How to use Addfields in MongoDB C# Aggregation Pip

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

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);