C# MongoDb custom field and value discriminator

CustomFieldDiscriminatorConvention.cs

    /// <summary>
    /// Map a resource "keyType" to polymorphic classes.
    /// </summary>
    /// <remarks>
    /// Followed example here:
    /// http://pastebin.com/9UweEKBe
    /// </remarks>
    public class CustomFieldDiscriminatorConvention : IDiscriminatorConvention
    {
        private readonly string elementName;
        private readonly Type defaultType;
        private readonly Dictionary<string, Type> typeMap;

        public CustomFieldDiscriminatorConvention(string elementName, Type defaultType, Dictionary<string, Type> typeMap)
        {
            this.elementName = elementName;
            this.defaultType = defaultType;
            this.typeMap = typeMap;
        }


        /// <summary>
        /// Element Name
        /// </summary>
        public string ElementName
        {
            get { return elementName; }
        }

        /// <summary>
        /// Return the type that matches the string
        /// </summary>
        /// <param name="bsonReader"></param>
        /// <param name="nominalType"></param>
        /// <returns></returns>
        public Type GetActualType(MongoDB.Bson.IO.BsonReader bsonReader, Type nominalType)
        {
            Type type = defaultType;
            var bookmark = bsonReader.GetBookmark();
            bsonReader.ReadStartDocument();
            if (bsonReader.FindElement(ElementName))
            {
                var value = bsonReader.ReadString();
                if (typeMap.ContainsKey(value.ToLower()))
                    type = typeMap[value];
            }
            bsonReader.ReturnToBookmark(bookmark);
            return type;
        }

        public BsonValue GetDiscriminator(Type nominalType, Type actualType)
        {
            return actualType.Name;
        }
    }

example usage

            var dis = new CustomFieldDiscriminatorConvention(
                "keyType",
                typeof(Resource),
                new Dictionary<string, Type>(3)
                {
                    {"domain", typeof (DomainResource)},
                    {"ip", typeof (IpResource)},
                    {"uri", typeof (UriResource)}
                }
                );
            BsonSerializer.RegisterDiscriminatorConvention(typeof(Resource), dis);

@razzeee That makes sense, but I couldn't find a way to provide a default value for documents saved to the database before the discriminators were implemented using ScalarDiscriminatorConvention.