Mongo Db C# Driver Method to create new computed fields

Question

I need to create new fields to all existing documents to store the lowercase value of emailAddress(which is an existing field).

await _mongoHelper.MongoDatabase.GetCollection<T>("users").UpdateOneAsync(
           (Builders<T>.Filter.Eq("userId", userId)),Builders<T>.Update.Set("fieldInLower", new BsonDocument(new BsonElement("toLower", "$firstName")))).ConfigureAwait(false);
        )

Result is :

"fieldInLower" : {
   "toLower" : "$firstName"
}

Answer 1

starting from mongodb server v4.2 you can use aggregation pipeline stages to refer to the existing fields as described here .

the following example c# code does the same using 'MongoDB.Entities' which is a more convenient way compared to official driver.

using MongoDB.Entities;

namespace StackOverflow
{
    public class Program
    {
        public class User : Entity
        {
            public string Email { get; set; }
        }

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

            var user = new User { Email = "Email@Domain.Com" };
            user.Save();

            DB.Update<User>()
              .Match(u => u.ID == user.ID)
              .WithPipelineStage("{ $set: { LowerCaseEmail: { $toLower: '$Email' } } }")
              .WithPipelineStage("{ $unset: 'Email'}") //if you need to remove the original field
              .ExecutePipeline();
        }
    }
}