MongoDB C# driver aggregation between dates returns null fields

Question

I am using the c# driver for mongodb and want to use an aggregation query for a web API I am creating. For my aggregation query I am concerned with a profile class that has usernames, dates and steps. I want to create a query that selects usernames and gets their total steps for a given week, in descending order by total steps. I only want to display their Username and their total steps.

When I attempt the aggregation query I am having an issue with some of my fields coming up as null. So, I believe my query is not structured correctly.

I have a “Profile” class that I am using for my data currently.

[BsonIgnoreExtraElements]
[DataContract]
public class Profile 
{   
    [DataMember]
    public string Username { get; set; }

    [DataMember]
    public DateTime Date { get; set; }

    [DataMember]
    public uint? Steps { get; set; }
}

I have created some test data with for example profiles using the following

//Test data
for (uint index = 1; index < 20; index++)
{      
    Profile aprofile = new Profile
    {   
        Username = string.Format("testuser{0}", index),
        Date = RandomDay(),
        Steps = (index + index + index)*2

    };
    AddProfile(aprofile);
}

If I run the code a few times and query a particular user, I get data that is like this:

[{"Username":"testuser1","Date":"2014-07-03T00:00:00Z","Steps":6},
{"Username":"testuser1","Date":"2014-07-07T05:00:00Z","Steps":6},
{"Username":"testuser1","Date":"2014-07-17T05:00:00Z","Steps":6},
{"Username":"testuser1","Date":"2014-07-18T05:00:00Z","Steps":6}]

Then, I have a couple static methods to find the earliest date and the latest date for my aggregation query.

//gets a datetime for the earlist date and time possible for the current week
public static DateTime GetStartOfCurrentWeek()
{
    int DaysToSubtract = (int)DateTime.Now.DayOfWeek;
    DateTime dt = DateTime.Now.Subtract(TimeSpan.FromDays(DaysToSubtract));
    return new DateTime(dt.Year, dt.Month, dt.Day, 0, 0, 0, 0);
}

//gets a datetime for the latest date and time possible for the current week
public static DateTime GetEndOfCurrentWeek()
{
    DateTime dt = GetStartOfCurrentWeek().AddDays(6);
    return new DateTime(dt.Year, dt.Month, dt.Day, 23, 59, 59, 999);
}

My attempt at an aggregation query is below.

//Here is my aggregation query to get all profiles within a given week, 
public IEnumerable<Profile> GetAllProfiles()
{
    DateTime StartofWeek = GetStartOfCurrentWeek();
    DateTime EndofWeek = GetEndOfCurrentWeek();


    var match = new BsonDocument 
                {{ "$match", new BsonDocument 
                     {{ "Date", new BsonDocument {
                            {"$gte", StartofWeek},
                            {"$lt", EndofWeek}
                      }}}
                }};

    var group = new BsonDocument
                {{"$group",
                    new BsonDocument
                    {{ "_id", new BsonDocument
                         {{"id", "$Username"},

                    }},
                         {"Steps",  new BsonDocument
                              {{"$sum", "$Steps"}}
                    }}
                }};


    var sort =  new BsonDocument
                {{"$sort", new BsonDocument
                    {{"Steps", -1}}

                }};    



    var pipeline = new[] {match, group, sort};      

    var args = new AggregateArgs { Pipeline = pipeline, OutputMode = AggregateOutputMode.Inline };

    // run the aggregation query and get a list of BsonDocuments
    IEnumerable<BsonDocument> documents = _profiles.Aggregate(args);
}

However, the results I am getting are showing the usernames as Null and the dates as null.

[{"Username":null,"Date":"0001-01-01T00:00:00","Steps":96},
{"Username":null,"Date":"0001-01-01T00:00:00","Steps":66},
{"Username":null,"Date":"0001-01-01T00:00:00","Steps":24}]

What do I need to do to get my array of documents so that the usernames display along with the total steps I have (which do seem to be working). I don't want the Dates in my query results. Just the Username and their total steps for that week.

Answer 1

I don't really understand why do you need to query your mongo with BSon. You do have your model class. Aggregation you are trying to perform will look like:

var result = collection.Aggregate()
    .Match(r => r.Date >= StartofWeek && r.Date < EndofWeek)
    .Group(r=>r.Username, r=> new {UserName = r.Key, Dates  = r.Select(d=>d.Date),  Steps = r.Sum(x=>x.Steps)})
    .ToList();

Note, that you could get not one date, but array of it, therefore i aggregate it as Dates.

Reason, why you don't see Date and username is that you don't query it, in your group statement i see just sum of steps. if you really want to continue to query this way, you should add it to your group stage, something like:

var group = new BsonDocument
             {
                    { "_id", new BsonDocument{{"id", "$Username"},
             }},
             {"Steps",  new BsonDocument{{"$sum", "$Steps"}}},
             { "Dates", new BsonDocument{{"$addToSet", "$Date"}} },
             { "Username", new BsonDocument{{"$first", "$Username"}} },
        };

This way i can see the Username and dates in resulting BsonDocument.

But as i said already, this way to query Mongo is not the best one to do in c#