Querying Aggregated collection with C# driver (v2.0.1)

[Eric Le Goff]

Hi,

I'm new to MongoDB and to C#. 

Please apologize if this is the wrong place to ask (rather a C# mongodb driver 2.0 question)

I currently have a main collection 'event' which I successfully aggregate into a 'myaggregate' collection, i.e 

> db.event.aggregate(

... [

...     { $match: {"txn_date_time" :{$gt : new Date(ISODate().getTime() - 1000 * 60 * 60 * 24)}}},

...     { $group: {"_id": "$account_ref", "amount_last_day":{$sum:"$amt"} }},

...     { $out: "myaggregate"}

... ])

> 

> db.myaggregate.findOne()

{ "_id" : "487383", "amount_last_day" : 44.31999999999999 }

The aggregation query above was ported to a C# class where code looks like 

 await evenCollection.aggregate()

                .Match(X => X.txn_date_time > DateTime.Now.AddHours(-24))

                .Group(x => x.account_ref, g => new { Account = g.Key, amount_last_day = g.Sum(x => x.amt)})

                .OutAsync("myaggregate");

I can confirm that this C# code provides the expected result, i.e correct "myaggregate" collection is created.

Now I can succesfullly query this collection (find by Id) from mongo shell : 

> db.myaggregate.find({"_id" : 487383"})

{ "_id" : 487383", "amount_last_day" : 44.31999999999999 }

BUT my C# translation of this trivial query does not work as expected :

I created a  ""Agg" C# class to represent a ""myaggregate" entity

i.e

     [BsonIgnoreExtraElements]

    public class Agg

    {

        [BsonId]

        public  string Id { get; set; }  // please note this is no ObjectId

        public double totalAmount { get; set; }

    }

and 

my c# query goes like this

[..]

 string criteria ="487383";

 Task<Agg> task =  coll.Find(x => x.Id == criteria).FirstOrDefaultAsync();

            Task.WaitAll(task);

            Agg result = task.Result;

I don't understand why my result value is always 'null' where I was expecting an instance mapping the same result we got above  : 

{ "_id" : 487383", "amount_last_day" : 44.31999999999999 })

Could you point me to where the problem is ? In My 'Agg' entity class ? In my c# Task for querying the "myaggregate" collection ?

Thanks

--
Eric Le Goff

[Craig Wilson]

Hi Eric,

In your shell query, it appears that there might be a typo as the shell syntax listed is illegal. I don't intend to be pedantic, but MongoDB considers strings and integers different values. It kind of appears as though your shell query may using integers, while your Agg class is using strings for the "_id" field. Could you clarify?

Craig

[Eric Le Goff]

Hi Craig,

My  "event" collection has a "account_ref" field which actually contains alphanumeric values. My example was unfortunate since the chosen account was "487383" but could have been "foo42" instead.

> db.myaggregate.find({"_id" : "foo42"})

{ "_id" : "foo42", "amount_last_day" : 42.5 }

This is the reason why  Agg class contains a 

public  string Id { get; set; } but I can't make any Find request on 'myagregate' so that I get the corresponding instance.

I hope this is more clear

Thanks

Eric

--
Eric Le Goff

http://fr.linkedin.com/in/elegoff

@elegoff

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[Craig Wilson]

Hi Eric,

Perhaps you could simply post a full program I can try? I have attempted to reproduce this and am having no issues.

Craig

[Eric Le Goff]

Craig,

Thanks for your help. I was finally able to have a successful result. The code samples I provided were simplified and were indeed running fine (as you also experienced)

It seems the problem was related to case sensitivity : the member name in the Agg class (public double totalAmount { get; set; }) had to match exactly the aggegrated field in my query

 await evenCollection.aggregate()

                .Match(X => X.txn_date_time > DateTime.Now.AddHours(-24))

                .Group(x => x.account_ref, g => new { Account = g.Key, TotalAmount = g.Sum(x => x.amt)})

                .OutAsync("myaggregate");

--
Eric Le Goff

http://fr.linkedin.com/in/elegoff

@elegoff

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