如何使用MongoDB C＃Driver Aggregate在2个字段中获取具有特定值的最新项目？

Question

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Basicly, I have a MongoDB with a collection of documents with these properties:

String group;
String key;
String value;
DateTime timestamp;

I want to get a list of all the most recent documents with unique (distinct) combination of group and key.

There are documents with the same group and key but different values; I only want the one with the most recent timestamp.

So if I have:

Document A: Group = 2; Key = Something; Value = 123; Timestamp = 20160621;

Document B: Group = 2; Key = Something; Value = 888; Timestamp = 20160622;

I want only document B (with value 888) to be retreived in the query.

This is what I got so far:

var aggregate = collection.Aggregate()
                .Match(new BsonDocument { { "deviceid", deviceid } })
                .Sort( new BsonDocument { { "timestamp", -1} })
                .Group(new BsonDocument { { "groupkey", "$group" }, { "latestvalue", "$first.value" } });

This however results in the following exception: "Command aggregate failed: the group aggregate field 'groupkey' must be defined as an expression inside an object"

1) Any tips on how to use Aggregate to do what I want? (basicly, sort descending by timstamp and distinct based on 2 fields)

2) Any tips on how to convert the aggregate to a list or similar? (I need to use linq on all the results to find the value of a document with specific group and key)

Answer 1

when using aggregation framework, as a part of training I will suggest execute same in monogo shell/robomongo to see correct syntax.

Please find below fixed syntax of your query

var aggregate = collection.Aggregate()
                .Match(x=>x.key== "k10")
                .SortByDescending(x=>x.timestamp)
               .Group( BsonDocument.Parse("{ '_id':'$group',  'latestvalue':{$first:'$value'} }")).ToList();