How to use Addfields in MongoDB C# Aggregation Pipeline

Question

Mongo DB's Aggregation pipeline has an "AddFields" stage that allows you to project new fields to the pipeline's output document without knowing what fields already existed.

It seems this has not been included in the C# driver for Mongo DB (using version 2.7).

Does anyone know if there are any alternatives to this? Maybe a flag on the "Project" stage?

Answer 1

I'm not sure all the BsonDocument usage is required. Certainly not in this example where I append the textScore of a text search to the search result.

        private IAggregateFluent<ProductTypeSearchResult> CreateSearchQuery(string query)
        {
            FilterDefinition<ProductType> filter = Builders<ProductType>.Filter.Text(query);
            return _collection
                .Aggregate()
                .Match(filter)
                .AppendStage<ProductType>("{$addFields: {score: {$meta:'textScore'}}}")
                .Sort(Sort)
                .Project(pt => new ProductTypeSearchResult
                {
                    Description = pt.ExternalProductTypeDescription,
                    Id = pt.Id,
                    Name = pt.Name,
                    ProductFamilyId = pt.ProductFamilyId,
                    Url = !string.IsNullOrEmpty(pt.ShopUrl) ? pt.ShopUrl : pt.TypeUrl,
                    Score = pt.Score
                });
        }

Note that ProductType does have a Score property defined as

        [BsonIgnoreIfNull]
        public double Score { get; set; }

It's unfortunate that $addFields is not directly supported and we have to resort to "magic strings"

Answer 2

As discussed here Using $addFields in MongoDB Driver for C# you can build the aggregation stage yourself with a BsonDocument.

To use the example from https://docs.mongodb.com/manual/reference/operator/aggregation/addFields/

{
  $addFields: {
    totalHomework: { $sum: "$homework" } ,
    totalQuiz: { $sum: "$quiz" }
  }
}

would look something like this:

BsonDocument expression = new BsonDocument(new List<BsonElement>() {
    new BsonElement("totalHomeWork", new BsonDocument(new BsonElement("$sum", "$homework"))),
    new BsonElement("totalQuiz", new BsonDocument(new BsonElement("$sum", "$quiz")))
});
BsonDocument addFieldsStage = new BsonDocument(new BsonElement("$addFields", expression));
IAggregateFluent<BsonDocument> aggregate = col.Aggregate().AppendStage(addFieldsStage);

expression being the BsonDocument representing

{
  totalHomework: { $sum: "$homework" } ,
  totalQuiz: { $sum: "$quiz" }
}

You can append additional stages onto the IAggregateFluent Object as normal

IAggregateFluent<BsonDocument> aggregate = col.Aggregate()
    .Match(filterDefintion)
    .AppendStage(addFieldsStage)
    .Project(projectionDefintion);