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MongoDB Aggregate function in C#

I am trying to display/list data after using aggregation function but it isn't happening.
This code works absolutely fine.
var connectionstring = "mongodb://localhost:27017";
var client = new MongoClient(connectionstring);
var db = client.GetDatabase("school");
var col = db.GetCollection<BsonDocument>("students");
var filter = new BsonDocument("type", "homework");
var filter2 = Builders<BsonDocument>.Filter.Eq("scores.type", "homework");
var myresults = await col.Find(filter2)
.Limit(2)
.Project("{name:1,scores:1,_id:0}")
.Sort("{score:1}")
.ToListAsync();
foreach (var result in myresults)
{
Console.WriteLine(result);
}
This code fetches document as it should however when I replace
var myresults = await col.Find(filter2)
.Limit(2)
.Project("{name:1,scores:1,_id:0}")
.Sort("{score:1}")
.ToListAsync();
with this
var myresults = await col.Aggregate()
.Unwind("{$scores}")
.Group(new BsonDocument { { "_id", "$_id" }, { "lowscore", new BsonDocument("$min", "$scores.score") } })
//.Group("{_id:'$_id',lowscore:{$min:'$scores.score'}}")
.ToListAsync();
No record is being pulled.
I do not want to use Pipeline method. I simply want to display the result obtained via aggregate function.
This is my Mongo Query (I want the same result as this in C#)-
db.students.aggregate([{$sort:{_id:-1}},{$unwind:"$scores"},{$group:{_id:"$_id", lowscore:{"$min":"$scores.score"}}}])

Building aggregation pipeline is bit tricky.
Try:
var pipeline = new BsonDocument[] {
new BsonDocument{ { "$sort", new BsonDocument("_id", 1) }},
new BsonDocument{{"$unwind", "$scores"}},
new BsonDocument{{"$group", new BsonDocument{
{"_id", "$_id"},
{"lowscore",new BsonDocument{
{"$min","$scores.score"}}
}}
}}
};
var result = collection.Aggregate<BsonDocument> (pipeline).ToListAsync();
If you do pipeline.ToJson(), you'll get following JSON equivalent string which is same as of your original and tested MongoShell query.
[
{
"$sort": {
"_id": 1
}
},
{
"$unwind": "$scores"
},
{
"$group": {
"_id": "$_id",
"lowscore": {
"$min": "$scores.score"
}
}
}
]

This is wrong... {$scores} isn't even valid json. Remove the curly braces and the dollar sign from the $unwind directive.
The parameter name is field, so you need to provide a field name to it.

Try with writing only $score instead of #scores.score. may be it helpful.
db.students.aggregate([{$sort:{_id:-1}},{$unwind:"$scores"},{$group:{_id:"$_id", lowscore:{"$min":"$score"}}}])

C# mongoDB wont update array in database

I want to update array in my mongoDB collection. Posting new document works just fine, but it won't update array. Structure of my document is:
var document = new BsonDocument
{
{ "post_title", model.Title },
{ "post_text", model.Text },
{ "post_author", model.Author },
{ "post_picture", model.Picture },
{ "post_comments", new BsonArray() },
};
And my update function is:
[HttpPost]
[Route("api/PostInfo/{Comment}")]
public async Task Post(Comment comment)
{
try {
BsonObjectId oldId = new BsonObjectId(new ObjectId(comment.id.ToString()));
var mongoDbClient = new MongoClient("mongodb://127.0.0.1:27017");
var mongoDbServer = mongoDbClient.GetDatabase("nmbp");
var collection = mongoDbServer.GetCollection<PostInfo>("post");
var filter = Builders<PostInfo>.Filter.Eq(e => e._id, oldId);
var update = Builders<PostInfo>.Update.Push("post_comments", comment.comment);
await collection.FindOneAndUpdateAsync(filter, update);
var test = oldId.GetType();
}
catch
{
}
}
When debugging, i can see that post controller is triggered, and comment values are correct, but when I take a look in database, value of "post_comments" array is empty. No error is thrown in catch block. Am I doing something wrong?

It looks like the problem was in this line:
var filter = Builders<PostInfo>.Filter.Eq(e => e._id, oldId);
The correct one should look like this:
var filter = Builders<PostInfo>.Filter.Eq("_id", oldId);

MongoDb Driver 2.0 C# Filter and Aggregate

I'm playing around with the new driver of mongodb 2.0, and looking for adding some facetted searchs (Temporary move ,before using elastic search).
Here is some method where I created to build the agreggation. I guess that it should work.
As parameter I passed also a filterdefinition in the method.
But I don't find how to limit my agreggation to the filter.
Any Idea ???
private void UpdateFacets(SearchResponse response, FilterDefinition<MediaItem> filter, ObjectId dataTableId)
{
response.FacetGroups =new List<SearchFacetGroup>();
SearchFacetGroup group = new SearchFacetGroup()
{
Code = "CAMERAMODEL",
Display = "Camera model",
IsOptional = false
};
using (IDataAccessor da = NodeManager.Instance.GetDataAccessor(dataTableId))
{
var collection = da.GetCollection<MediaItem>();
var list = collection.Aggregate()
.Group(x => ((ImageMetaData) x.MetaData).Exif.CameraModel, g => new { Model = g.Key, Count = g.Count() })
.ToListAsync().Result;
foreach (var l in list)
{
group.Facets.Add(new SearchFacetContainer()
{
Code = l.Model,
Display = l.Model,
Hits = l.Count,
IsSelected = false
});
}
}
response.FacetGroups.Add(group);
}

I haven't used facet, but with Mongo driver Aggregate has .Match operation that accepts a filterdefinition.
collection1.Aggregate().Match(filter)

MongoDB result set for Aggregate()

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.

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"},
}
}
};

MongoDB C# query for timestamp less than the current time

I'm trying to return documents from the collection where id = 1 and date created is less than the current time.
I tried this, but it isn't working:
var collection = database.GetCollection("test");
var time = DateTime.Now;
var query2 = new QueryDocument
{
{ "id", 1},
{{"created_on", {"$lt",time}}
};
What's wrong with this query?

You need an embedded document for the $lt sub-object, but you forgot to create it:
var query = new QueryDocument {
{ "id", 1 },
{ "created_on", new BsonDocument { { "$lt", time } } }
}
Also consider using the Query builder, which might make things simpler:
var query = Query.And(
Query.EQ("id", 1),
Query.LT("created_on", time)
);