to create a new field in all my documents in a collection. That field will be an array with another field's value as (for now) the single value.

This is my code:

var builder = new UpdateDefinitionBuilder<Member>();
var update = builder.AddToSet(f => f.Sids, "$Sid");

var models = new WriteModel<Member>[]
{
    new UpdateManyModel<Member>(FilterDefinition<Member>.Empty, update)
};

new MongoClient().GetDatabase("mydb").GetCollection<Member>().BulkWrite(models);

It almost worked: the new field (Sids) was created as an array, but with the literal value $Sid instead of the value from the Sid field.

What am I missing?

Answer 1

great answer by mickl...

starting with mongodb server v4.2 you can refer to existing fields of the documents using aggregation pipeline stages as described here.

in case anybody's interested here's a sample program using MongoDB.Entities doing the same:

using MongoDB.Entities;
using System;
using System.Collections.Generic;

namespace StackOverflow
{
    public class Program
    {
        public class Member : Entity
        {
            public string Sid { get; set; }
        }

        private static void Main(string[] args)
        {
            new DB("test");

            var members = new List<Member>();

            for (int i = 1; i <= 10; i++)
            {
                members.Add(new Member
                {
                    Sid = Guid.NewGuid().ToString()
                });
            }

            members.Save();

            DB.Update<Member>()
              .Match(_ => true)
              .WithPipelineStage("{ '$set': { 'Sids': ['$Sid'] } }")
              .WithPipelineStage("{ '$unset': ['Sid'] }")
              .ExecutePipeline();
        }
    }
}