When you create an aggregation using the fluent api, it seems that the BsonElement attribute on an element property is ignored.

See the following fluent aggregation:

var aggregation = Collection.Aggregate()
.Group(key => new Key

{ Name = key.Name, }

,
g => new Root

{ Id = g.Key, }

);

This generates the following aggregation command:

aggregate([{ "$group" : { "_id" :

{ "Name" : "$name" }

} }])

Note the capital "N" on "Name".

I'm grouping on a class of the Key, and here's the code for that class.

public class Key
{
[BsonElement("name")]
public string Name

{ get; set; }

}

I'm clearly mentioning "name" here, without any capital.
The generated aggregation should therefore be:

aggregate([{ "$group" : { "_id" :

{ "name" : "$name" }

} }])

A fix is required because for best practice, field names should not be long (because they are repeated over every document in the collection).
But POCO classes in C# might have long a descriptive names.

A field in C# can be named "ArticleNumber", while in mongo I would like to name it "artNr".

Kind regards