MongoDB result set for Aggregate()

Question

I started out with Mongo client doing some nifty queries and aggretations.. but now that I want to use it in .NET/C#, I see that I can't simply run the query as text field..

Furthermore, after resorting to building an Aggregation Pipeline, and running the collection.Aggregate() function, I'm getting a result set, but I have no idea how to traverse it..

Can anyone help guide me here?

Here's my code:

var coll = db.GetCollection("animals");
var match = new BsonDocument {
    { "$match",   new BsonDocument {{"category","cats"}} }
};

var group = new BsonDocument{
    {
        "$group", new BsonDocument{
            {"_id", "$species"}, 
            {"AvgWeight", new BsonDocument{{"$avg", "$weight"}}} }
    }
};

var sort = new BsonDocument{{"$sort", new BsonDocument{{"AvgWeight", -1}}}};
var pipeline = new[] { match, group, sort };
var args = new AggregateArgs { Pipeline = pipeline };
var res = coll.Aggregate(args);

foreach (var obj in res)
{ 
   // WHAT TO DO HERE?? 
}

Also, I should say that I'm a little rusty with C# / ASP.NET / MVC so any room for simplification would be much appreciated.

Answer 1

Your result is IEnumerable of BsonDocument, you can Serialize them to C# objects using the BSonSerializer. And this code snippet just writes them to your console, but you can see that you have typed objects

 List<Average> returnValue = new List<Average>();
 returnValue.AddRange(documents.Select(x=> BsonSerializer.Deserialize<Average>(x)));

 foreach (var obj in returnValue)
 { 
    Console.WriteLine("Species {0}, avg weight: {1}",returnValue._Id,returnValue.AvgWeight);
 }

And then have a class called Average, where the property name match the names in the BSonDocument, if you want to rename then (because _Id is not so nice in c# terms concerning naming conventions), you can add a $project BsonDocument to your pipeline.

 public class Average
 {
      public string _Id { get; set; }
      public Double AvgWeight {get; set; }
 }

$project sample (add this in your pipeline just before sort

 var project = new BsonDocument 
            { 
                { 
                    "$project", 
                    new BsonDocument 
                        { 
                            {"_id", 0}, 
                            {"Species","$_id"},
                            {"AvgWeight", "$AvgWeight"}, 
                        } 
                } 
            };