How can I complete a string concatenation inside a project stage using C# and the Mongo aggregation pipeline?

Question

I'm trying to perform a string concatenation within a project stage of a MongoDb aggregation pipeline operation.

I need to add a field called "coid" to a document, which is to be the result of a string concation between 2 strings:

1. the string literal: "prefix-"
2. the string coming from the "SecId" field in the first document of the "values" array field.

My attempt is below, but it keeps generating a compiler error. Does anyone know how I can accomplish this string concatenation, within the aggregation pipeline project stage?

    new BsonDocument("$project", 
                    new BsonDocument
                    {
                        { "_id", 1 },
                        { "coid",
                        new BsonDocument("$concat",new BsonDocument[
                              new BsonDocument("prefix-"),
                              new BsonDocument("$first", "$values.SecId")])
                        }
                    })

Edit: Here is an example of one string concatenation: If the value of $values.Secid is "12345", then the concatenation should be "prefix-12345".

Update here is an enlarged view of my pipeline

            new BsonDocument("$lookup",
                new BsonDocument
                {
                    {"from","accounts"},
                    { "localField", "ig" },
                    { "foreignField", "ipi" },
                    { "as", "accounts" },
                }),
            new BsonDocument("$project",
                new BsonDocument
                {
                    { "_id", 1 },
                    {
                        "coid",
                        new BsonDocument("$first", "$accounts._id")
                    },
                    { "newField",
                        new BsonDocument("$concat","prefix-" + [from first element of $accounts array, take the _id value]
                    },
                }),
           new BsonDocument("$out", LocalOutputCollection)

Answer 1

There are a couple problems with that code:

• The literal passed in the $concat array should be string, not BsonDocument
• The $first operator is only available in a group stage, you probably need to use $arrayElemAt

new BsonDocument("$concat",
     new BsonDocument[
                      "prefix-",
                      new BsonDocument(
                            "$arrayElemAt", 
                            new BsonDocument[ "$values.SecId", 0]
                      )
     ]
)
                        

Answer 2

you can easily do $concat with the AsQueryable interface like so:

var result = collection
    .AsQueryable()
    .Select(x => new
    {
        x.Id,
        coid = "prefix-" + x.Values.First().SecId
    })
    .ToList();

it generates the following projection:

{
    "$project": {
        "Id": "$_id",
        "coid": { "$concat": ["prefix-", { "$arrayElemAt": ["$Values.SecId", 0] } ]
        }
    }
}